NAVAL POSTGRADUATE SCHOOL · 2019. 10. 4. · MEXSAT performance in order to evaluate the...

NAVAL POSTGRADUATE

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THESIS

FEASIBILITY STUDY OF OPTIMIZATION MODELS FOR THE MEXSAT SYSTEM

by

Armando Cruz Ojeda

June 2019

Thesis Advisor: Charles M. Racoosin Co-Advisor: Steven J. Iatrou

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6. AUTHOR(S) Armando Cruz Ojeda

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A 13. ABSTRACT (maximum 200 words) The Mexican Satellite (MEXSAT) system has a limited lifetime; therefore, it is crucial to evaluate and compare the performance of alternative techniques to achieve space communications, which could possibly improve the Mexican space communication capabilities. Furthermore, when compared to MEXSAT, the O3b constellation concept seemed to offer several benefits. The purpose of this thesis was to investigate the feasibility of acquiring satellite communications with more efficient performance for the Mexican Navy (SEMAR) as the main stakeholder, by adopting this O3b constellation concept. This research used STK software to develop, analyze, and compare different proposed satellite constellation models and their communication links with the current MEXSAT performance in order to evaluate the feasibility of getting a more capable and functional satellite constellation for Mexico. When modeling and comparing both communication capabilities, the research found antenna gain values were crucial to the efficiency of the links. Moreover, rain attenuation values were critical and significantly affected the performance of the modeled links at Medium Earth Orbit. Ultimately, our simulated results showed that the performance of the current MEXSAT system was superior to the modeled O3b constellations.

14. SUBJECT TERMS Mexican Navy, space, JC4I systems, C4I, satellite, constellation, model, RF, communications, MEXSAT, SATCOM, link budget, Mexico, O3b

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Approved for public release. Distribution is unlimited.

FEASIBILITY STUDY OF OPTIMIZATION MODELS FOR THE MEXSAT SYSTEM

Armando Cruz Ojeda Lieutenant Junior Grade, Mexican Navy

BEE, Escuela de Ingenieros de la Armada de México, 2012

Submitted in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE IN SYSTEMS TECHNOLOGY (COMMAND, CONTROL, AND COMMUNICATIONS)

from the

NAVAL POSTGRADUATE SCHOOL June 2019

Approved by: Charles M. Racoosin Advisor

Steven J. Iatrou Co-Advisor

Dan C. Boger Chair, Department of Information Sciences

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ABSTRACT

The Mexican Satellite (MEXSAT) system has a limited lifetime; therefore, it is

crucial to evaluate and compare the performance of alternative techniques to achieve

space communications, which could possibly improve the Mexican space communication

capabilities. Furthermore, when compared to MEXSAT, the O3b constellation concept

seemed to offer several benefits. The purpose of this thesis was to investigate the

feasibility of acquiring satellite communications with more efficient performance for the

Mexican Navy (SEMAR) as the main stakeholder, by adopting this O3b constellation

concept. This research used STK software to develop, analyze, and compare different

proposed satellite constellation models and their communication links with the current

MEXSAT performance in order to evaluate the feasibility of getting a more capable and

functional satellite constellation for Mexico.

When modeling and comparing both communication capabilities, the research

found antenna gain values were crucial to the efficiency of the links. Moreover, rain

attenuation values were critical and significantly affected the performance of the modeled

links at Medium Earth Orbit. Ultimately, our simulated results showed that the

performance of the current MEXSAT system was superior to the modeled O3b

constellations.

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TABLE OF CONTENTS

I. INTRODUCTION..................................................................................................1 A. PROBLEM STATEMENT .......................................................................1 B. RESEARCH QUESTIONS .......................................................................2 C. C4I SYSTEMS AND SPACE SYSTEM CAPABILITIES.....................3 D. MEXICAN NAVY’S SPACE COMMUNICATIONS

CAPABILITIES .........................................................................................5 E. POTENTIAL BENEFITS .........................................................................6 F. LIMITATIONS ..........................................................................................8 G. RESEARCH METHODS ..........................................................................8 H. THESIS OUTLINE ....................................................................................9

II. CURRENT MEXSAT SYSTEM ........................................................................11A. POLITICAL OVERVIEW .....................................................................11 B. TECHNICAL OVERVIEW ....................................................................12

1. Advantages of GEO .....................................................................172. Disadvantages of GEO .................................................................193. Ku-band HTS Systems Performance .........................................21

C. THE USAGE OF THE MEXSAT SYSTEM BY SEMAR ...................24

III. OPTIMIZATION MODELS ..............................................................................25A. PROPOSED MEXICAN NAVY REQUIREMENTS ...........................25 B. DESIGN OF THE PROPOSED SATELLITE

CONSTELLATION .................................................................................27 1. O3b Satellite Constellation Concept...........................................282. Proposed STK Models and Their Satellite

Communications Capabilities .....................................................32

IV. COMMUNICATIONS PERFORMANCE OF THE PROPOSEDMODELS ..............................................................................................................61 A. LINK VARIABLES CLASSIFICATION..............................................61 B. COMPARISON METHODOLOGY ......................................................62

1. Downlink Performance ................................................................632. Uplink Performance.....................................................................67

V. CONCLUSION AND FUTURE WORK ...........................................................71

LIST OF REFERENCES ................................................................................................75

INITIAL DISTRIBUTION LIST ...................................................................................83

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LIST OF FIGURES

Figure 1. Global O3B coverage anywhere 45 degrees North/South of the Equator. Source: [10]. ..................................................................................2

Figure 2. Project Echo 2 passive communications satellite, 1965. Source: [15]. .......4

Figure 3. GSO vs. GEO. Source: [27]. ......................................................................13

Figure 4. L-band coverage of the MEXSAT system. Source: [19]. ..........................14

Figure 5. Ku-band coverage of the satellite Bicentenario. Source: [30]. ..................14

Figure 6. Wide beams coverage vs. HTS technology coverage. Source: [32]. .........15

Figure 7. Morelos 3 L-Band system coverage is distributed on spot beams. Source: [18]. ...............................................................................................16

Figure 8. Covered geographical areas change with respect with the frequency used. Source: [31]. .....................................................................................16

Figure 9. Geostationary satellite coverage. Source: [37]. .........................................20

Figure 10. Total, dry air and water-vapor zenith attenuation from sea level with surface pressure: 1013 hPa, surface temperature: 15° C, and surface water-vapor density: 7.5 g/m3. Source: [39]. ............................................22

Figure 11. Mexican territory, coastlines, and its EEZ. Source: [43]. ..........................26

Figure 12. O3b constellation system. Source: [46]. ....................................................29

Figure 13. Continuous street of coverage from a single orbital plane. Source: [48]. ............................................................................................................30

Figure 14. O3b HTS satellite final assembly at Thales, in Cannes, France. Source: [46]. ...............................................................................................30

Figure 15. Map of Mexico. Source: [49]. ....................................................................31

Figure 16. Mexican territory and its EEZ modeled in STK ........................................34

Figure 17. Coverage of a Ka-band HTS ......................................................................35

Figure 18. Coverage of a Ku-band HTS......................................................................36

Figure 19. Modeled current MEXSAT system ...........................................................37

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Figure 20. Geometric relation between satellite and user. Source: [58]. ....................38

Figure 21. View of two satellites’ spot beams overlapping the same region ..............40

Figure 22. Time when three satellites overlap occurred in a non-cost efficient modeled constellation. ...............................................................................40

Figure 23. Isometric view of the proposed Walker constellation................................42

Figure 24. Walker constellation grid statistics 24-hour report ....................................43

Figure 25. Proposed MEO satellite constellation with one orbital plane and five satellites......................................................................................................43

Figure 26. Proposed five-satellite O3b constellation grid statistics 24-hour report ..........................................................................................................44

Figure 27. Modeled five-satellite O3b constellation with non-continuous coverage on the targeted area .....................................................................45

Figure 28. Proposed MEO satellite constellation with one orbital plane and seven satellites ...........................................................................................45

Figure 29. Proposed seven-satellite O3b constellation grid statistics 24-hour report ..........................................................................................................46

Figure 30. Modeled seven-satellite O3b constellation with non-continuous coverage on the targeted area. ....................................................................47

Figure 31. Proposed MEO satellite constellation with one orbital plane and eight satellites.............................................................................................47

Figure 32. Proposed eight-satellite O3b constellation grid statistics 24-hour report ..........................................................................................................48

Figure 33. Modeled eight-satellite O3b constellation with non-continuous coverage on the targeted area .....................................................................49

Figure 34. Typical BER vs. Eb/No curves. Source: [21]. ...........................................52

Figure 35. AER report of the proposed O3b MEO satellite constellation ..................55

Figure 36. Rainfall rate exceeded for 0.01% of an average year. Source: [69]. .........56

Figure 37. Representation of schemes of QPSK and 8PSK signals. Source: [67]. .....57

Figure 38. Schematic showing the change in path length through the atmosphere. Source: [21]. ..........................................................................72

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LIST OF TABLES

Table 1. Cost of the proposed satellite constellations based on the number of satellites......................................................................................................39

Table 2. Diameter of the reflector antennas per type of facility. .............................59

Table 3. Achieved gain values of the reflector antennas per type of facility. ..........60

Table 4. Achieved gain values of the satellites’ reflector antennas. ........................60

Table 5. Achieved performance the link on the aircraft facility. .............................64

Table 6. Achieved performance the link on the expeditionary station facility. .......65



Table 9. Achieved performance the link on the aircraft facility. .............................68




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LIST OF ACRONYMS AND ABBREVIATIONS

8PSK Eight-Phase Shift Keying AER Azimuth, Elevation, and Range BER Bit-Error-Rate C2 Command and Control C4I Command, Control, Communications, Computers, and Intelligence EEZ Exclusive Economic Zone EIRP Effective Isotropic Radiated Power FSS Fixed Services Satellite GEO Geostationary Orbit GPS Global Positioning System GSO Geosynchronous Orbit HTS High Throughput Satellite IEEE Institute of Electrical and Electronics Engineers IFL Inter-Facility Link ITU International Telecommunications Union LFS Free Space Loss LO Local Oscillator MEO Medium Earth Orbit MEXSAT Mexican Satellite system MSS Mobile Services Satellite NASA National Aeronautics and Space Administration NOC Network Operations Center PSK Phased-Shift Keying QPSK Quadra-Phase Shift Keying RF Radio Frequency SATCOM Satellite Communication SCT Secretariat of Communications and Transport SEMAR Mexican Navy STK Systems Tool Kit

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ACKNOWLEDGMENTS

First, my greatest gratitude goes to my lovely wife, Iyanu. Her infinite patience,

love, and support were essential for facing all the tough challenges we encountered

within these two years in Monterey. Thank you for tolerating my absences at home when

I had to work on this thesis. Thank you for being there for better, for worse, in sickness,

and in health. I love the family that you gave me, and I love you more.

This work is also dedicated to my beautiful daughter, Sofia. You might not be

aware now, but you inspire me to become better every day. As any other Dad, I want the

best for you. Thank you for being such a good girl and for always cheering me up.

I am very thankful to my parents, Armando and Maria del Carmen, and my

brothers, Pablo and Rodrigo. They persuaded me to always force myself and do the best I

can, no matter the task. Without their guidance, I would have never achieved this

triumph. Thank you for your love and for always being there when I needed it. Thank

you, Dad, for ALWAYS having a good answer to all the questions I have.

Distinctive appreciations go to my advisor, Professor Charles M. Racoosin, and

my co-advisor, Professor Steven J. Iatrou. Thank you for your kind instruction, wisdom,

patience, and perseverance. Your infinite help and hard work permitted this project to

succeed.

I will be eternally grateful to the Mexican Navy for giving me this huge

educational and once-in-a-lifetime opportunity. I will always be in your debt.

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I. INTRODUCTION

A. PROBLEM STATEMENT

Unfortunately, as all other nations around the world, the United Mexican States

(Mexico) possesses limited economic resources. This motivates it to spend resources on

new military technology wisely whenever there is a necessity for technological

investment. An informed decision maker is predisposed to make the right decisions,

based on the priorities, requirements, and limitations.

New technologies emerge at a great pace, obeying Moore’s Law which proposes

an exponential growth in the world’s technological pace [1]. Now, several innovative

space concepts seem to be suitable for the Mexican Navy (SEMAR). These new

technologies were not available when the Mexican Satellite system (MEXSAT) was

designed, and need to be evaluated properly to see if they fit Mexico’s necessities and

boundaries. The MEXSAT system was implemented in 2012, when satellite Bicentenario

was launched on December 18, 2012 [2]. A second satellite, Satellite Morelos 3, was

launched on October 2, 2015 [3]. Both satellites are expected to have a 15-year life span

[4]. This means that the MEXSAT is halfway through its life expectancy and today

Mexico should be looking forward to examine if the current geostationary orbit (GEO)

satellite constellation configuration is still the right option.

We envisioned that modifying several characteristics of the current MEXSAT

system could both improve the performance and decrease the cost of building the actual

satellites, launching them, and establishing their required ground infrastructure. Recently,

commercial firms have built different types of satellite constellation systems using a

different range of frequencies and launching them in orbits other than GEO. Additionally,

commercial companies have made space systems more affordable to launch [5].

Furthermore, compared with other types of space systems, GEO satellites present several

technical and economical disadvantages compared to lower orbit satellites [6]. These

disadvantages could culminate in a gap that another different type of satellite

constellation could close if implemented by the SEMAR.

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Emerging technology allows leaders to multiply their strengths and greatly

enhances their authority over their assets [7]. The SEMAR recognizes that taking care of

its most valuable asset, its sailors, represents a broad range of actions. One of these

actions is providing the proper technology to develop the ordered operations within any

field and conditions. The expiration of the current MEXSAT system is unavoidable.

Therefore, a formal evaluation of the current system must be conducted to increase its

technological capabilities, be in conditions to contend with the external and internal

threats, and possibly decrease the costs associated with it.

B. RESEARCH QUESTIONS

This study explored the feasibility of acquiring a better quality and faster satellite

communications for SEMAR by adopting the O3b constellation concept. The O3b system

is a Medium Earth Orbits (MEO) constellation designed for telecommunications under a

coverage footprint of +/- 45 degrees latitude [8]. O3b coverage is illustrated in Figure 1.

Moreover, the O3b concept offers continuous coverage worldwide. When one satellite

leaves a specific facility, the following one takes over without transmission interruption

[9]. Consequently, MEO satellite constellations could be functional and suitable for

SEMAR’s purposes.

Figure 1. Global O3B coverage anywhere 45 degrees North/South of the Equator. Source: [10].

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Therefore, we envisioned that this study could potentially permit answering the

following questions in a more informed way.

• Based on the current Mexican operational requirements, could Mexico

improve its communications satellite capabilities?

• How profitable is to apply the O3b constellation concept to replace the

current MEXSAT and improve the communication capabilities?

• Is it possible to achieve more satellite communication capabilities with the

usage of a different type of orbit satellite constellation?

• Could the O3b constellation concept represent a more economical

alternative if implemented for Mexico?

C. C4I SYSTEMS AND SPACE SYSTEM CAPABILITIES

Any military must acquire and maintain optimally ready the required tools to

properly plan and execute military operations when and where they are ordered. Those

tools include Command, Control, Communications, Computers, and Intelligence (C4I)

systems, which provide a commander the essential information to design, direct, and

control all assigned military operations properly [11]. Commanders must gain

information superiority, situational awareness, and intelligence dominance to achieve

operational success in the battle space [7]. Generally, C4I systems would permit

commanders to become more effective by allowing them to acquire that required

situational awareness across any military operation and communicate with the troops in

order to improve either planning or making decisions regardless the type of mission [12].

R. C. Olsen described the different capabilities that have been addressed by either

military or commercial space technologies, which include communications,

reconnaissance, surveillance, remote sensing, and meteorology fields [13]. The scope of

this thesis focuses on space-based communication systems, which pushes the military to

continuously acquire new and more capable systems as consistently as possible.

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In 1960, The National Aeronautics and Space Administration (NASA) launched

Echo I, shown in Figure 2, an experimental communication satellite, which allowed users

to share radio signals between two points on Earth that were too far apart to establish

terrestrial communications [14]. The success of this project can be considered the

beginning of the information age. Today’s space communications capabilities, through

the use of a space communication relay, allow people around the world to connect with

any other connected point of the Earth, using either a personal or a non-personal

electronic device for either civilian or military purposes.

Figure 2. Project Echo 2 passive communications satellite, 1965. Source: [15].

To achieve an exceptional performance, C4I systems must remain interconnected

no matter the location of the troops or terminals and the time when the operation takes

place. Therefore, space communications systems represent the dominant technology

available that could satisfy the basic connectivity requirements for C4I systems either

worldwide or locally. Space technologies could allow every military user to have a global

view of any area of interest [14]. The Committee to Review DoD C4I Plans and

Programs recognized this when the U.S. military was using the U.S. Global Positioning

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System (GPS) in 1993, as friendly armed forces conducted well-orchestrated actions

within tactical operations in order to attack and defeat Iraqi enemy forces [12]. Space

communications systems allow the military to exponentially increase their situational

awareness on the battlefield by seeing what is happening in real-time.

Essentially, C4I systems with space communications capabilities allow the

military to deploy troops and establish complex military operations where there was not

any previously established infrastructure. This advantage makes it possible to defeat any

adversary with fewer human resources, and it also provides the proper tools, sufficient

information, and tactics to achieve information and intelligence superiority.

D. MEXICAN NAVY’S SPACE COMMUNICATIONS CAPABILITIES

SEMAR, as an internally regulated armed force of Mexico, has to accomplish

actions to safeguard national sovereignty and defend the nation from external threats

[16]. Additionally, it should guarantee compliance with the legal order in the Mexican

marine zones by fighting against terrorism; smuggling of contraband; and illegal

trafficking of persons, weapons, narcotics, and psychotropic drugs under the terms of the

applicable legislation [16]. Hence, SEMAR currently employs C4I systems to constantly

conduct strategic, operational, and tactical military operations within both the Mexican

territorial seas and the Mexican Exclusive Economic Zone (EEZ). Clearly, the SEMAR

must regularly look to upgrade its C4I systems, and should consider it a national security

matter. Acquiring innovative C4I solutions will finally permit SEMAR to widen its

situational awareness within all levels of operations.

Currently, the SEMAR only accounts for space communications capabilities by

using the MEXSAT. The development of the MEXSAT system was a project conducted

by the Mexican Federal Secretariat of Communications and Transport (SCT) [4], in

which the satellite constellation was built and is currently used by several Mexican

Federal dependencies. The MEXSAT system consists of two GEO satellites. One satellite

is for fixed services (FSS), the Satellite Bicentenario, and the second satellite is for

mobile services (MSS), the Satellite Morelos 3. Along with these two satellites, there are

two ground stations located in Iztapalapa, Mexico City, and Hermosillo, Sonora [17].

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Satellite Bicentenario transponders work within the Ku and C frequency bands, and were

designed to primarily provide telecommunication services to fixed terminals within

located facilities [18]. Moreover, satellite Morelos 3 transponders work within the Ku and

L frequency bands [18]. Morelos 3 was mainly designed to deliver mobile satellite

communications.

Today, SEMAR employs L-band and Ku-band terminals within its assets to

provide communication capabilities using the MEXSAT when an operation of any type is

ordered. The hardware and software specifications of the installed terminals depend on

the type of facility on which they are installed. Moreover, the SEMAR installs these

terminals in a wide variety of ships, aircrafts, ground vehicles, and ground facilities.

The nature of the MEXSAT improves the capabilities of the Mexican C4I

systems. The SEMAR uses the Ku-band terminals as the main medium for sharing voice

and text within large and medium range. The satellite Bicentenario provides both services

such as institutional e-mail, institutional cloud, VoIP telephony, and access to Internet

and coverage to large ships, naval stations, and aircrafts [19].

Additionally, the SEMAR employs L-band terminals as an alternate voice and

text communication system within large and medium range. L-band systems provide the

troops with sharing data and VoIP capabilities.

E. POTENTIAL BENEFITS

This study was developed to evaluate if the O3b MEO concept could offer several

benefits due to its essential characteristics, such as lower launching costs, more

reasonable costs for satellite building, lower latency, possible higher data rates, smaller

antennas, coverage flexibility, and less power needed to close the link to MEO.

Due to the MEO satellites being the four times closer to GEO constellations,

lower costs associated with launching are implicit [14]. Furthermore, commercial

contractors have provided more affordable prices for launching both classified and

unclassified payloads to space [5]. Likewise, MEO satellites are expected to have a lower

cost than GEOs. We projected that Mexico could build a small constellation by

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employing only a few MEO satellites which would cover continuously the main interest

areas where SEMAR develops most of its operations. These aggressive changes provide

an enormous opportunity for Mexico to decrease the cost of building and launching the

required satellite constellation.

Additionally, comparing MEO with GEO satellites, MEO satellites achieve a

lower latency, similar to fiber optic. This lower latency is due to the shorter distance that

electromagnetic waves have to travel through space in order to arrive to their destinations

[10]. This huge advantage makes the O3b concept especially convenient and suitable for

military operations where there is a low latency network requirement but there is no

physical network infrastructure established previously.

O3b telecomm system is reported to have the possibility to supply multiple

gigabits of throughput worldwide [9]. Furthermore, O3b satellites carry transponders that

work with Ka-band, providing 20 GHz for the downlink (satellite to Earth) and 30 GHz

for the uplink (Earth to satellite) [9]. This Ka-band provides high quality throughput up to

1.2 Gbps per satellite and up to four times faster data rates than current GEO satellites

[9]. This O3b concept could improve the current MEXSAT data rates. We therefore

evaluated emerging space communication concepts like Ka-band and Ku-band in MEO

satellite constellations.

Moreover, the usage of Ka-band could theoretically lead to use smaller antennas

when needed. Generally, the antenna sizes are designed based on its working frequencies

[20]. Scaling the frequency to higher values will allow to the use of smaller antenna sizes

and probably achieve the same required performance achieved with lower frequencies

[20]. SEMAR, as a military force, is highly interested in portable and mobile

communications systems. Therefore, this feature represents a significant possible benefit.

O3b satellites carry transponders with steerable beams that can be placed

anywhere 45 degrees north/south of the equator [9]. This feature provides significant

flexibility of coverage, allowing Mexico to have wider coverage than just the Mexican

national territory. Additionally, SEMAR commonly participates in international maritime

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warfare exercises. Consequently, Mexican sailors could acquire a more accurate

situational awareness on these naval exercises from this coverage flexibility feature.

Finally, MEO satellites require less power to complete communications than GEO

satellites need. This is due to the smaller gap between the MEO satellites and ground

facilities, and theoretically MEO satellite terminals would need less power to close the

link, no matter their location.

F. LIMITATIONS

Due to clearance issues, the current MEXSAT information such as distances, used

frequencies, data rates, antenna sizes, or transmission power values of satellites and

ground terminals required to obtain communications with satellites Bicentenario and

Morelos 3 were not available. Therefore, simulations were used to obtain a reasonable

approximation of those values.

G. RESEARCH METHODS

This study was accomplished primarily by using Systems Tool Kit (STK) to

validate the feasibility of acquiring a better quality and faster satellite communications

for SEMAR compared with the current performance of only Ku-band MEXSAT systems.

L-band MEXSAT systems were excluded from this study.

STK is a powerful software with limited land, air, sea, and space-based analysis

features. For this study we used STK for both simulating the required space constellation

models and executing the required link budget analysis to ratify the proposed satellite

communication links were feasible and better than the current performance of the Ku-

band systems of MEXSAT.

Furthermore, we conducted numerous link budget analyses. The link budget

analysis is the first step when designing or proving the feasibility of a determinate space

communication system [21]. The link budget equation conducts a mathematical test used

to calculate the required antenna gain and the transmit power to close a satellite link [21].

This equation considers all the losses and gains generated from the transmitter to the

receiver in space-based communications [21].

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After having analyzed the proposed satellite communication links and collecting

all the data, we conducted different comparisons between Ku and Ka band performances

combined with different simulated constellations in order to obtain the best performance

from all the models.

H. THESIS OUTLINE

Chapter II explains the essence of the current MEXSAT system by providing a

political and technical overview, defines the characteristics and performance of it, and

states how SEMAR uses it. Chapter III describes the proposed architecture dividing it

into two major areas of expertise, the satellite constellation design and the space-earth

links achieved by that constellation. Next, Chapter IV uses the linkbudget equation

methodology to assess the efficiency between the current MEXSAT system and the

proposed satellite communication systems. It describes the comparison made between

space communication links from both MEXSAT GEO and the chosen MEO using Ku-

band and Ka-band transponders, which helps to find the constellation with the best

performance. Finally, Chapter V concludes and provides recommendations for future

research.

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II. CURRENT MEXSAT SYSTEM

Mexico has an official territory of 1.96 million square kilometers of landmass

area, 5,127 square kilometers of insular surface, and 3.1 million square kilometers of

EEZ [22]. Therefore, the total area of Mexico exceeds 5 million square kilometers.

Moreover, Mexico has economic rights about the EEZ within a maximum width of 684

kilometers [22].

Due to the vast area of this territory, Mexico possesses a wide variety of

geographic ecological systems and mountain ranges [23]. These different chains of

mountains and valleys that exist within the Mexican territory make the communication

links susceptible to losses and natural interference due to long distances. This could affect

the range and quality of military terrestrial communications and, therefore, decrease the

effectiveness of the tactical operations that the Mexican Navy performs. Moreover,

Mexico’s large EEZ, makes the task of covering the whole zone by placing

communication repeaters or stations nearly impossible [23]. The necessity to have a

reliable military communications system represents an exhausting effort, due to this vast

and diverse territory, yet is necessary for protecting the national sovereignty.

A. POLITICAL OVERVIEW

Satellite communications technology is advancing at a great pace [4]. This

dramatic pace encourages each country’s leadership to keep updated and synchronized

with their executive manager and technical leaders in order to achieve the technological

military requirements and system compatibility that every country or organization needs.

To maintain the national security and keep telecommunication services inside the

Mexican territory, the SCT and other Mexican Federal dependencies, including the

SEMAR, announced the creation of the MEXSAT system [4]. The SCT conducted and

currently manages the MEXSAT project, but all the involved federal dependencies

provided part of the costs and infrastructure required to complete the project [4].

MEXSAT is a satellite communication system that provides satellite

communication services in the Mexican landmass, territorial waters, and some regions

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bordering the national territory [17]. The MEXSAT system strengthened the national

telecommunications sector with its offered services and with the fact that it is a system

owned by the Mexican government [24]. The MEXSAT system is currently employed by

the SEMAR within the tactical, operational, and strategic levels of operations [24].

MEXSAT also helps other federal dependencies to achieve communications in cases

where remote communication services are needed such as disaster relief, emergency

response, telemedicine, and remote mode education within rural communities inside the

national territory [24].

The MEXSAT system helps SEMAR to achieve its mission because, as an armed

military force, it has to accomplish actions to safeguard national sovereignty and defend

the nation from external threats [16]. Additionally, the MEXSAT system should provide

to SEMAR enough C4I capabilities to guarantee compliance of the legal order in the

Mexican marine zones by fighting against terrorism; smuggling of contraband; and illegal

trafficking of persons, weapons, narcotics, and psychotropic drugs under the terms of the

applicable legislation [16]. Conclusively, the MEXSAT system is an efficient

telecommunications tool that provides a broad range of services inside the coverage

footprint of that system.

B. TECHNICAL OVERVIEW

Using Newton’s law of gravity, Li showed that GEO satellites are able to stay on

their orbit because they balance its centrifugal force while moving and the gravitational

force [25]. It is possible to know that the radius of a perfect GEO is 42164.2 kilometers,

by solving

(1)

from [25].

The GEO concept can only be accomplished based on the main assumption that

the Earth is a spherical symmetric body [26]. GEO satellites remain about 35,000

kilometers far apart from the Earth [26]. Additionally, to be called geostationary orbits,

Li defined that “the spacecraft should orbit on the equatorial plane, flying in the same

13

direction as the rotation of the Earth” [25]. Otherwise, inclined orbital planes with a

35,000 km altitude are called geosynchronous orbit (GSO). Figure 3 visually

demonstrates the inclination difference between GSO and GEO.

Figure 3. GSO vs. GEO. Source: [27].

The current MEXSAT system includes two GEO satellites, satellite Morelos 3

and satellite Bicentenario, and two gateway stations located in Iztapalapa, Mexico City

and Hermosillo, Sonora, respectively [17]. MEXSAT satellites currently provide

communication capabilities within L-Band, Ku-Band, and C-Band [17].

Satellite Morelos 3 is a GEO satellite, placed in the orbital position 113º W with a

communications payload that operates in both L-Band frequency and Ku-Band

frequency. It provides mobile communication services to mobile and semi-fixed users

[28]. The Institute of Electrical and Electronics Engineers (IEEE) stated that the nominal

frequency range for the L-band is from 1 GHz to 2 GHz and for the Ku-Band is from

12.0 GHz to 18 GHz [29]. Additionally, the L-band coverage provided by the satellite

Bicentenario is demonstrated in Figure 4.

14

Figure 4. L-band coverage of the MEXSAT system. Source: [19].

The satellite Bicentenario is a geostationary satellite placed in the orbital position

116.8º W with a communications payload, which operates at Ku-Band frequency and C-

Band frequency, provides services to fixed location terminals [17]. The IEEE states that

the nominal frequency range for the Ku-Band is from 12.0 GHz to 18 GHz, and C-band is

from 4 GHz to 8 GHz [29]. The Ku-band coverage provided by the satellite Bicentenario

is illustrated in Figure 5.

Figure 5. Ku-band coverage of the satellite Bicentenario. Source: [30].

15

Both satellites, Bicentenario and Morelos 3, are not conventional FSS or MSS, as

they also fall in the high throughput satellite (HTS) category [4]. FSS and MSS possess

one or few beams that cover large regions of ground. An HTS, however, is a satellite that

possesses multiple spot beams instead of having one single beam to cover a massive area.

Each beam covers a much smaller area and “re-uses” the same frequency in each spot

beam [31]. Therefore, it could cover a considerable region with many such spot beams.

Moreover, if a spot beam is far enough from the closest beams, there will not be

interference and those spot beams can re-use the same frequency. This HTS technology

increases the coverage, throughput, and efficiency of the links. The performance of the

HTS technology is shown in Figure 6.

Figure 6. Wide beams coverage vs. HTS technology coverage. Source: [32].

Figure 7 shows how the Morelos 3 L-Band antenna reflector delivers 122 spot

beams of coverage to mobile terminals in the Mexican territory as well as the Gulf of

Mexico and the Pacific Ocean.

16

Figure 7. Morelos 3 L-Band system coverage is distributed on spot beams. Source: [18].

Additionally, the size of the spot beam will depend on the antenna size and the

frequency used [31]. The working frequency drives the size of the geographical area that

the beam can cover [31]. This is demonstrated in Figure 8.

Figure 8. Covered geographical areas change with respect with the frequency used. Source: [31].

17

In addition to these satellites, MEXSAT has two communication control centers.

Both centers are similar in capability and infrastructure. Each center includes a Network

Operations Center (NOC) and both are able to transport, process, administrate, monitor,

and analyze all the traffic from the mobile users. Both sites are connected by a dedicated

ground link called Inter-Facility Link (IFL) which provides coordinated operations

between the two centers [17]. This IFL is a support mechanism for each communication

control center when partial failures occur.

During normal operations, the center located in Mexico City is considered the

main control center and the center located in Hermosillo, Sonora provides access to the

Ku-Band capabilities when the weather does not allow the main control center to achieve

a link to the satellite [17]. The Hermosillo center can also achieve additional L-band

traffic capacity because it provides additional access to more of the “L” satellite band

spectrum [17].

1. Advantages of GEO

Since people started to build space systems, they had to decide the final position

of those systems. According to Vatalaro, mobile telecommunication companies have

implemented either GEO or non-GEO satellite constellations to provide their services

locally or worldwide, depending on the resources, requirements, and limitations of the

user [6].

The MEXSAT constellation uses only GEO satellites. GEOs possess a constant

relative geometry between the Earth’s surface and the satellite, and this allows a satellite

positioned there to permanently monitor the same wide areas of the Earth and achieve

satellite communications [33].

Guillemin discussed some of the advantages of the GEO versus lower altitude

orbits, such as coverage, efficiency, no tracking needs, frequency coordination, time to

market, and drag force [34].

The first two stated advantages, coverage and efficiency, depend on one another.

GEO satellites seem to be fixed from Earth’s perspective. The coverage is always

18

constant, which makes them efficient because they always cover the same region and

seem to be stationary from any point inside the satellite’s footprint. This means that the

satellite will focus its resources permanently over a certain region [34]. Not needing a

worldwide coverage is a clear advantage for Mexico. All civilian duties and military

operations commonly take place inside the Mexican national territory and the EEZ,

which perfectly suits acquiring a GEO constellation. Any GEO satellite pointed to

Mexico could easily fulfill the requirements for many of the Mexican operations.

Additionally, the ground antennas used to achieve a link to GEO satellites will not

need to track a set of satellites that are constantly moving around the Earth at great speed

[34]. Ground terminals for GEO satellites need to be pointed in a fixed direction, which

makes it easier to decrease costs when using fixed located facilities. Those types of

antennas do not need reorientation [34]. Furthermore, the users and designers do not have

to worry about the Doppler-effect due to this lack of satellite motion [6].

Moreover, the frequency coordination process is absent when using GEO

satellites. This process permits the satellite communication systems to avoid frequency

overlapping wherever they are working [34]. Frequency coordination is a difficult

process that is managed mostly in non-GEO constellations, because satellites are

overlapping geographically everywhere due to the current space saturation. Conversely,

GEO do not require to coordinate the frequency that the satellites will be using [34].

Furthermore, GEO satellites once launched, are ready to provide the promised

services to the final users [34]. GEO satellites do not rely on other satellites for the

coverage that the system is designed to achieve. The coverage is always constant. In case

contractors are not able to build, test, and deploy a lower orbit system at once, any single

GEO satellite can provide a large coverage by itself [34]. This is convenient because it

generates revenues as soon as it is deployed in space. Therefore, a GEO satellite system

can be deployed quickly, depending on the launching conditions, number of satellites,

and type of constellation.

Finally, null atmospheric drag force allows GEO designers to worry little about

keeping the spacecraft on the same orbit [34]. As previously mentioned, GEO orbits

19

remain in a fixed position relative to the Earth because the Earth’s atmospheric drag at

GEO is null. Therefore, they have the advantage that the lifetime of any GEO satellite

could be higher than any other satellite in a lower type of orbit [34]. Typically, GEO

satellites usually require less propellant fuel required to keep the satellite in its orbit when

re-orientation is needed.

2. Disadvantages of GEO

Vatalaro and Guillemin discussed the disadvantages of using GEO constellations,

such as latency, free space attenuation, cost, challenging performance with mobile

terminals, unsuitability for high latitudes, and difficulty to cover an extra region [6] [34].

Berlocher defined latency as the time that a digital bit travels from the source to

its destination through a digital network [35]. Due to the nature of GEO, latency times

can be large because the electromagnetic waves need to travel farther than lower orbit

networks, this is a critical disadvantage [34]. Usually choosing how much latency affects

our systems relies on the trade-off between the application of the satellite and the distance

from the satellite to the Earth [34]. Nevertheless, it is always convenient to search the

smallest latency possible. Berlocher determined that GEO satellites were working with a

latency of around 540 milliseconds [35].

Secondly, GEOs face a huge free-space loss due to the long path length to the

Earth [6]. Moreover, there are more probabilities for the signal to obtain more losses.

This can be challenging when designing a communication system for those types of

orbits, and the developed system would become very complex.

Overall, GEO satellites are more expensive due to the highly complex systems

they need to carry. The complexity to build a system that deals with this enormous free

space attenuation results in a significantly expensive and complex project [6]. The

antennas used for GEO orbits need to deal with the complexity of achieving a signal

strong enough to close the link over this enormous distance. Finally, the cost for building

a GEO satellite is much higher than any lower orbit satellite.

20

Moreover, this large free space attenuation draws a limit line with the usage of

portable terminals, as directional antennas must be used with GEO [6]. Consequently, the

environment or surroundings of a satellite control center or communication center matter

[6]. Either high buildings or high mountains will limit the quality of the link between the

satellite and the ground. Besides, GEO satellite systems need to generate an enormous

amount of energy in order to transmit a high power signal in order to maintain the link

[6].

Finally, GEO satellites might not be the best solution for polar or high latitude

applications because, usually, GEO or GSO cannot cover Polar Regions [36]. Whenever

Polar Regions coverage is desired, the implementation of a highly inclined circular or

elliptical orbit may provide good visibility for high latitudes [36].Figure 9 shows the

normal coverage for geostationary satellites.

Figure 9. Geostationary satellite coverage. Source: [37].

In the specific case that users need to extend their satellite coverage in a specific

region from an existing footprint of a constellation, the user’s constellation could either

look for building and launching another satellite to space that will cover the additional

coverage requirements or moving the orbit inclination of one of the existing GEO

satellites [34]. This defines the downside of how slow the process could be of

implementing any additional coverage using a determined GEO constellation.

21

3. Ku-band HTS Systems Performance

As initially designed, transponders of the MEXSAT HTS satellites work with Ku-

band [17]. When evaluating the performance of the Ku-band, a comparison between the

characteristics of Ku-Band over Ka-band was required. This comparison results because

modern HTS systems currently operate with either one of those ranges of frequency. It is

important to view the advantages and disadvantages looking at the same system

characteristics, which in this case are the HTS systems. Consequently, here we discuss

the advantages and disadvantages of Ku-band over Ka-band for SEMAR purposes.

A comparison of Ku-band over Ka-band HTS systems have shown several

advantages such as cost, service reliability, and better recovery time from satellite failure.

On the other side, Ka-band was superior on the antenna gain, antenna size, and efficiency

of frequency allocation for military purposes.

Ku-band HTS systems are cheaper than Ka-band systems because they are more

common on the commercial side [38]. The “mass market” offers services to provide

Internet directly to home or small offices [38]. Therefore, the required Ku-band terminals

and VSATs could be cheaper. Moreover, companies that build satellites are commonly

more comfortable with Ku-band, thus the invoice for using Ku-band would be much

lower. For government purposes, Ku-band represents a more economical option because

Mexico has to cover the costs of building, launching, maintaining, and purchasing the

needed terminals for a satellite.

Ka-band HTS systems can be designed to be reliable, but their Ku-band

counterpart’s performance could surpass Ka-band while using the same characteristics or

specifications in both systems [38]. Ka-Band offers satellite links far more vulnerable to

disruption from rain attenuation and atmospheric disturbances [39]. Water heavily affects

Ka-band as illustrated in Figure 10. To achieve the same availability and data rates, Ka-

band HTS systems would require an enormously greater amount of power. Therefore,

Ku-band can easily achieve more reliable HTS systems.

22

Figure 10. Total, dry air and water-vapor zenith attenuation from sea level with surface pressure: 1013 hPa, surface temperature: 15° C, and

surface water-vapor density: 7.5 g/m3. Source: [39].

Specifically, this study employed systems that worked with both Ku-band and Ka-

band. Moreover, this study used different frequencies for the uplink and the downlink

depending on used band. Systems that worked with Ku-band used 14 GHz for the uplink

and 12 GHz in the downlink. Conversely, in this research, Ka-band systems functioned

with 30 GHz for the uplink and 20 GHz in the downlink. Figure 10 shows that the zenith

attenuation for the uplink Ku-band is about 0.07 dB and for the uplink, Ka-band gets 0.25

dB attenuation. Both uplinks differ about 0.21 dB, which determines that when working

with Ka-band on the uplink, the vapor attenuation at zenith happens to be more than three

times greater than the attenuation achieved with Ku-band systems. When comparing the

downlinks, Ka-band attenuation was the same, but Ku-band attenuation happened to be

smaller than the uplink, about 0.06 dB total. These values helped this research to

conclude that at the zenith, Ka-band systems achieve three times more water-vapor

attenuations than Ku-band systems. It is important to note that non-geostationary

23

satellites experience variable look-angles, which alter the calculated attenuation values at

the zenith. These types of satellites are moving relatively fast with respect to a fixed

location on Earth. Therefore, employed antennas need to constantly change their look-

angles to properly point the satellite. This makes the path length longer, and consequently

the water-vapor attenuation is greater than when measured at zenith. Look-angle values

are inversely proportional to the water-vapor attenuations.

Organizations with satellite communications requirements rarely implement a Ka-

band HTS system [38]. These existing systems frequently work in closed networks not

available to the “mass market” [38]. Therefore, in case a Ka-band HTS fails, it will be

difficult to find how to commercially mitigate this lack of service. On the other hand, due

to the huge commercial availability of Ku-band systems, it is possible to acquire a leased

Ku-band satellite relay to cover the failure.

On the contrary, Ka-band HTS systems obtained more optimal antenna sizes and

gain. Assuming we evaluate two antennas of the same size, and one is Ku and the other is

Ka, the Ka-band antenna achieves almost four times more gain than the other [40].

Therefore, it is possible to acquire the same gain values with Ka antennas that are a

quarter of the size than Ku antennas. Because of this, Ka-band antennas could either

achieve more gain or have a smaller antenna size. This is convenient because it allows

reduced antennas for VSAT applications, which makes the installation easy.

Additionally, Ku-band HTS systems are less efficient for military communication

purposes because Ka-band is a frequency where the commercial and military bands are

separated and are assigned contiguous to each other (29.0-30.0 GHz and 30.0-31.0 GHz

respectively) [40].

In conclusion, Ku-band HTS systems provide more advantages compared to Ka-

band HTS systems when applied to GEO. Later in this study, applying the O3B concept

to the requirements and limitations of Mexico, as a final user, will eventually help to

decide what working frequency is more suitable.

24

C. THE USAGE OF THE MEXSAT SYSTEM BY SEMAR

The Department of Defense Dictionary of Military and Associated Terms defines

the term Defense Satellite Communications System as the set of geosynchronous

satellites that provides high data rate communications for military forces, diplomatic

corps, and the executive [11]. Therefore, the MEXSAT system can be considered as a

Defense Satellite Communications system because it possesses two geostationary

satellites with which it can provide data rate communications for several Mexican federal

dependencies and organizations.

By using MEXSAT satellite systems during naval operations, SEMAR generally

obtains the capability of doing voice calls, data sharing, video conferences, and geo-

localization of tactical mobile units [17]. SEMAR has exploited the MEXSAT system in

order to provide mobile, voice, and data services to all sailors who need satellite

communication capabilities to accomplish their specific military missions. It has greatly

helped to improve performance within operational and strategic missions [41].

Additionally, SEMAR has assisted the Mexican population in humanitarian efforts such

as disaster relief and emergency services [41]. The MEXSAT system allows SEMAR to

better protect and defend both long coastlines, along the Pacific and the Gulf of Mexico,

where quality communications are used by joint air-surface-ground tactical operations,

frequent among SEMAR forces.

Generally, SEMAR uses satellite Bicentenario to primarily provide institutional e-

mail, institutional cloud storage, VoIP, and access to the Internet [19]. These services are

provided to satellite terminals generally installed on battleships, naval stations, advanced

naval stations, aircrafts, unmanned aerial vehicles systems, and manned ground vehicles

[19].

Similarly, SEMAR uses satellite Morelos 3 to share voice and text, which allows

SEMAR to supply command and control (C2) and VoIP services on small patrol ships,

naval stations, advanced naval stations, aircrafts, unmanned aerial vehicles systems, and

manned ground vehicles [19].

25

III. OPTIMIZATION MODELS

The current MEXSAT system will operate for an in-orbit life-time of at least 15

years since both Bicentenario and Morelos 3 satellites were launched back in 2012 and

2015, respectively [17]. Additionally, the MEXSAT system was initially designed back in

2010 [42]. Inevitably, new satellite technology concepts, such as the O3b constellation

system, have emerged [8]. This new concept represents a new path for Mexico to possibly

achieve satellite communications, once the current MEXSAT system no longer works.

Moreover, space-based systems exemplify an optimal solution that will provide

better communication services for SEMAR’s assignments such as fighting against illegal

drug, human, and arms trafficking; enforcing the law; helping on military ISR missions; and

providing disaster relief within the national territory [19]. Therefore, it is necessary to

strengthen the current MEXSAT system. This could relieve the constant need for faster

communications and enhance the current MEXSAT capabilities over the numerous

mountainous terrains and coastlines around the country as well as the EEZ.

A. PROPOSED MEXICAN NAVY REQUIREMENTS

For this study’s purposes, we defined the SEMAR as the main stakeholder. We were

aware that other Mexican federal dependencies or institutions employ the system, and they

have their own requirements. The scope of this study, however, analyzed many scenarios,

which could possibly fill those unknown requirements.

This study envisioned several main requirements for a satellite communications

constellation, such as coverage of the Mexican territory, HTS features, spectral efficiency,

low latency, the same current network redundancy feature, and the lowest cost possible.

From this start point, the communications satellite constellation proposed models

must cover the entire Mexican territory, including mid-low regions, mountainous terrains,

and national coastlines, where naval units could be located. Figure 11 shows the interested

territory along with the required coastlines and the Mexican EEZ.

26

Figure 11. Mexican territory, coastlines, and its EEZ. Source: [43].

The MEXSAT system currently comprises an HTS constellation. This type of

satellite constellation allowed SEMAR to achieve higher data rates and bandwidth [32].

Therefore, Mexico’s next satellite constellation must consider to keep using this technology,

which increases the employment of frequency re-use techniques through multiple narrow

spot beams within a determinate area [44]. Spot beams enhance satellite antenna gain values

and promote spectrum reusability, but the disadvantage is the increment in the payload’s

complexity and the total weight of the HTS system [44]. On the other hand, HTS technology

will permit the next version of the system to provide either the same or better satellite

communication capabilities. Due to the nature of the military operations within the country,

and the necessity to increase the situational awareness over the national territory and the

coastlines, it is important to guarantee the best performance to make communications

available everywhere.

Consequently, it is necessary to consider, as a minimal requirement, how to

maximize the satellite spectral efficiency and data rates in every modeled scenario when

designing the next proposed system. Additionally, it is important to consider that the

terminals in the proposed models should resemble the currently implemented terminals on

MEXSAT, analyzing both fixed naval stations and mobile terminals such as ships, naval

airplanes, expeditionary stations, and manned ground vehicles.

27

Another positive requirement is latency. To design advantageous satellite

constellations, latency should be reduced. Latency highly depends on the current altitude of

the satellite and on the type of orbit. The current MEXSAT system provides the highest

latency values for satellite communications [34]. By proposing a non-GEO orbit, the

suggested system could provide smaller latency standards.

Similarly, the implementation of the upcoming satellite constellation by itself, which

is designed in this study, should keep the currently provided level of redundancy of

MEXSAT. As a system with military applications, redundancy is required. The level of

redundancy is commonly proportional to the size of the infrastructure and, therefore, to the

total cost of the system. Consequently, more redundancy is not required in these models.

Inevitably, the proposed design must look to reduce the cost of the new system

because as a developing economy [45], Mexico is making great efforts to distribute its

economic resources as efficiently and wisely as possible. To achieve the lowest possible

cost, the design models should maximize their satellite overpass cycle and minimize the

number of satellites and proposed orbits [36].

The proposed models coming from this study take into account all these previous

requirements and should permit SEMAR to share text and data, conduct video calls,

transmit High Definition (HD) videos, perform teleconferences, and know the real-time

location of the tactical units. Therefore, they are modeled using the O3b concept and

evaluated to appreciate how viable they are. They should enhance the communications

provided by the current MEXSAT system, empower the technology of the Mexican Federal

Forces, fulfill the space gap that Mexico has experienced compared to other developed

nations, and if locally built, could become the avenue for space-based development.

B. DESIGN OF THE PROPOSED SATELLITE CONSTELLATION

The proposed models of communications satellite constellation were designed

according to the previously mentioned satellite constellation requirements. Additionally, a

MEO satellite constellation initially represented a feasible option of improvement because

changing from GEO to MEO should increase the constellation’s performance due to the

shorter distance that satellite links would have to deal with. Furthermore, the O3b MEO

28

constellation concept was applied in the proposed constellation models, and the suggested

constellation should upgrade the actual MEXSAT system strengths. Additionally, the

models presented in this study not only either matched or did not exceed the cost of the

current MEXSAT system, but also took into account that it is critical to minimize the

number of satellites by maximizing the amount of time each satellite covers a ground station

[36].

Moreover, to develop these proposed constellations models, two main expertise

areas divided this study: the orbital design of the actual satellite constellations and the Earth-

space satellite communication capabilities, each one is described in this chapter, further in

subsections III.B.2.a STK Satellite Constellation Models and III.B.2.b Satellite

Communication Links, respectively. Each subsection describes each model and its

performance from the respective expertise point of view. Using the O3b concept, several

MEO constellation models were first built and compared to find the constellation with the

best coverage capabilities. Furthermore, once the proposed MEO satellite constellations

were described in subsection III.B.2.a, STK Satellite Constellation Models, the subsection

III.B.2.b, Satellite Communication Links, described the link budget equation and how it was

used to evaluate the feasibility of the proposed space communications links. Additionally,

Satellite Communication Links subsection addressed the requirements, variables, and

constraints of those proposed space’s connections.

1. O3b Satellite Constellation Concept

This subsection describes what the O3b satellite constellation concept is and

discusses the advantages of these types of orbits against GEO. O3b is a MEO HTS

constellation, whose transponders work within the Ka-band [46]. The O3b system is capable

of delivering multiple terabits of throughput at the same time when using all of its satellites

inside their covered regions [8]. Moreover, O3b has currently sixteen satellites, which build

a revolutionary satellite constellation that provides massive capacity, flexibility, and

coverage [8]. The O3b constellation concept is able to cover an area with a range of +/- 45

degrees latitude with only one zero-inclination orbital plane [46]. Mexico’s requirements,

29

however, narrowed the desired covered region for this constellation. The original O3b

constellation concept is shown in Figure 12.

Figure 12. O3b constellation system. Source: [46].

The O3b constellation and the proposed models had only one zero-inclined orbital

plane and used the Street-of-Coverage technique. This technique uses a series of satellites

followed by each other located on the same orbital plane to cover a continuous linear

straight area [47]. The objective of the Street-of-Coverage is to determine the minimal

number of orbital planes needed to cover the required region, and Figure 13 illustrates it

[48].

30

Figure 13. Continuous street of coverage from a single orbital plane. Source: [48].

The O3b system is flexible because every O3b HTS has twelve antennas, two for

gateways and ten for customers, which provide steerable beams that can be shaped,

routed, shifted, and switched as needed to cover any desired terrain within the covered

area [8]. These antennas will create hundreds of spot beams, which together will build a

large region of coverage [8]. Figure 14 illustrates one of the O3b HTS satellites and its

steerable antennas.

Figure 14. O3b HTS satellite final assembly at Thales, in Cannes, France. Source: [46].

31

Finally, Mexico falls seamlessly into the coverage of the O3b satellite

constellation concept. The O3b MEO constellation system provides a full coverage of the

Earth anywhere within +/- 45 degrees of Latitude of the equator line [8]. Figure 15 shows

that Mexico has minimum and maximum latitude values of 14°31’ 58” N and 32°43’0”

N, respectively [49]. This indicates that the O3b constellation could cover Mexico

flawlessly. Moreover, O3b HTS satellites are located on an equatorial plane with zero

degree inclination. The O3b covered area excludes higher latitudes. This permits O3b to

offer a wide coverage of approximately 400 million square kilometers (km2) [8].

Figure 15. Map of Mexico. Source: [49].

MEO satellite constellations provide three main advantages over GEO

constellations due to this lower distance feature: fiber-like latency, lower Free Space Loss

(LFS), and beam susceptibility [50]. O3b MEO satellites represent an emerging satellite

technology that provides cutting-edge technology to support millions of users worldwide.

Due to MEO’s lower altitudes compared with GEO, MEO constellations represent a

feasible option for improving the MEXSAT system.

O3b latency has a substantial advantage over GEO. Due to its lower altitude

(8,062 km) compared to GEO’s altitude (35,000 km), O3b is four times closer than GEO

satellite systems [46]. Generally, GEO satellites provide latency values of more than 400

32

milliseconds (ms.) in a round trip [51], which are enormous compared to the values of

less than 150 ms. provided by the round trip latency values of O3b [46]. O3b latency

values are similar to optical fiber technology [9]. These faster links could highly improve

the quality of the military communications where low latency is required.

In addition to the latency enhancement, MEO satellites face lower LFS because

the satellites remain in lower orbits than GEO and the link has to travel less distance in

open space. This low-LFS advantage permits the use of antennas with lower gain; closes

the link while dealing with greater rain, clog, or fog attenuations; and achieves greater

throughput at a given power or accomplishes the same throughput at lower power [50].

This advantage could provide an optimal solution for mobile receivers, which, due to

today’s urgent need for constant update of battlefield technologies, suits military

applications perfectly. This combination of lower latency and LFS will empower the

troops with a tremendously greater situational awareness.

Narrow Ka-band beams are less susceptible to jamming [50]. The O3b MEO

concept works with Ka-band transponders, which produce narrower beams than those

achieved by Ku-band transmitters. Since the proposed constellation uses bidirectional

links, possible attackers can easily identify the location of both transmitters and receivers.

Therefore, if maliciously planned, enemies could jam the MEXSAT received and

transmitted links.

A proposed MEO satellite constellation concept seems to be a feasible and

cheaper solution. The previous statements mean that the next system would require less

power, fuel, and resources to arrive to the final orbit position. From there, it can be kept

within the desired orbit and achieve telecommunications. Therefore, this study evaluated

the feasibility that SEMAR could take advantage of this idea and build a MEO satellite

constellation to fulfill its communication requirements.

2. Proposed STK Models and Their Satellite Communications Capabilities

This subsection develops the proposed satellite constellation model, in which two

main expertise areas divided this project: the orbital design of the modeled satellite

constellations and the Earth-space satellite communication capabilities.

33

This study used the Systems Tool Kit (STK) software for developing the proposed

satellite constellation models and demonstrating that the O3b concept is suitable for

Mexico’s satellite communication requirements. STK is a four-dimensional simulation

and modeling software, which permits the modeling of objects in land, air, sea, and space

environments for analyzing the modeled system’s performance within a specific range of

time [52]. STK is appropriate for this study because it models in time and space a precise

representation of the Earth, satellites, and RF environment attenuations [52]. Moreover, it

supports many kinds of maps, elevation files, and geographic information system (GIS)

data [52]. Fortunately, it is possible to obtain the results of the model’s performance in a

report or graph representation and export them to other data formats.

Although this study did not use all the included features, STK offers a wide

variety of advanced modeling and analytical functions with different possible tracks

focused on different application areas [53]. For modeling and evaluating the viability of

innovative satellite constellation models, we took advantage of the space trajectory tool

of STK. Furthermore, the STK communications/radar module was the employed tool to

measure the performance of the modeled links.

In the following subsection III.B.2.a STK Satellite Constellation Models, several

constellations are first modeled using the O3b concept and then compared to find the

optimal MEO satellite constellation with the best coverage capabilities. Furthermore, the

subsection III.B.2.b Satellite Communication Links describes the link budget equation

and how its variables evaluate the feasibility of the proposed space communications

links.

a. STK Satellite Constellation Models

Before modeling several satellite constellations to prove coverage efficiency, this

study simplified the method for evaluating the coverage of those HTS constellations with

a “one spot beam” assumption. Additionally, it was essential to evaluate visually if a

single MEO HTS, with ten spot beams and using either Ku-band or Ka-band, could cover

the entire interested region for Mexico.

34

For simplicity, taking into account that each HTS, no matter the constellation, has

several spot beams; we assumed that the performance of one spot beam over a specific

location would represent the performance of the entire set of spot beams of an HTS over

the entire Mexican territory and the Mexican EEZ. Therefore, evaluating a single beam

continuously covering a fixed location could show that the entire constellation offers

continuous full coverage for the targeted area. Hence, it was necessary to establish that

one HTS with both Ka-band and Ku-band transponders could cover the required area for

this study. However, by assuming the performance of one spot beam would represent the

others. The characteristics of all the spot beams of a single satellite would have to be

generalized to make this “one spot beam” assumption work.

As previously stated, it is possible to change the width of the spot beams by using

either the Ku-band or Ka-band. Figure 16 illustrates the required region that one MEO

HTS has to cover. Figure 16 also shows Mexico and its EEZ as the considered “targeted

area” when modeling the constellations in STK.

Figure 16. Mexican territory and its EEZ modeled in STK

35

Dimov mentioned that half-power angle 𝜃𝜃𝐻𝐻𝐻𝐻 is the beamwidth defined when the

antenna radiates half the maximum power (-3 dB) [54]. Additionally, STK calculated the

beamwidth values of a parabolic antenna in [55] by

𝜃𝜃3 𝑑𝑑𝑑𝑑 = 58.5 �𝜆𝜆𝐷𝐷� . (2)

The Ka-band MEO HTSs worked with a frequency of 20 GHz and had an antenna

diameter of 0.66 meters, which resulted with each beam having a beamwidth of 1.3295

degrees. These calculated beamwidth values were sufficient to cover the required area

using 10 Ka-band spot beams. Figure 17 shows the coverage of a Ka-Band MEO HTS

satellite.

Figure 17. Coverage of a Ka-band HTS

The Ku-band MEO HTSs worked with a frequency of 12 GHz and had an antenna

diameter of 0.66 meters, which resulted with each beam having a beamwidth of 2.2159

degrees. These calculated beamwidth values were sufficient to cover the required area

using 10 spot beams. Figure 18 shows the coverage of a Ku-Band MEO HTS satellite.

36

Figure 18. Coverage of a Ku-band HTS

Modeling the coverage of two satellites, each with either a Ku-band or a Ka-band

transponder and employing 10 spot beams for customers per satellite, allowed us to verify

the feasibility of both systems using either of the frequencies. Therefore, it is possible to

evaluate a single spot beam and generalize its performance through all the other beams of

that satellite. When comparing the Ka-band and Ku-band coverages, the Ku-band HTS

coverage was superior because its beamwidths were larger and covered more area due to

their maximum gain values.

(1) Current MEXSAT Model

As previously mentioned, the MEXSAT system primarily consists of two GEO

HTSs (Bicentenario and Morelos 3) and two gateway stations located inside the Mexican

territory in the cities of Hermosillo, Sonora and Mexico City [17]. This system was

modeled in STK for further comparison purposes with the proposed O3b satellite

constellation. Figure 19 demonstrates the model with the included satellites and

gateways.

37

Figure 19. Modeled current MEXSAT system

The total Mexican project investment to acquire the MEXSAT system was U.S.

$1.7 billion, which was divided in the federal budget from the year 2010 to 2015 [56].

(2) Proposed Satellite Constellation Design

This section describes the design of the proposed MEO HTS constellations. Three

common concepts influence the characteristics of those constellations: the geometric

relation of the satellite-Earth scenario, overall cost of the proposed system, and

overlapping satellite coverage.

Figure 20 defines the geometric relation of a satellite-Earth scenario, which

permits us to see that facilities or ground mobile satellite terminals at high latitudes will

have to deal with longer distances. Therefore, when evaluating the coverage performance

of every proposed model, the efficiency of those constellations was determined using the

location of the highest-latitude facility within the targeted area. The city of Mexicali,

Mexico, was chosen as the best-case location because it proved to be the city with the

highest latitude within the country (32.62781N, 115.45446W) [57], and, therefore, was

used as the test location when proving coverage. If any of the proposed constellations

provide full and continuous coverage to this location, it will easily provide full

continuous coverage inside the targeted area on any other location.

38

Figure 20. Geometric relation between satellite and user. Source: [58].

Moreover, the proposed constellations were designed primarily so that their

overall cost would match or not exceed the cost of the current MEXSAT system. There is

no reason to support an equally or more expensive project than the current system if that

project provides a lower performance. Additionally, based on the overall cost, the

minimal number of satellites required to achieve continuous and full coverage determined

the main features of the modeled constellations.

This study defined three variables of the over-all cost of a modeled constellation:

the price for building the satellite, launching it, and purchasing the required ground

terminals. For this study, we assumed that each MEO HTS would cost U.S. $100 million,

which is a very conservative number based on [59] and [60]. Next, each MEO HTS is

supposed to weight 700 kg [61]. Therefore, it is possible to hire a complete “Falcon 9”

from the company Space X for U.S. $62 million, which is able to launch 8,300 kg to

GEO [62]. Those 8,300 kg represent a maximum payload of 11 MEO HTS going to

GEO. However, Space X does not provide the specific amount of weight that a “Falcon

9” is able to carry up to MEO, but the study assumed that a MEO satellite constellation

with 11 satellites was the limit when using the Falcon 9 to place the entire constellation in

its final orbit. Finally, the cost for purchasing the required ground terminals was

considered as the same price required to build and launch the required satellites of a

specific constellation. Based on these assumptions, the proposed costs for the new

constellations are indicated in Table 1

39

Table 1. Cost of the proposed satellite constellations based on the number of satellites

Number of Satellites

Cost for building the satellites

Cost for launching those satellites

Cost of ground terminals

Total cost of the proposed system

Eight $800 million U.S.

$62 million U.S.

$862 million U.S.

$1.724 billion U.S.

Seven $700 million U.S. $762 million U.S.

$1.524 billion U.S.

Six $600 million U.S. $662 million U.S.

$1.324 billion U.S.

Five $500 million U.S. $562 million U.S.

$1.124 billion U.S.

Based on Table 1, only the constellation with eight satellites matched the cost of

the current MEXSAT system, and the rest represented an improvement in cost. Table 1

defines that MEO constellations with eight satellites or below should be evaluated to find

if they are competitive enough compared to performance of GEO constellations.

Furthermore, the need to lower the cost of the whole system created a trade-off between

the numbers of satellites and required terminal’s look-angle on the ground, which

affected the performance of the constellation. Changing one parameter, such as the

number of satellites or the value of the look-angle in the ground terminal, would affect

the performance of the proposed constellation. The value of the look-angle was found to

be proportional to the needed number of satellites to continuously cover the area of

interest.

Finally, overlapping coverage between satellites is necessary to continuously

cover the targeted area. Minimizing the number of satellites, however, will minimize the

overlapping time. As one of the established requirements was to minimize costs of the

proposed constellation, modeled MEO constellations offered overlapping times with at

most two satellites to reduce the number of required satellites. This architecture suggests

having two antennas on the ground, which would manage the handover of the two

satellites at the overlapping moment. One overlapping period is shown in Figure 21.

40

Figure 21. View of two satellites’ spot beams overlapping the same region

Figure 22 shows how a modeled constellation with satellites using low look-

angles, which provided periods of overlapping with more than two satellites at the same

time. For this study’s purposes, this model was not efficient, because it did not minimize

the number of satellites per constellation. This model had five degrees of look-angle,

which provided the disadvantage of an extensive range, about 12,339.5 km, in the worst-

case scenario. This massive range deteriorates the performance of the communications

link with respect to other MEO links because it provides more path losses and

attenuations, which is discussed with detail later in this chapter.

Figure 22. Time when three satellites overlap occurred in a non-cost efficient modeled constellation.

41

The following subsections define different satellite constellations and evaluate

their coverage performance. First, a six-satellite Walker-Delta constellation was proposed

using MEO constellations to test different kinds of satellite constellations in order to

possibly find another way to improve the current MEXSAT system. Secondly, three O3b

satellite constellation systems were modeled each with the same orbital plane but a

different number of satellites (five, seven, and eight). The performance of these

constellations was stated, and the O3b constellation models showed why the O3b concept

is the constellation that works best for continuously covering areas over +/- 45 latitude

regions.

1. Walker Constellations

A useful technique for designing a global coverage constellation is the Walker

constellations [47]. Although this study is not looking for full global coverage, Walker

constellations offer highly competent local or global coverage using inclined orbits [63].

Walker constellations also provides valuable properties of satellite distribution, which

could be helpful because this technique searches for both maximizing the provided

coverage and minimizing the number of satellites and orbital planes [64]. There are two

main variants of Walker constellations: Walker Delta constellations and Walker Star

constellations [64]. A Walker Delta constellation type was chosen because STK specified

that “in Walker-Delta constellations the ascending nodes of the planes are distributed

over the full range of 360 degrees” [64], which optimize the satellite distribution by

evenly distributing the modeled orbital planes and the satellite true anomalies. This even

distribution does not permit reducing the number of satellites to five, but establishes the

minimum to be six.

Richharia defined the notation for expressing the number of satellites and orbital

planes as: number of satellites, number of orbital planes and harmonic factor (N, P, m)

[63]. When designing this Walker constellation in STK, these variables were defined and

tested. After several iterations of the models, the best-proposed Walker constellation

model was found. The notation of the proposed constellation was (6, 2, 1/3), which had

six satellites in total, two orbital planes, and three satellites in each orbital plane (120

degrees of spacing from one to the other in the same orbit). Both orbital planes had an

42

inclination of 25 degrees, but they had a RAAN spacing of 180 degrees with each other,

which put them on the exact opposite ascending nodes. Each orbital plane had a circular

shape and an altitude of 8,000 km and a semi-major axis of 14,378 km. The range in the

worst-case scenario rose to 10,987.63 km. Figure 23 shows the modeled Walker

constellation.

Figure 23. Isometric view of the proposed Walker constellation

This Walker-Delta constellation provided continuous coverage over Mexico as

the targeted area. Figure 24 illustrates that testing this constellation over 24 hours

provided simultaneous coverage over Mexico with two satellites on average. Figure 24

also demonstrates that it never lost coverage on a simulated day. The red line displays the

times when at least one or two satellites were covering Mexico as the targeted area. The

blue line represents the times when two or three satellites had access to the targeted area.

Finally, the green line represents an average of those two previous lines.

43

Figure 24. Walker constellation grid statistics 24-hour report

2. O3b Constellation with Five Satellites

The modeled O3b constellation with five satellites is shown in Figure 25. All the

satellites are in the same equatorial plane. This constellation promised to decrease the

cost of the next iteration of the MEXSAT system most.

Figure 25. Proposed MEO satellite constellation with one orbital plane and five satellites

After several iterations of trade-offs within the proposed model, an optimal trade-off

value was found to be five satellites and 18 degrees of look-angle ground terminals. At this

combination, the five-satellite O3b constellation provided the largest possible range (about

11,059.28 km) between the ground transmitter and the satellite receiver, but the smallest

number of required satellites to continuously cover the required area.

44

Figure 26 demonstrates that the proposed O3b constellation never lost coverage on a

simulated day in STK. The green line displays the times when at least one or two satellites

were covering the targeted area. The red and blue lines are overlapping with the green one.

Based on the data extracted from Figure 26, we can see that the satellite overlapping while

covering the same region represents 0.6245% of the complete simulated day.

Figure 26. Proposed five-satellite O3b constellation grid statistics 24-hour report

Figure 27 shows the performance of the proposed optimal constellation when the

value of look-angle was changed from 18 degrees to 19 degrees. We are able to appreciate

how the constellation lost coverage for some minutes because the ground terminals cover

less of the path of the satellites moving in space. Extracting the data from the chart, we

know that the constellation lost coverage for 60 minutes in the entire day, which represents

almost 4.167% of the entire coverage. This results in a non-continuous coverage

constellation.

45

Figure 27. Modeled five-satellite O3b constellation with non-continuous coverage on the targeted area

3. O3b Constellation with Seven Satellites

The modeled O3b HTS constellation with seven satellites is shown in Figure 28.

As the previous model, the satellites are in the same equatorial plane. The cost of this

constellation is barely below the cost of the current MEXSAT system. If the achieved

performance is determined to be only slightly inferior, it may be beneficial to explore that

financial trade-off.

Figure 28. Proposed MEO satellite constellation with one orbital plane and seven satellites

46

After several iterations of trade-offs within the proposed model, an optimal trade-

off value was found to be seven satellites and 25 degrees of look-angle ground terminals.

This combination provided a maximum range of about 10,464.36 km between the ground

transmitter and the satellite receiver within this seven-satellite O3b concept. This

constellation represents a feasible limit if it is required to decrease the cost of the next

iteration of the MEXSAT system.

Figure 29 demonstrates that the proposed seven-satellite O3b constellation never

lost coverage on a simulated day in STK. The red and blue lines are overlapping with the

green one, and the green line displays the times when at least one or two satellites were

covering the targeted area. Based on the data extracted from Figure 29, we can see that

the satellite overlapping while covering the same region represents 3.53% of the

complete simulated day.

Figure 29. Proposed seven-satellite O3b constellation grid statistics 24-hour report


value of look-angle was changed from 25 degrees to 26 degrees. We are able to

appreciate how the constellation lost coverage for some minutes and extracting the data

from the chart, we know that the constellation lost coverage for 30 minutes in the entire

day, which represents 2.08% of the entire coverage. This results in a non-continuous

coverage constellation.

47

Figure 30. Modeled seven-satellite O3b constellation with non-continuous coverage on the targeted area.

4. O3b Constellation with Eight Satellites

The modeled O3b HTS constellation with eight satellites is shown in Figure 31.

As the previous models, the satellites are in the same equatorial plane. This constellation

achieves the same cost of the current MEXSAT system. As previously stated, any MEO

HTS constellation with more than eight satellites does not represent a cost-advantage

constellation. This eight-satellite O3b concept would require a better performance than

the current GEO system to justify migrating.

Figure 31. Proposed MEO satellite constellation with one orbital plane and eight satellites.

After several trade-off iterations within this proposed model, the ideal look-angle

value was 25 degrees. This combination provided a maximum range of about 10,154.39

km between the ground transmitter and the satellite receiver, which was the lowest

maximum range of all the modeled constellations. This constellation required a superior

48

link performance to represent a feasible option for the next iteration of the MEXSAT

system.

Figure 32 demonstrates that this proposed eight-satellite O3b constellation never

lost coverage on a simulated day in STK. The green line displays the times when at least

one or two satellites were covering the targeted area. Based on the data extracted from

Figure 32, we can see that the satellite overlapping while covering the same region

represents 0.48% of a complete simulated day.

Figure 32. Proposed eight-satellite O3b constellation grid statistics 24-hour report


value of look-angle was changed from 28 degrees to 29 degrees. We are able to

appreciate how the constellation lost coverage for some minutes, and extracting the data

from the chart, we know that the constellation lost coverage for 100 minutes in the entire

day, which represents 6.94% of the entire coverage. This also results in a non-continuous

coverage constellation.

49

Figure 33. Modeled eight-satellite O3b constellation with non-continuous coverage on the targeted area

Both types of satellite constellation models, the Walker-Delta and the O3b

concept worked well and provided continuous and full coverage within the required area

of Mexico and its EEZ. However, we clearly see that the modeled O3b constellations

provided the possibility to change the number of satellites more easily and achieved

lower ranges at the edges of each satellite coverage, which is a clear advantage for

Mexico’s requirements.

b. Satellite Communication Links

Satellite communication capabilities provide the medium to effectively share data

from one Earth station transmitter to another station that works as the receiver, and this

station could be located an enormous distance apart from the transmitter. Pelton defined

that the most basic satellite system works with a transmitter on the ground sending an

uplink signal to a satellite receiver, which processes that signal and retransmits it on a

downlink signal to another ground station located in the desired location on Earth [21]. In

a basic satellite communications system, the information modulates the RF carrier, uses a

specific medium to reach the receiver, and fights against losses and attenuations [65].

In the space communications community, it is common to say that the link is

closed when a satellite connection is established. Any functional satellite communication

system must be designed to overcome the losses and attenuations faced in the pathway

between the receiver and the transmitter. When designing a satellite communications

(SATCOM) system, engineers must provide a link margin, which is a limited excess of

50

signal power above of what it is required [21]. Furthermore, the link budget equation

evaluates the feasibility of SATCOM systems because it accounts for all the gains and

losses involved in closing a link [21].

This section is divided in two subsections. The first defines the behavior of the

link budget equation and its variables. The second subsection defines the specifications of

proposed ground terminals. Different types of terminals were used in the STK

simulations to obtain the quality values of the link budget variables.

(1) Link Budget Equation

The link budget equation is described in [65] as

𝐸𝐸𝑏𝑏𝑁𝑁0

= 𝑃𝑃𝑡𝑡𝐺𝐺𝑡𝑡 �1𝛼𝛼� � 𝜆𝜆2

(4𝜋𝜋𝑑𝑑)2� �1𝐿𝐿1� 1𝑘𝑘𝑘𝑘

𝐺𝐺𝑟𝑟𝑇𝑇

(3)

where 𝐸𝐸𝑏𝑏𝑁𝑁0

is the ratio of Energy per bit to the spectral noise density, 𝑃𝑃𝑡𝑡 is the required

power at the transmitter, 𝐺𝐺𝑡𝑡 is the gain of the antenna on the transmitter, and the product

of 𝑃𝑃𝑡𝑡𝐺𝐺𝑡𝑡 represents the Effective Isotropic Radiated Power (EIRP). Moreover, 𝛼𝛼

represents the atmospheric attenuations, 𝜆𝜆 is the wavelength of the used RF, 𝑑𝑑 is the

range or distance between the satellite and the ground terminal, 𝐿𝐿1 represents the receiver

losses, 𝑘𝑘 is the Boltzmann’s constant, 𝑅𝑅 is the data rate, 𝐺𝐺𝑟𝑟 is the gain of the receiver’s

antenna, and 𝑇𝑇 is the equivalent noise temperature of the system.

The link budget equation helps to evaluate the feasibility of a possible

improvement in the performance of the current MEXSAT system. The link budget

equation provides the adequate engineering algorithm necessary to guarantee functional

performance of the required space communications capabilities. Any change in the value

of variables in the link budget equation affects the efficiency of the link, increases/

decreases complexity of the required hardware and software, changes the cost of the

design and implementation, or demands more power from the transmitter. Engineers

employ the link budget equation to offer the user the available options to decide, based on

the communication requirements and budget limitations, which system characteristics to

build.

51

Commonly, the feasibility of closing the link is measured by the amount of link

margin a satellite connection has [21]. The link margin is the received excess signal

above the required threshold required to close the link [21]. Usually, the common

systems are designed to experience a minimum link margin of ~3 dB in the worst-case

scenario [65]. The achieved link margin will permit to link closure no matter how much

noise, losses, and attenuations take place in a specific environment or case. Therefore, the

higher the link margin, the better the link quality.

Three variables measure the strength of the signal after arriving to the receiver

end of the link. Variables such as Carrier-to-Noise ratio (C/N), energy per bit to the

spectral noise density ratio �𝐸𝐸𝑏𝑏𝑁𝑁0�, and Bit-Error-Rate (BER) help us measure the strength

of the transmitted signals after or before modulation in the receiving end. C/N is defined

as the level of carrier to noise ratio, which is measured before the demodulation processes

[21]. 𝐸𝐸𝑏𝑏𝑁𝑁0

represents the signal-to-noise level after demodulating a signal [21]. Finally,

BER defines how many bit errors were received per given unit of transmitted bits at the

receiving edge and after demodulation [65]. Factors such as interference, distortions,

noise, or bit synchronization errors alter the digital data. Generally, high C/N, 𝐸𝐸𝑏𝑏𝑁𝑁0

, and

BER values provide optimal, reliable, and efficient communications.

Modulation is the process where information signals, known as the baseband

signal, are used to modify a single and usually higher RF wave that is known as the

“carrier” [21]. Modulation draws the limit when information in any form (audio, data,

etc.) is ready to be transmitted or not [65]. After the digital signal arrives to the receiver

and is demodulated, BER is compared with the required 𝐸𝐸𝑏𝑏𝑁𝑁0

to evaluate the performance

of satellite links. The lower the required 𝐸𝐸𝑏𝑏𝑁𝑁0

to operate, the more interference within the

arriving signal is allowed to close the link. Figure 34 illustrates that performance

according to several modulations.

52

Figure 34. Typical BER vs. Eb/No curves. Source: [21].

For this study, the minimum acceptable BER value was defined as 10−6 or less,

which represents one error occurring in one million transmitted bits. When achieving this

value of BER, the link is properly closed. Hence, the goal when designing and comparing

the proposed satellite links with the current MEXSAT links will primarily aim to look for

achieving that minimum BER threshold. However, when comparing the performance of

those links, C/N and 𝐸𝐸𝑏𝑏𝑁𝑁0

were also imperative in deciding which performance was better.

Additionally, for this study’s purposes, a link analysis exploited the link budget equation

to determine the achieved BER values in every link of the built satellite constellation

models. In the same way that the previous evaluation of the built HTS constellation

models happened, the link budget equation only analyzed one beam per satellite for

simplicity purposes. The performance of that beam was generalized to all of the spot

beams a single HTS has.

53

Furthermore, the link budget variables were divided in three different categories

depending on how their values were determined: assumed for simplicity, estimated based

on the infrastructure/requirements of the current Mexican users, and determined by the

performance of the modeled satellite constellation. All those variables determined the

performance of the link and contributed to finally calculate the achieved 𝐸𝐸𝑏𝑏𝑁𝑁0

, C/N, and

BER of a specific link.

1. Variables Assumed for Simplicity

The variables assumed for simplicity were 𝐿𝐿1 as the receiver losses and 𝑇𝑇 as the

equivalent noise temperature of the system.

𝐿𝐿1 represents all the sources of signal degradation at the receiver point. Losses

such as modulation loss, local oscillator (LO) phase noise, random loss and noise,

polarization loss, interference noise, and thermal noise affect the performance of the

receiver. The calculation of these losses were assumed to be equal for all links in this

study. To reflect this, 𝐿𝐿1 had a value of one.

𝑇𝑇 is the thermal noise triggered by the natural motion of the electrons within the

hardware of the system [70]. It is a different manner to define the noise introduced by the

transmitter to the system. Sklar stated that 290 Kelvin (K) is commonly selected as a

reasonable estimate of the source temperature of numerous satellite links [65]. For

simplicity purposes in this study, 𝑇𝑇 had the value of 290 K.

2. Variables Estimated Based on the Infrastructure/Requirements of the Current Mexican Users

The variables estimated based on the infrastructure/requirements of the current

Mexican users were the 𝑃𝑃𝑡𝑡 as is the required power as the transmitter, 𝐺𝐺𝑡𝑡 as the gain of

the transmitter’s antenna, 𝐺𝐺𝑟𝑟 as the gain of the receiver’s antenna, 𝜆𝜆 as the wavelength of

the working frequency, and 𝑅𝑅 as the wanted data rate.

𝑃𝑃𝑡𝑡 is the amount of electrical power required to feed the transmitter and close a

specific link. For this investigation, it varied according to the proposed type of facility

and terminals. The 𝑃𝑃𝑡𝑡 values ranged from 10 to 250 Watts (W).

54

Antennas are one of the most valuable assets in a satellite system. An antenna

captures and radiates RF signals through the free space with direction to another antenna

[21]. The transmitter’s antenna and the receiver’s antenna both possess the same formula

for calculating their gain value, 𝐺𝐺𝑡𝑡 and 𝐺𝐺𝑟𝑟. The antenna gain is described in [66] as

𝐺𝐺 = 𝜂𝜂 �𝜋𝜋𝐷𝐷𝜆𝜆�2 (4)

where 𝜂𝜂 is the efficiency of the antenna, which value for this study came to be 55%, D is

the diameter of the antenna, and 𝜆𝜆 is the wavelength of the working frequency, which is

obtained by

𝜆𝜆 = 𝑐𝑐𝑓𝑓 (5)

where c is the speed of light and f the frequency in which the satellite connection is

working.

However, in this study the obtained values did not match with each other because

the diameters of the antennas were always different from each other. The diameter of

each proposed antenna is determined in subsection III.B.2.b “(2) Proposed Terminals for

Different Environments and Usages.”

Finally, R or data rate is defined as the speed at which data is being transferred

from one device to another [67]. For this study, R had different values and they depended

on the type of facility. Some facilities require higher data rates than others, based on the

number of users and communication requirements. This value changed according to the

type of facility, but ranged from 5 to 500 Mbps per facility.

3. Variables Determined by the Performance of the Modeled Satellite Constellations

The variables determined by the performance of the modeled satellite

constellations were 𝑑𝑑 as the range or distance between the satellite and the ground

terminal and 𝛼𝛼 that represents the atmospheric and free space attenuations.

The distances from the satellites and the ground terminals are determined by the

performance of the satellite constellation. Figure 35 shows that the distance of the

55

proposed O3b MEO links vary from 9604 to 11059 km. The MEXSAT links have a static

distance of 35,800 km [17].

Figure 35. AER report of the proposed O3b MEO satellite constellation

Although 𝛼𝛼 represents attenuations due to dry air, atmospheric gases (clouds and

fog), and rain, this study only considered, modeled, and simulated rain attenuations

affecting the performance of the modeled link through a slant propagation path because

rain attenuation produces more attenuation than atmospheric gases. The International

Telecommunications Union (ITU) model ITU-R P.618-13, which describes the method

for predicting precipitation and clouds attenuation along a specific slant path [68], was

used. This recommendation represented a reliable method to acquire the rain attenuation

values needed to achieve more reliable simulations.

One of the main parameters to calculate the rain attenuation along a specific path

was the point rainfall rate for the location for 0.01% of an average year (R0.01), which for

this research was considered to have the value of 90 mm/h. This is because Figure 36

demonstrated that Mexico got places with high R0.01 values and 90 mm/h, which could

56

represent the worst-case scenario when achieving satellite communications inside the

targeted area of this study.

Figure 36. Rainfall rate exceeded for 0.01% of an average year. Source: [69].

Finally, the free space loss (LFS) means the diminution of the electrical field

power when it travels above the Earth’s atmosphere through space [65]. LFS is

proportional to the distance between transmitter and receiver. LFS is usually the largest

loss that space links experience [65]. LFS is described in [70] as

𝐿𝐿𝐿𝐿𝐿𝐿 = �4𝜋𝜋𝑑𝑑𝜆𝜆�2 (6)

where d is the distance involved in the link.

4. Bandwidth and Modulation

Hranac defined spectral efficiency as the boundary value that defines how much

data could be transmitted in a given bandwidth [71]. Spectral efficiency is measured in

units of transmitted bits per second per hertz (bps/Hz). Moreover, the O3b constellation

57

provides ten fully steerable Ka-band antennas per satellite for customer beams, which for

maritime applications, offer 216 MHz of bandwidth per direction, over 500 Mbps, and

round-trip latency of less than 150 milliseconds. These values result in an expected

spectral efficiency of at least 2.3 bps/Hz [72].

These spectral efficiency values determined the required modulation. As

previously stated, modulation transforms digital data into compatible RF waveforms for

transmission over the desired channel [65]. To satisfy the required spectral efficiency,

this study used Eight Phase Shift Keying (8-PSK) modulation. Phased-shift keying (PSK)

modulation uses “M” phase shifts to code the carrier. Each phase shift represents “N”

baseband bits depending on the type of used modulation [73]. This scheme is known as

M-ary PSK. PSK is the most commonly exploited digital modulation [73]. Moreover,

Figure 37 shows the modulation schemes of Quadra-phase shift keying (QPSK) and

8PSK, and presents how QPSK and 8PSK are able to transmit 2 and 3 bps/Hz,

respectively. Therefore, QPSK did not meet the minimum required spectral efficiency

and 8PSK was chosen as the employed modulation.

Figure 37. Representation of schemes of QPSK and 8PSK signals. Source: [67].

Moreover, the communications/radar module included in STK is described

because it was employed to evaluate the efficiency of the models and its terminals.

58

5. STK Communications/Radar Module

STK offers a wide variety of tools that mostly help to solve air and space

analyses. Specifically, the STK communications/radar module was used to define and

analyze the proposed satellite communication systems. Proposed links took advantage of

the possibility to get link budget reports; model the rain attenuation, transmitters, and

receivers; and simulate the employed modulation and polarization.

The STK communications module offers a link budget tool for reporting and

graphing the performance of the proposed links [52]. This research modeled a basic link

because it defined the access between a transmitter and a receiver per satellite when

covering the targeted area. The link budget report considers all the constraints and

features defined previously in the model, transmitters, and receivers and displays a

detailed report with parameters such as EIRP, C/N, 𝐸𝐸𝑏𝑏𝑁𝑁0

, BER, Pt, Gt, Gr, and some others

which influence the performance of the link in the design process.

Additionally, the STK communications module can incorporate the ITU

Recommendation P.618-13 to properly design space-earth links because it considers the

critical atmospheric effects for variable elevation angles [68]. For the purpose of

enhancing the performance of the design, the satellite link modeled rain that degrades the

signal when passing through it. As evaluated before, rain attenuation is proportional to

the working frequency and look-angles. Water particles highly degrade the transmitted

signals.

Next, the communications module allows the user to choose the type of receiver

or transmitter. Values like the type of antenna, gain, and diameter of the dish were

defined accordingly to the requirements of the model.

(2) Proposed Terminals for Different Environments and Usages

This study considered and modeled four types of ground facilities, which all

worked with the same satellites (O3b and MEXSAT satellites) and with the same Ku-

band and Ka-band frequencies when evaluating the proposed modeled links. The four

types of facilities used and modeled in STK were aircrafts, expeditionary stations, patrol

59

ships/vehicles, and ground stations. Each type of facility determined the hardware and

software specifications of the modeled satellite terminals. Those types of terminals were

intended to provide the same services that the current MEXSAT system does, such as

institutional e-mail, institutional cloud, VoIP telephony, and Internet access. In this

research, proposed Ka-band terminals worked at 20 GHz for the downlink and 30 GHz

for the uplink. Moreover, all proposed Ku-band terminals worked at 12 GHz for the

downlink and 14 GHz for the uplink. Each of the modeled facilities possessed only one

satellite terminal, which was always a parabolic reflector antenna and performed as both

the transmitter and receiver.

To provide realistic link models, and after extensive research in the commercial

market, the average characteristics of commercial satellite terminals determined the

diameter of the reflector antenna on the proposed terminals. Due to a huge diversity on

commercial terminals, however, this study only included one terminal per type of facility,

and it was determined by which better represented that type of facility.

Table 2 defined the employed reflector diameter that each type of facility used in

the models. Data rates and transmitted power output values varied depending on the

direction of the link (uplink or downlink).

Table 2. Diameter of the reflector antennas per type of facility.

Diameter of the reflector (m) Aircrafts 0.3

Expeditionary stations 0.415 Patrol ships/vehicles 1

Ground stations 7

According to Equation (4), the gain of a specific antenna changes with the

working frequency. Table 3 describes the achieved gains of all the types of terminals, in

which the great advantage of Ka terminals against Ku terminals is shown.

60

Table 3. Achieved gain values of the reflector antennas per type of facility.

Diameter of the reflector

(m)

Ku Uplink Gain (dB)

Ku Downlink Gain (dB)

Ka Uplink Gain (dB)

Ka Downlink Gain (dB)

0.3 30.28 28.93 36.88 33.37 0.415 33.09 31.75 39.71 36.18

1 40.73 39.39 47.35 43.82 7 57.63 56.29 64.25 60.73

Moreover, the MEXSAT system uses one 2.3-meter antenna on the receiver in

each satellite [1]. As previously stated, an O3b satellite has 10 antennas for customer

beams and 2 antennas for gateways. Each antenna has an approximated size of 0.66

meters, and this approximation was assumed based on Figure 14. The antenna reflectors

of MEXSAT satellites (2.3 m) and the proposed of the O3b constellation (0.66 m)

accomplished the gain values shown in Table 4. These values illustrate both the clear

advantage of Ka-band over Ku-band. Moreover, the MEXSAT antennas provide a ~11

dBi gain in improvement compared to the O3b constellation concept.

Table 4. Achieved gain values of the satellites’ reflector antennas.

Diameter of the reflector (m)

Ku Uplink Gain (dB)

Ku Downlink Gain (dB)

Ka Uplink Gain (dB)

Ka Downlink Gain (dB)

0.66 37.12 35.78 43.74 40.22 2.3 47.96 46.62 54.58 51.06

61

IV. COMMUNICATIONS PERFORMANCE OF THE PROPOSED MODELS

This chapter defines the performance of the modeled links and compares the

results with the performance of the current MEXSAT system. The last two subsections of

this chapter define the performance of each modeled link, classifying the performance of

those proposed links by either uplinks or downlinks. Inside each subsection, the results

are grouped by the type of facility: aircraft station, expeditionary station, patrol ships/

vehicle station, and ground station. Each facility had only one terminal with different

simulated links, which belonged to all the modeled O3b constellations that were using

either Ku-band or Ka-band transponders. Once the performances are defined,

comparisons are made between the performances of the modeled links according to a

specific output variable. Next, the efficiency of each proposed link is compared with the

efficiency of the modeled MEXSAT links in order to demonstrate which link

accomplishes the best performance. Finally, each facility subsection concludes which link

on a specific facility achieves the best performance.

Generally, O3b modeled constellations have a common disadvantage relative to

the MEXSAT system. Table 4 permits us to see that O3b constellations have much

smaller satellite antennas than the MEXSAT system, which resulted in lower antenna-

gain values, approximately ~11 dB. This is a clear benefit of the MEXSAT system when

comparing its performance to the O3b concept. Additionally, the modeled MEXSAT

links had static performance because they are in geostationary orbit, but also were

modeled using only Ku-band transponders for this study.

A. LINK VARIABLES CLASSIFICATION

This study determined a binary classification of the involved variables when

employing the link budget tool from STK using input and output variables. Values of the

input variables include the number of satellites, size of the employed antenna, data rates,

and the required power. Each change represents a trade-off between the communication

capabilities achieved versus the hardware and software required to close the link. These

62

input variables influence the output variables. This study considered the LFS, rain loss,

and BER as output variables. The output variables represent the performance of the

modeled link and the results that were employed to make comparisons with other

modeled links. As stated before, when designing and comparing the proposed satellite

links with the current MEXSAT links, a successful link will achieve a minimum BER’s

threshold value of 10−6.

B. COMPARISON METHODOLOGY

All the models in this study followed the same method for comparing the obtained

results of the simulated links from the modeled satellite constellations. This method

comprised of two phases in the context of a common worst-case scenario. The city of

Tapachula, Mexico, located at 14.9056° N, 92.2634° W, represented the common worst-

case scenario and was the location where all terminals were set. Figure 36 illustrates how

Tapachula city proved to be one of the most susceptible Mexican cities to suffer the

highest amount of rain attenuation when performing satellite communications. Rain

attenuations proved to be an important factor when evaluating the performance of the

links. This worst-case scenario represents the scenario where a specific link is closed and

considers the highest possible losses and attenuations. This means that if the satellite

links are established at the worst-case scenario, the constellation will be able to assure the

achievement of satellite communications elsewhere. Therefore, Tapachula City was

considered the worst-case scenario when evaluating all the proposed links within this

study.

Phase 1 established the values for both the required power to link closure and the

data rates employed by all simulated links. When simulating a specific type of link,

considering a particular direction of the link and the type of facility. In this case, we

simulated the modeled MEXSAT links first. The modeled MEXSAT links adjusted their

performance to obtain the higher possible data rates, but used an appropriate amount of

power according to the employed type of facility. The data rates of those modeled

MEXSAT links became the basis for the O3b proposed links. As previously stated, this

study evaluated the feasibility of achieving at least the same performance than MEXSAT,

63

if not better. Therefore, establishing the performance of the current MEXSAT prior to the

O3b constellations is strongly required. Once the MEXSAT links are calculated, the same

used values for power and achieved data rates were considered in the simulated O3b

constellations to obtain the results and compare both outcomes.

Phase 2 compares the results obtained in the simulations by analyzing the

obtained LFS and rain attenuations, and looks at which link properly closed by achieving

the required minimum BER’s threshold value of 10−6. Once a link was able to close, the

study computed the percentage of link closure during a specific coverage period. In

addition to the BER values, the coverage percentage also helped to determine which

satellite constellation performed better than the others.

1. Downlink Performance

This subsection evaluates and compares the efficiency of the modeled O3b

downlinks with the modeled MEXSAT performance. The results of this research are

classified by the type of facility: aircraft, expeditionary station, patrol ships/vehicle, and

ground station.

a. Aircraft Facility

The aircraft facility type was the first to be evaluated because it represents the

“worst-case scenario” due to its achieved antenna-gain values. Table 4 shows that the

aircraft 0.3-meter antenna had the lowest gain values, which means that if the link closes

using that antenna size, the performance of the other facilities is likely to be superior.

Furthermore, when using high-gain antennas, the links are more easily closed. In other

words, the higher the antenna gain, the easier it is to close the link in a specific scenario.

Table 5 shows the performance values of one period of coverage between the

aircraft facility and the modeled constellations, each with a different number of satellites.

64

Table 5. Achieved performance the link on the aircraft facility.

When working with a 0.3-meter antenna, the MEXSAT modeled link was able to

close the link with a minimum BER of 10−6 while using 20 W of power, and up to 8

Mbps. These values were the common basis for aircraft O3b downlinks.

Table 5 demonstrates that O3b had a LFS advantage over MEXSAT. The free

space losses experienced by the MEXSAT system were greater than the O3b concept by

~13 dB at zenith and ~10 dB when at the rising and setting satellite location. Moreover,

the MEXSAT links had a maximum disadvantage of ~8 dB over the Ka-band links.

Therefore, the O3b constellations working with Ku-band transponders had a clear

advantage over the other types of systems.

Table 5 also illustrates that the MEXSAT system experienced rain attenuation

advantages when compared with O3b. Overall, rain attenuations were greater when using

the O3b concept. The number of satellites was inversely proportional to the rain losses

achieved by the downlinks. The more satellites a constellation has, the more the slant

path through the rain is minimized, and this helps to reduce the rain attenuations.

Furthermore, the link never closed at all on the O3b constellations when working with

Ka-band links because they suffered more severe rain losses than Ku-band links, with

approximately ~20 dB more rain attenuation at the rising and setting look-angles, which

defines a huge disadvantage when comparing the performance of Ku-band transponders.

Additionally, on the setting and rising points, the five-satellite and seven-satellite Ku-

band O3b constellations had a weakness of ~6 dB and ~0.55 dB more rain attenuation

than the eight-satellite constellation, respectively. Moreover, MEXSAT had a maximum

Power (W)

LFS (dB) Rain loss (dB)C/N (dB)

Eb/N0 (dB) BER

initial 20 199.347 32.4666 -8.503 -10.2636 2.58E-01 0Mid 20 197.046 15.306 10.958 9.1974 2.29E-03final 20 199.347 32.4664 -8.503 -10.2635 2.58E-01 100 % does notinitial 20 194.91 12.7959 6.7307 4.9697 3.22E-02 64.516Mid 20 192.478 4.9513 17.007 15.2458 1.96E-08final 20 194.91 12.7952 6.7313 4.9704 3.22E-02 35.484 % does notinitial 20 194.429 7.437 12.57 10.8093 3.80E-04 74.074

0.3 m 8 Mid 20 192.478 4.9513 17.007 15.2458 1.96E-08final 20 194.429 7.4118 12.595 10.8345 3.67E-04 25.926 % does notinitial 20 194.232 6.8689 13.335 11.5741 1.27E-04 76.923Mid 20 192.495 4.9492 16.992 15.2309 2.06E-08final 20 194.232 6.8689 13.335 11.5741 1.27E-04 23.077 % does not

20 205.298 4.8687 14.951 13.1905 6.23E-06

% of the time that achieves more BER 10^-6




5 Sats MEO

Ka Rain

7 Sats MEO Ku

Rain8 Sats

MEO Ku Rain

DataRate (Mbps)

5 Sats MEO Ku

Rain

GEO Rain

AirCraftAntenna size

Ground terminal

65

~2 dB advantage over the eight-satellite Ku-band O3b constellation when at the setting

and rising points, while at zenith the difference was zero.

Table 5 demonstrates that using the aircraft facility, all proposed O3b

constellations did not close the link continuously. Moreover, the eight-satellite O3b

constellation did not overcome the performance of the current MEXSAT system.

Therefore, it was discarded for the following facility comparisons because of its poor

quality, inferior coverage performance, and its slightly higher cost than MEXSAT. There

is no reason to look further in this model because it costs more and its performance is

lower.

This aircraft facility evaluation demonstrates that MEXSAT downlinks defeated

O3b downlink’s in performance. Despite the LFS disadvantage, MEXSAT had a constant

benefit from the larger antenna-gain values and lower rain attenuations, which permitted

MEXSAT to achieve the required BER values.

b. Expeditionary Station, Patrol Ships/Vehicle Station, and Ground Station Facilities


expeditionary station facility and the modeled constellations. In this facility, a 0.415-

meter antenna allowed the MEXSAT modeled link to close with a minimum BER of

10−6 while using 20 W of power, and up to 16 Mbps. Therefore, these values were the

common basis for expeditionary stations O3b downlinks.

Table 6. Achieved performance the link on the expeditionary station facility.

Power (W)


Eb/N0 (dB) BER

initial 20 199.347 32.4666 -5.684 -7.4451 2.30E-01 0Mid 20 196.915 15.4826 13.731 11.9703 6.67E-05final 20 199.347 32.4664 -5.684 -7.445 2.30E-01 100 % does notinitial 20 194.91 12.7959 6.5389 4.778 3.47E-02 61.29Mid 20 192.478 4.9513 16.815 15.0541 3.77E-08

0.415 m 16 final 20 194.91 12.7952 6.5396 4.7787 3.47E-02 38.71 % does notinitial 20 193.514 5.5874 15.143 13.3823 4.04E-06 100Mid 20 192.479 4.9512 16.815 15.0536 3.78E-08final 20 193.514 5.5874 15.143 13.3823 4.04E-06 0 % does not

20 205.298 4.8064 14.822 13.0612 8.26E-06

Exp. Station

Antenna size Ground terminal

DataRate (Mbps)

5 Sats MEO Ku

Rain

GEO Rain

7 Sats MEO Ku

Rain

5 Sats MEO

Ka Rain




66


patrol ship/vehicles type facility and the modeled constellations. This type of facility used

a one-meter antenna, which allowed the MEXSAT modeled link to close with a minimum

BER of 10−6 while using 20 W of power, and up to 75 Mbps. Therefore, these values

were the common basis for patrol ship/vehicles O3b downlinks.



ground station facility and the modeled constellations. This type of facility used a seven-

meter antenna, which allowed the MEXSAT modeled link to close with a minimum BER

of 10−6 while only using 10 W of power, and up to 500 Mbps. Therefore, these values

were the common basis for ground station O3b downlinks.


Table 6, Table 7, and Table 8 demonstrate that O3b kept the LFS advantage over

MEXSAT. The MEXSAT system experienced more LFS than the O3b constellations by

~13 dB at Zenith and ~12 dB when at rising and setting satellite location. Moreover, the

MEXSAT links experienced a minimum disadvantage of ~5 dB from the Ka-band links.

Power (W)


Eb/N0 (dB) BER

initial 20 199.347 32.4666 -7.765 -9.5258 2.51E-01 0Mid 20 197.147 15.2466 11.655 9.8935 1.14E-03final 20 199.347 32.4664 -7.765 -9.5257 2.51E-01 100 % does notinitial 20 194.91 12.7959 7.4685 5.7076 2.35E-02 74.194

1 m 75 Mid 20 192.478 4.9513 17.745 15.9837 1.19E-09final 20 194.91 12.7952 7.4692 5.7083 2.35E-02 25.806 % does notinitial 20 193.514 5.5874 16.073 14.312 3.72E-07 100Mid 20 192.479 4.9512 17.744 15.9832 1.19E-09final 20 193.514 5.5874 16.073 14.3119 3.72E-07 0 % does not

20 205.297 4.9088 15.655 13.8936 1.16E-06

Patrol Ship/ Vehicle

Antenna size Ground terminal 5 Sats

MEO Ku Rain

GEO Rain

7 Sats MEO Ku

Rain

DataRate (Mbps)

5 Sats MEO

Ka Rain




Power (W)


Eb/N0 (dB) BER

initial 10 199.347 32.4666 -2.113 -3.8738 1.83E-01 61.29Mid 10 196.979 15.3795 17.341 15.5802 5.82E-09final 10 199.347 32.4664 -2.113 -3.8737 1.83E-01 38.71 % does not

7 m 500 initial 10 194.91 12.7959 13.121 11.3603 1.76E-04 93.548Mid 10 192.478 4.9513 23.397 21.6363 3.60E-30final 10 194.91 12.7952 13.122 11.361 1.75E-04 6.4516 % does notinitial 10 193.514 5.5874 21.726 19.9646 3.35E-21 100Mid 10 192.479 4.9512 23.397 21.6359 3.63E-30final 10 193.514 5.5874 21.726 19.9645 3.35E-21 0 % does not

10 205.297 4.9241 21.351 19.5904 1.27E-19

Ground Station


5 Sats MEO Ku

Rain

DataRate (Mbps)

GEO Rain

5 Sats MEO

Ka Rain

7 Sats MEO Ku

Rain




67

Table 6, Table 7, and Table 8 demonstrate that MEXSAT kept its rain attenuation

advantages when compared to O3b. Overall, rain attenuations were greater when using

the O3b concept. The number of satellites was inversely proportional to the rain losses

experienced by the downlinks. Additionally, when using Ku-band transponders, five-

satellite and seven-satellite O3b constellations had a maximum disadvantage of ~7 dB

and ~0.7 dB over MEXSAT, respectively. Moreover, the highly variable slant paths

affect severely the modeled links when at rising or setting points. Due to the

approximately ~20 dB greater rain losses, at the rising and setting look-angles, the Ka-

links did not close at all on the O3b constellations. For simplicity, the following uplink

evaluations did not consider using Ka-band transponders because it is shown that they

need more power or more antenna gain values than Ku-band transponders to achieve the

same required throughputs.

Although previously shown to be less suitable for the aircraft terminal cases,

Table 6, Table 7, and Table 8 demonstrate that when using the expeditionary station,

patrol ships/vehicle, and ground station facilities, the seven-satellite O3b constellation

achieved continuous coverage and reached higher BER values than MEXSAT.

2. Uplink Performance

This subsection describes and analyzes the performance of the modeled O3b

uplinks against the modeled MEXSAT efficiency. The type of modeled facility classified

the results of this research: aircraft, expeditionary station, patrol ships/vehicle, and

ground station.

a. Aircraft Facility


aircraft facility and the modeled constellations, each with a different number of satellites.

When working with a 0.3-meter antenna, the MEXSAT modeled link proved to close the

link with a minimum BER of 10−6 while using 25 W of power, and up to eight Mbps.

Therefore, these values were the common basis for aircraft O3b uplinks.

68

Table 9. Achieved performance the link on the aircraft facility.

b. Expeditionary Station Facility


expeditionary station facility and the modeled constellations. In this facility, a 0.415-

meter antenna allowed the MEXSAT modeled link to close with a minimum BER of

10−6 while using 200 W of power, and up to 135 Mbps. Therefore, these values were the

common basis for expeditionary stations O3b uplinks.


c. Patrol Ship/Vehicles Facility


patrol ship/vehicles type facility and the modeled constellations. This type of facility used

a one-meter antenna, which allowed the MEXSAT modeled link to close with a minimum

BER of 10−6 while using 30 W of power, and up to 100 Mbps. Therefore, these values

were the common basis for patrol ship/vehicles O3b uplinks.

Power (W)


Eb/N0 (dB) BER

initial 25 196.249 17.3854 4.4496 2.6887 6.71E-02 61.29032Mid 25 193.817 7.1848 17.082 15.3207 1.50E-08final 25 196.249 17.3856 4.4492 2.6883 6.72E-02 38.70968 % does notinitial 25 194.853 7.9135 15.317 13.5564 2.68E-06 100

0.3 m 8 Mid 25 193.818 7.1845 17.081 15.3204 1.51E-08final 25 194.731 7.736 15.617 13.8556 1.28E-06 0 % does not

25 206.637 7.0316 15.101 13.3403 4.45E-06



DataRate (Mbps)

5 Sats MEO Ku

Rain

GEO Rain

7 Sats MEO Ku

Rain

AircraftAntenna size

Ground terminal

Power (W)


Eb/N0 (dB) BER


0.415 m 135 Mid 200 193.818 7.1845 16.658 14.8974 6.32E-08final 200 194.853 7.9135 14.894 13.1333 7.07E-06 0 % does not

200 206.637 6.942 14.767 13.0065 9.29E-06



GEO Rain

7 Sats MEO Ku

Rain


DataRate (Mbps)

5 Sats MEO Ku

RainExp. Station

69


d. Ground Station Facility


ground station facility and the modeled constellations. This type of facility used a seven-

meter antenna, which allowed the MEXSAT modeled link to close with a minimum BER

of 10−6 while only using 250 W of power, and up to 500 Mbps. Therefore, these values

were the common basis for ground station O3b downlinks.


Table 9, Table 10, Table 11, and Table 12 demonstrate that O3b modeled

constellations experienced an LFS advantage over MEXSAT. The MEXSAT system

experienced more LFS than the O3b constellations by ~13 dB at zenith and ~12 dB when

at rising and setting satellite location.

Table 9, Table 10, Table 11, and Table 12 demonstrate that O3b had rain

attenuation disadvantages when compared to MEXSAT. Overall, rain attenuations were

smaller when modeling MEXSAT. Additionally, when using Ku-band transponders, five-

Power (W)


Eb/N0 (dB) BER


1 m 100 Mid 30 193.818 7.1845 17.362 15.6007 5.39E-09final 30 194.853 7.9135 15.598 13.8366 1.34E-06 0 % does not

30 206.636 7.0896 15.324 13.5634 2.64E-06




DataRate (Mbps)

5 Sats MEO Ku

Rain

GEO Rain

7 Sats MEO Ku

Rain

Patrol Ship/ Vehicle

Power (W)


Eb/N0 (dB) BER


7 m 500 Mid 250 193.818 7.1845 36.482 34.7208 1.00E-30final 250 194.853 7.9135 34.718 32.9567 1.00E-30 0 % does not

250 206.636 7.1116 34.423 32.6618 1.00E-30



GEO Rain


DataRate (Mbps)

5 Sats MEO Ku

Rain7 Sats

MEO Ku Rain

Grnd Station

70

satellite and seven-satellite O3b constellations had a maximum disadvantage of ~10 dB

and ~0.8 dB over MEXSAT, respectively.

Although less suitable on the downlink, Table 9, Table 10, Table 11, and Table 12

demonstrate that using all the facilities, the modeled seven-satellite O3b constellation

accomplished the same coverage performance but achieved a better link quality by

getting higher BER values than MEXSAT. This means that the modeled seven-satellite

O3b constellation was able to finally achieve a better uplink performance than the

modeled MEXSAT system.

71

V. CONCLUSION AND FUTURE WORK

This study has compared the performance between the O3b constellation concept

using Ka-band and Ku-band transponders versus the MEXSAT system using only Ku-

band transponders. The analysis revealed that the current MEXSAT system is the

modeled constellation with the highest functional performance because it achieved a

constant link closure and provided continuous coverage over the entire target area. The

modeled seven-satellite O3b constellation also provided continuous coverage and

accomplished links with a better quality at its zenith when communicating with facilities

that used antennas with diameters of at least 0.415 meters. However, the O3b

constellation did not achieve constant link closure when using only the modeled aircraft

facilities. Therefore, the modeled O3b constellation would not work continuously with

facilities with antennas smaller than 0.415 meters. Therefore, the MEXSAT system

achieved the most functional, continuous, and optimal performance compared to the

modeled O3b constellation.

There are four indications that the modeled MEXSAT system is superior to the

modeled O3b constellations. The first indication of MEXSAT’s superiority over O3b is

that, whereas the total O3b’s RF attenuations varied in value with the facility-satellite

path length, the total MEXSAT attenuations were constant and often lower in value due

to its constant facility-satellite path length. The MEXSAT links had constant path

lengths, but the modeled O3b constellations had their path lengths continuously changing

The LFS and rain attenuation values on the O3b links, on the other hand, were

proportional to the path length from Earth to the satellite, and were not constant when

covering the targeted area. This permitted the MEXSAT links to achieve a constant and

consistently superior performance, while the O3b links performance were variable and

sometimes superior only at the zenith. In these O3b models, the simulated LFS and rain

attenuations mitigated the propagation of RF waves along the Earth-space path, which

varied according to the determined physical characteristics such as the working

frequency, look-angle, and geographic location [68]. All the compared O3b links were

communicating with facilities located in the same geographic location and worked with

72

the same frequency (Ka-band or Ku-band). Therefore, the look-angle was the only

parameter that inversely influenced the path length from Earth to the satellite. If the look-

angle is reduced, the rain and LFS attenuations increase through a longer path length

[21]. Figure 38 shows the relationship between the short “A” and the long “B” path

lengths with high and low look-angles, respectively. Nevertheless, the MEXSAT system

had constant and functional performance, which made it superior to the modeled O3b

constellations. The modeled O3b links were superior only at zenith, which made the

constellation performance variable.

Figure 38. Schematic showing the change in path length through the atmosphere. Source: [21].

The second indication of MEXSAT’s superiority was that its Ku-band transponders

were less vulnerable to strong rain attenuation than the O3b’s Ka-band transponders were.

Originally, this study discarded the modeled O3b constellation working with Ka-band

systems due to the obtained poor quality links. Ka-band systems suffered from large rain-

attenuation values of ~20 or 30 dB, which did not allow them to close the link at all, and

resulted in a significant disadvantage when compared to the MEXSAT system [21]. In

addition to this, the modeled O3b constellations that worked with Ku-band continued to

suffer from an increased amount of rain attenuation relative to the geostationary MEXSAT.

This resulted in a disadvantage discussed previously in the first indication. In case the Ka-

band working frequency is required, this disadvantage would oblige the system’s designers

73

to implement additional fade mitigation techniques, which would add more complexity and

could represent a higher cost when designing and building such a Ka-band system.

The third indication that established the superiority of MEXSAT system was its

higher satellite antenna-gain values. This resulted in consistently closed links no matter the

size of the facility’s antenna on the ground. The MEXSAT system employs a 2.3-meter

antenna to close the link within the targeted area. Due to the large gain achieved by that

diameter, the MEXSAT system was able to close the link no matter the size of the employed

antenna on the ground. This 2.3-meter antenna achieves a larger gain value than the O3b

satellite’s antenna. When comparing the performance of both modeled constellations, the

MEXSAT system takes advantage of the achieved antenna gain values and closes the link

more easily than the modeled O3b constellations.

Finally, the MEXSAT system achieved a superior performance than the modeled

O3b constellation in its ability to close links continuously when working with facilities that

used small antennas. The O3b, in contrast, did not achieve link closure uninterruptedly when

employing these facilities. When evaluating the modeled O3b constellations in this study,

the results demonstrated that facilities with the smallest modeled antennas achieved poor

link availability and did not receive continuous coverage. Surprisingly, when evaluating the

seven-satellite O3b constellation, all the links were able to close uninterruptedly except

when assessing the downlink where the modeled links did not achieve continuous coverage

in the targeted area. This lack of complete coverage represents the unique link performance

disadvantage of the seven-satellite O3b constellation over the MEXSAT system. Hence, the

MEXSAT system achieved greater functional performance than O3b because MEXSAT is

able to work with all the modeled facilities. Usually, the SEMAR would require continuous

and full coverage at the targeted area while using the whole range of required facilities to

successfully execute any operation at the strategic, operational, and tactical level.

This study reveals that the modeled seven-satellite O3b constellation shows the

potential to be improved in order to accomplish full and continuous coverage over the

desired Mexican territory, and this could be accomplished by changing one of the variables

of the link budget equation. Adopting limited modifications in the values of any variable

would represent either more complexity or greater cost for the proposed next system. We

74

recommend further evaluation of the feasibility of employing antennas with higher gain

values on the satellite or utilize fade mitigation techniques.

Redesigning the O3b satellites by resizing the antennas in order to obtain higher gain

values may culminate in a more expensive design with higher costs. This could, however,

permit the modeled O3b constellation to accomplish a performance superior to the current

MEXSAT system with features such as fiber-like latency and either higher data rates, or

power requirements equal to or less than those of MEXSAT.

Moreover, fade mitigation techniques employ adaptive systems that regulate and

change as desired the power, coding modulation, data rates, and frequency to be used by the

SATCOM system to provide continuous communication coverage [21]. Nevertheless, this

option also represents a possible increase in cost and complexity that would require further

feasibility evaluations

The next phase for assessing the feasibility of the O3b concept is to calculate

different and not previously evaluated attenuations and noise that could affect the signal

transmission. In particular, the atmospheric effects for satellite links affected by RF

attenuations from atmospheric gases, clouds, rain, scintillation, and depolarization due to

rain [21]. This study only measured the rain attenuation values as the atmospheric losses

happening in propagation losses. The attenuation effects due to gases, clouds, scintillation,

and depolarization due to rain should be calculated and considered.

Moreover, different kinds of noises could be considered to improve the quality of

this feasibility study, such as adjacent noise, noise temperature, and sun outage. Adjacent

noise happens when non-desired nodes illuminate one of the employed terminals, which

needs to be considered when designing the satellite constellation [21]. The noise

temperature was estimated to be 290 K for this study’s purposes. Future work should instead

consider calculating a non-approximated value, which would include the noise both caught

by the antenna and generated in the feeder [21]. Finally, the sun represents a constant source

of noise in deep space and could cause interference that should be calculated [21].

75

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