Enabling Inter-satellite Communi

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ENABLING INTER-SATELLITE COMMUNICATION AND RANGING FOR SMALLSATELLITES

R. Sun (1) , D. Maessen (1) , J. Guo (1), E. Gill (1)

(1) Chair of Space Systems Engineering, Faculty of Aerospace Engineering, Delft University of

Technology, Kluyverweg 1, 2629 HS, Delft, the Netherlands, +31152786098, [email protected]

ABSTRACT

As a first step in the development of a fully functional, reliable and efficient inter-satellitecommunication and ranging system for small satellites, various key system drivers in terms oflayer-based communication architectures and technologies are analysed based on the differentoperational needs for different missions. The lower three layers, being the physical, date link, andnetwork layers, are of primary concern. After analyzing the current existing inter-satellite rangingmethods, focuses have been put on Global Navigation Satellite System (GNSS)-like rangingmethods with respect to its signal characteristics, achievable ranging accuracy, and the feasibility ofits use to inter-satellite links.

1. INTRODUCTION

To increase mission return, future space missions can utilize multiple miniaturized spacecraft flyingtogether in a coordinated formation. These spacecraft will likely communicate with each other viaan Inter-Satellite Link (ISL) to enable advanced functions, e.g. relative navigation and formationcontrol, clock synchronization, science and spacecraft health data exchange. Flying two or morespacecraft in a precisely controlled formation presents high requirements for ISLs, because inter-satellite sensors are needed not only to support communication, but also to enable formationacquisition and maintenance in a precise relative position using inter-satellite tracking. Direct inter-satellite ranging is one of the tracking strategies. It can be applied both in Low Earth Orbit (LEO) asan ancillary to Global Positioning System (GPS)-based relative positioning, and in Medium EarthOrbit/Geosynchronous Earth Orbit (MEO/GEO) or non-Earth orbits that are out of the range of GPSsignals. Therefore, the combination of inter-satellite communication and ranging into a single

package with its flexible use of inter-satellite sensors on small satellites allows a wide range ofapplications related to distributed spacecraft systems.

A series of missions have been flown or proposed with high demands on the inter-satellite rangingaccuracy. This includes the GRACE mission with its radiometric K/Ka-band inter-satellite trackingat micrometer-level accuracy [1], the PRISMA mission with its S-band Radio frequency (RF)-basedmetrology at centimeter-level ranging accuracy [2], and their follow-ups such as PROBA-3 [3]. The

success of these missions depends on the ability to achieve formation keeping by autonomous on- board control, as well as on their accurate relative positioning in a precise manner. Some othermissions like NASAs New Millennium Program missions ST-3 (Starlight) [4], ST-5 [5], andTechsat-21 [6], although aborted or heavily modified, their technologies regarding ISLs are stillvaluable and inspiring. The Autonomous Formation Flying (AFF) sensor developed for ST-3 has

been modified with the intention to reuse it for the future Terrestrial Planet Finder (TPF) mission[7]. Magnettospheric Multiscale Mission (MMS) [8] and MicroArcsecond X-ray Imaging Mission(MAXIM) [9] are other examples that rely on ISL-based technologies to ensure mission success.Apart from mission specific applications, inter-satellite communication and ranging allows for areduced ground segment, and with that reduced operation costs, as well as enhanced systemrobustness and real-time operations. Therefore, this technology is of high interest both for currentand future formations.When designing the inter-satellite sensor, it is wise to take advantage of existing hardware andsoftware used in commercial terrestrial communication networks. However, the space environment

Small Satellite Systems and Services Symposium (4S), Funchal, Portugal, 31 May 4 June 2010 1
mailto:[email protected]:[email protected]:[email protected]


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does pose some challenges, such as multiple mobile nodes forming a dynamic network topology,intermittent communication links, line of sight ranging, limited on-board power and computingresources, and bandwidth constraints, which make the design of an inter-satellite sensor achallenging task. With these considerations, this paper analyzes various key system drivers in orderto provide recommendations for the development of a fully functional, reliable and efficient inter-satellite communication and ranging system for small satellites. This paper consists of three main

parts. The first one regards the basic inter-satellite communication issues, including Radiofrequency or optical links, frequency allocations, data rate, and power control mechanism for smallsatellites. The second one is about inter-satellite communication network architectures andtechnologies, which emphasize on how to share the channels and how to route data. The third partof the paper summarizes the existing inter-satellite ranging methods, focusing on GNSS-likeranging. The ranging signal characteristics, achievable ranging accuracy, signal acquisition andtracking, and the feasibility of their use via ISLs are discussed. Concerning the limitations of power,mass and cost for small satellites, this paper gives recommendations at the end of each part.

2. LAYER-BASED INTER-SATELLITE COMMUNICATIONS

Different operational needs for missions trigger different ISL communication requirements andarchitectures. A layer-based Open Systems Interconnection (OSI) model serves as a good referencefor understanding what is needed to make communication work, as depicted in Fig. 1 [10]. As such,the requirements levied on inter-satellite communications systems will be compatible with the basictenets of networking. The physical layer, related to the trade-offs among several system drivers suchas power, frequency, data rate and bandwidth, has been found to be of primary concern, since it hasthe largest role to play when it comes to reliable and efficient communication via ISL.

PhysicalMedia, Signal , and

Binary Transmission

Data Link M A C a n d L L C

(Physical Addressing)

Network Path D etermination and IP

(Logical Addressing)

TransportEnd-to-End C onnections

and Reliability

SessionInterhost Co mmunication

PresentationDa ta Representation

and Encryption

Application Ne tw or k A c ce ss t o

Application Data

Data

Data

Segments

Packets

Frames

Bits

Data uni t Layer

H o s t L a y e r s

M e d i aL

a y e r s

Fig. 1. OSI model [10]

Radio Frequency or Optical Inter-satellite Links

Radio frequency (RF) and optical/laser are the two primary communication media for an ISL.Optical links have the ability to provide very high speed communications with data rate on the order

of Gbps[11], which is suitable for image transfer. Optical sensors are also used once the involvedspacecraft reach very short distances with respect to each other, guaranteeing very high positionaccuracy. The narrow beam and high directivity of the laser affords interference-free signal transfer



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and low possibility of interception. However, optical sensors tend to have a relatively small field ofview. Hence, to obtain spherical coverage one must either have a large number of them or extendtheir field of regard by scanning. A more challenging part of an optical sensor is that a highlyaccurate Acquisition, Tracking and Pointing (ATP) subsystem is required in order to establish andmaintain the communication link, as depicted in Fig. 2. In the process of scanning for target andmaintaining link, a direct line-of-sight is needed and a rapid relative motion between two

spacecraft will make this process more complex. Although transmission of an image by laser linkfrom one satellite to another has been demonstrated in SILEX [12] (Semiconductor Laser Inter-satellite Link Experiment) with a data rate of 50 Mbps, optical communication is still a very newtechnology for spacecraft.

Yes

No No

Scan fortarget

Wait forconfirmation

Generate beacon

laser

Getconfirmation?

Yes Maintainlink

If needmaintain link?

Fig 2. ATP assembly in optical links

Compared to optical sensors, RF sensors have problems of their own, but they appear moremanageable. The data rates achievable with an RF link are lower than with an optical link, but if ina distributed spacecraft mission the link message traffic only consists of navigation data, spacecrafthealth and status, and some science data (not including imaging information such as ininterferometric and optical mapping missions), data rates less than 10Mbps are more than adequate.RF links can provide omni-directional coverage when considering multi-antenna combination.Although RF equipment is subject to co-channel interference, multipath, atmospheric and man-made noise, careful system design and use of technologies such as spread spectrum modulation cansignificantly reduce interference effects in most cases. Furthermore, long term experience withradio transmission for space-to-ground links makes RF-based inter-satellite communication more

reliable and easier to implement in space. On balance, once there is a need for a very high data rateor a very accurate positioning requirement, optical sensors can be the solution. Otherwise, an RFsensor is preferable for a small satellite mission.

Frequency Allocation

The appropriate frequency band (or bands) is an important part of any recommendation for inter-satellite communications. The choice of frequency bands depends upon the spectrum regulationsspecified by the International Telecommunication Union (ITU), technical characteristics andconstraints (including availability of hardware), and mission requirements.

Table 1. Frequency allocation candidates for inter-satellite communications [14]Band Frequency Range Service Examples

S 2025 - 2110 MHz2200 - 2290 MHzSRSSRS

PRISMATPF

Ku 13.75 - 14.3 GHz14.5 - 15.35 GHzSRSSRS

Ka22.55 - 23.55GHz25.25 - 27.5 GHz32.3-33.4 GHz

ISSISSISS, RNSS

IridiumGRACE (K/Ka band)StarLight

W 59 - 64 GHz65 - 71 GHzISSISS

International and national spectrum regulations are an important consideration when identifying the proper spectrum or when making frequency allocations for distributed spacecraft inter-satellitecommunications. Based on the service designation by ITU, several frequency allocation optionsmay be available for inter-satellite communications [13, 14]: allocations to Space Research Service(SRS); Inter-satellite Service (ISS); Radionavigation and radionavigation-satellite service (RNSS).



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Table 1 provides several frequency bands that are appropriate for implementing ISLs between 1 and100GHz as recommended by ITU. Examples of several distributed spacecraft missions withdifferent frequency allocations are also listed in Table 1.

Besides these spectrum regulations, the system designer needs to consider the availability ofhardware and the technical characteristics of the frequency bands. Several technical parametersinfluence the selection of frequency bands for ISLs including:

Available bandwidth and data rates;It is well understood that the higher the frequency allocation, the wider the bandwidth available.Based on the Shannon theorem, the maximum theoretical data rate is proportional to the bandwidth.So, high speed data can be achieved with high frequency bands for which wide band use is

permitted. In the process of frequency selection, the different amount of transmitted data in differentmissions determines the required bandwidth, which will consequently influence the frequencyallocation, as shown in table 2.

Table 2 bandwidth and data rates equivalences

Bandwidth Maximum data rate

Recommended

Frequencyallocation

narrow 100Mbps Ku, Ka, W

Number of channels and/or number of links in the local distributed spacecraft network andmultiple access techniques;

Multiple spacecraft in the local distributed spacecraft network will share communication links. For

code division multiple access (CDMA), because it employs spread-spectrum technology and aspecial coding scheme to allow multiple users to be multiplexed over the same physical channel, thetotal data rate can be equivalent to several times the single access bandwidth, depending on themodulation and the number of links. Therefore, sufficient spectrum is needed to accommodate thisincreased data transmitted simultaneously by multiple users. For a frequency division multipleaccess (FDMA) system, which assigns a sub-frequency band for each spacecraft, the bandwidth foreach channel contributes to the total spectrum. In addition, the frequency isolation between sub-frequency bands should also be considered to mitigate mutual interference and facilitate the

bandpass filters to separate channels. When the number of channels is large, lower frequencyallocation bands (e.g. 2025-2110 MHz in S band has only 85MHz bandwidth available) may not

provide sufficient bandwidth alone when using FDMA technology. For time division multiple

access (TDMA), whose strategy is to share a single carrier frequency with multiple users atdifferent time slots, has the least influence to frequency allocation compared to CDMA and FDMA.

Link performance, associated with required transmitter power, propagation, and antennacharacteristics, which can vary greatly depending upon frequency;

The free-space loss L FS is inversely proportional to the square of the carrier frequency f [16]:

2( ) ( )4 4 FS

c L

d d 2

f

(1)

where d is the distance between the transmitter and receiver, c is the speed of light, and is the

wavelength. Thus, lower frequency has a smaller space loss.

http://en.wikipedia.org/wiki/Spread-spectrumhttp://en.wikipedia.org/wiki/Proportionality_(mathematics)http://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Proportionality_(mathematics)http://en.wikipedia.org/wiki/Spread-spectrum


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In addition, the RF frequency also affects the satellite transmitter power, antenna size and beamwidth. In turn, these factors affect satellite size, mass, and complexity. Their relationship can be expressed by the following equations [16]:

2 2 2 2 2

2 2t

D DG

c

f (2)

21GHz f D

(3)

l t EIRP PL G (4)

t G is the transmitter antenna gain, is the half-power beamwidth in degrees (-3dB lower than the

peak gain), D is the antenna diameter, is the antenna efficiency, P is transmitter power, l L istransmitter-to-antenna line loss, and EIRP is the transmitter effective isotropic radiated power.

When assuming constant EIRP and decreasing carrier frequency, the satellite antennas diameterincreases to maintain the specified beamwidth until it reaches a maximum size (or mass) limit.Reducing the carrier frequency further requires more transmitter power to compensate for the loss

in antenna gain. On the other hand, going to higher frequencies makes the antenna beam narrower,which means that higher frequencies are incompatible with having an omni-directional coverage.

The overall link performance can be expressed by the link budget using the received signal-to-noise power density ratio C/N 0 of the communication system. If we assume identical transmitter power,antenna gains, and implementation losses in both the lower and the higher frequency bands, thereceived signal will be much weaker at higher frequency band.

In general, higher frequency bands enable smaller antenna size, but require more transmitter powerto compensate for all the attenuation effects, such as free-space loss, reduced effective C/N 0, andultimately make the overall communication system more complex. Distributed spacecraft missions,who usually consist of multiple small satellites with mass, power and cost constraints, arerecommended from link performance perspective to use lower frequency allocation (e.g. S band).

Ionospheric errors, Multipath, and Doppler shifts effects;ISLs are normally used both for the distribution of data among spacecraft and for navigation

purposes. Performing pseudorange measurements is a common way to realize relative navigation.From J.A. Avila-Rodriguez work on the feasibility of using C-band (2-4GHz) as future GNSSfrequencies [17], we conclude that frequency allocation has an effect on the pseudorange error

budget, including ionospheric effects, carrier multipath and the signal acquisition process.Ionospheric effects are inversely proportional to the square of the carrier frequency, and carriermultipath is inversely proportional to the carrier frequency. Therefore, a higher frequency allocation

helps to reduce the errors caused by ionosheric path delay, and to mitigate the carrier multipath aswell. However, due to higher maximum Doppler shifts at higher frequency, the Doppler searchregion increases, which negatively influences signal acquisition. Assuming identical code length,signal acquisition takes a longer time at high frequency bands.

The carrier frequencies for inter-satellite links must be assigned so as to avoid interferencewith other onboard communication systems like the TT&C subsystem.

If the inter-satellite sensor and the TT&C subsystem work in the same frequency band (e.g. S band,sufficient frequency separation between the inter-satellite link, and the TT&C uplink and downlinkis necessary to reduce the risk of disturbance. The PRISMA mission is a good example whenassigning carrier frequencies to its Formation Flying RF (FFRF) sensor [2]. Table 3 shows thefrequency allocation for PRISMA. Except for the consideration of separation between FFRF andTT&C, the carrier frequency selection for FFRF also guarantees to maximize the isolation betweenISL bandwidth and deep space research bandwidth (2290 2300MHz), and provides a reasonable



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inter-frequency separation (between S1 and S2) in order to optimize the integer ambiguityresolution functionality in the phase measurement for inter-satellite ranging.

Table 3. Frequency allocation for PRISMAFFRF

S1: 2275 MHz S2: 2105 MHzTT&C

Carrier frequency(S band)

TM: 2214 MHz TC: 2035 MHz

Data type, Data rates and Bandwidth

The required information to be distributed between the satellites falls into several data types ortraffic, each of which has different levels of data rate and bandwidth requirements: Navigation data Payload data Spacecraft health & status

Navigation data can contain the measured absolute position, relative distance, velocity, attitude, andtime information. The volume and transmission frequency of the navigation data is tightly coupledto the nature of the mission. In case of tightly cooperating spacecraft in close formations (separationdistances < 1 km) with high positioning accuracy and tight control windows, the typical maneuvercycle for maintenance of the formation may be too short for a ground-controlled formation and thusmany require a fully autonomous on-board control approach with the help of inter-satellitecommunication. In this scenario, the frequency of broadcast of navigation data over an ISL can beon the order of seconds or even continuously, which enables approximately real-time relativenavigation corrections to command the drifting spacecraft back within its control window. The tightcommunication link requires significantly higher data rate and greater bandwidth requirements than

those missions requiring kilometer level positioning accuracy.The need for payload data transmission via ISLs depends on knowledge of how the payload datawill be processed in the distributed spacecraft system [18]. If the common science data are

periodically dumped to ground stations without space segment processing, no science data needs to be transmitted to other spacecraft. On the other hand, some collaborative missions may require theexchange of science data to facilitate distributed space-based computing. Therefore, estimates of thedata rates and bandwidth for payload data present the greatest challenge because payload data typeand the degree of onboard distributed computing can be significantly different for differentmissions. For missions collecting imaging information, the data rate can be up to 10Mbps, but thefrequency of transmission can be low, on the order of hours. Some technique for reducing the datarate can be used, e.g. transmitting only the changes of the payload data [16]. The bandwidthoccupation of payload data is also related to the network topology of the communication system.The mostly used distributed computing topology is the star topology [19], which facilitates the datacollecting, comparison and processing within the master spacecraft, but is not as bandwidthefficient as a distributed topology.

Spacecraft health & status data is very low volume and would be broadcast less frequently than thenavigation data. The amount of this information depends upon the complexity of the equipment onthe spacecraft and the need to share the information with other spacecraft. In general, 10 3 bit issufficient for spacecraft health & status data, and the frequency of broadcast is on the order ofminutes [18].

Taking account of navigation, payload and spacecraft health & status data together, missionrequirements will decide the major contributors for the overall data rate and bandwidth. Forautonomous formation flying like PRISMA, navigation data dominates the inter-satellite



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communication bandwidth; while for the scientific missions, science data can be the main factordetermining the communication strategy.

Furthermore, the date rate determination is also affected by the power constraints and the separationdistance between the spacecraft. Higher data rate or larger distance means higher powerconsumption, and therefore higher system costs. Fortunately, for missions with large separationdistances between spacecraft, positioning accuracy and date rate requirements are normally lowerthan close formations, because the control windows are getting larger along with the separationdistance. If a specific formation flying mission is divided into several phases during its lifetime,variable data rate communication and variable position accuracy requirements can be implemented.Table 4 shows two examples of this situation [8, 20].

Table 4. Ranging accuracy and data rate requirements at different separation distances onPRISMA mission and MMS mission

Inter-satellitetransceiver

Spacecraftseparation

3 RangingAccuracy

Data Rate

10m 500m1m (LOS*>45)

20cm (6


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Furthermore, if CDMA signal structures are used to support simultaneous communications andranging, the well-known near-far interference problem exists. Near-far interference is a form ofmultiple access interference (MAI) in which the signals from other users will appear as noise to thesignal of interest and interfere with the desired signal in proportion to the number of users. Theinterference of a signal from a transmitter that is in close proximity to the receiver reduces theeffective received signal to interference power density ratio (SIR) several times more than the signal

from a remote transmitter.The objective of power control for inter-satellite links is to implement dynamically adjustable

power attenuation in the transmitters to provide a minimum SIR that satisfies the signal acquisitionand tracking requirements at the receiver. Taking into account both the noise and interference, anequivalent SIR can be expressed as [21]:

00

1

( ) Ri Ti ii N R Ri

Tx x Ti i x

P P GSIR

P P N B P G P G N B

(5)

where P is the power, G is the channel gain (loss), B is the bandwidth, N is total number ofspacecraft, and the subscript R indicates the received, T means the transmitted, and i, x indicatedifferent links.

Equation (5) shows that the equivalent SIR is a function of various transmitted powers and channelgains. What's more, the channel gain is inversely proportional to the square of the distance betweentransmitter and receiver. Therefore, in order to provide a minimum equivalent SIR, the transmitted

power P Ti should be adjusted along with the changing relative distances between any twospacecraft. For most missions, the formation geometry does not change rapidly during the time ofthe two sequential transmission duty cycles. Therefore, the information provided by the prior dutycycle can be used as reference to estimate the current relative distances. The transmitter power P Ti can then be adjusted to an optimal level, as depicted in Fig. 4. Other power control algorithms exist,such as SIR-based and position-based methods that are proposed for use in CDMA cellular mobile

radio networks [22]. Further research is needed to demonstrate their feasibility for ISLs.

Modulator TX

Data Demodulator RX

dt=i-1

, vt=i-1

d t=i Desired SIR

Power adjustment

Data

# S/C noise interference

Fig. 4. Adaptive power control mechanism for inter-satellite links

3. INTER-SATELLITE COMMUNICATION NETWORK ARCHITECTURE

The architectural complexity of the inter-satellite communication network is depends on its multipleaccess technologies and topology schemes. The data link layer and the networking layer in the OSImodel support their implementations. For the data link layer, we focus on different multiple access

technologies with respect to their specific advantages and disadvantages. For the network layer,several topologies are compared, and the need for multi-hop routing is discussed concerning largeseparation distance and strict timing constraints.



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Multiple Access Technology

Three basic techniques for sharing links in distributed spacecraft systems are FDMA, TDMA andCDMA. Table 5 summarizes their capabilities applied to inter-satellite communications, as well as a

brief overview of their advantages and disadvantages with an attempt to find the candidates for

small satellites with limitations on power, mass and cost. FDMA is not an economic choice becauseit needs a large frequency bandwidth to guarantee multiple transmitters or channels assigned tomultiple sub-frequencies. TDMA with its simple timing schedule is suitable for small satellites,especially in the situation that the number of spacecraft is not large (


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the Applied Physics Laboratory (APL) in Johns Hopkins University considers to apply hybridFDMA/CDMA for full duplex inter-satellite communications. This combination can support ascalable number of spacecraft and meanwhile achieve sufficient isolation between the transmitsignal and receive signal [24].

Topology

The communication topology is tightly coupled to the nature of the mission and the number ofspacecraft. Canonical topologies generally used for communications within distributed spacecraftsystems include centralized (star), fully distributed (mesh), and hierarchical forms.

The centralized formation flying topology consists of a center or mothership spacecraft andseveral daughter spacecraft, such as in the MAXIM mission [9]. The mothership acts as thecontrol point or data collector point for the formation, since it normally has a stronger data

processing capability. Information is passed from the daughter spacecraft to the mothership viaISLs. However, the mothership is a single point of failure due to its unique capabilities and itscentralized role within the formation. The distributed formation flying topology consists of several

spacecraft that have similar capabilities, such as in the MMS mission [8]. A hierarchical topology isthe combination of the previous two topologies and divides the formation into manageable subsets.Due to this, it works well for larger formations. A comparison of the different topologies is providedin Table 6 [24].

Table 6 Comparison of different communication topologies used in distributed spacecraft systemTopology Advantages Disadvantages Examples

Centralized(Star)

Simple design N-1 links for N nodes;

Relies on the capability of the centralresource, which is responsible for primarycontrol and information dissemination Potential faults within the central

resource greatly influences theimplementation of whole mission

DARWINMAMIXTPF

Distributed(Mash)

Support direct interaction among alldistributed assets Fault tolerate for each node Real-time communication to each

node

Topology architecture is N(N-1)/2 for Nnodes Rapid growth in complexity as N

increases Resource limitations such as

communication bandwidth and processingcapability.

GRACEMMS

Hierarchical

Robustness is supported Control structure complexity

depends on the functional relationships between S/C

Needs multilevel approach; Not necessary for small distributed

missions.

Multi-hop Routing

Whereas the aforementioned topologies with direct inter-satellite communications are the primaryand relative simple solutions, some indirect communication strategies such as dynamic networkrouting with multiple hops are potentially applicable to communicate over long distances that aregreater than the normal transmit power can support. Data can be relayed through various spacecraftin order to achieve the desired end-to-end communication objectives that cannot be realized bydirect, line-of-sight inter-satellite communications.

However, both the changing formation geometry and the choice for best paths to relay data posechallenges to the routing strategy. Distributed missions are consequently taxed with relativelydemanding on-board processing requirements. For some missions with strict time constraints,especially in a formation where the satellites need to maintain a precise relative position andorientation, a possible end-to-end latency requirement may be imposed (e.g. an upper bound ofsignal transmission latency is required in real-time control systems). This implies constraints on the



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multi-hopping because the signal end-to-end delay depends on the number of traversed satellites(processing, queuing, and propagation delay) [18].

Other indirect inter-satellite communication links exist, such as using a ground station or via thecommon commercial LEO communication constellations (Iridium, Orbcomm, or Globalstar) as arelay. Their feasibility depends on the communication architectures and differs from mission tomission.

4. INTER-SATELLITE RANGING

Inter-satellite Ranging Methods

Depends on the mission requirements, inter-satellite ranging can be performed in many ways. Fig. 5 depicts a breakdown of the various options. First, a choice needs to be made whether or not to

perform the ranging in a direct or an indirect manner. The indirect method comprises the subtractionof two measurements for which the difference yields the inter-satellite range. This can be doneusing GNSS measurements from onboard GNSS receivers, tracking data via ground station/Tracking and Data Relay Satellite System (TDRSS)/ Precise Range and Range Rate Equipment(PRARE) [15], or two-line elements as provided by North American Aerospace Defense Command(NORAD).

Inter-satelliteranging

Indirect Direct

One-way Two-wayGNSS

Ground stationtrackin GNSS-like Pulseg phase or code

PRAREtrackin

Range rate/Do Transponderg ppler shift

TDRSStrackin g

Two-lineelements

Fig. 5. Inter-satellite ranging options

If indirect measurements are not available, not exact enough, or if it is desired to obtain the range ina more autonomous manner, a direct range measurement between the satellites needs to be made.This can be done using a one-way signal or a two-way signal. The latter can be split into pulsed(radar or lidar) and transponder-based methods while the former can be divided into GNSS-like orrange rate/Doppler shift methods. Note that if the clocks on the two satellites are perfectlysynchronized, accurate one-way ranging using pulses is possible. However, since this is very hard toachieve in practice, this method is not considered to be a feasible option.

The one-way ranging methods suffer from ambiguity (GNSS-like, phase or code measurements) ordo not directly provide a range measurement (range rate measurements e.g. interferometric). Aninterferometric measurement namely provides the range rate and needs to be processed using an

estimator to yield the inter-satellite range. Due to the repetitive nature of the GNSS-like signals, phase and code measurements on the other hand suffer from ambiguity, which needs to be resolvedusing a proper system design. This can be done by using multiple signals with different frequencies



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to resolve the ambiguity for phase measurements and by using very long codes or by addingadditional timing data to resolve the ambiguity for code measurements.

GNSS-like Inter-satellite Ranging Signal Characteristics

Inter-satellite ranging is based on locally generated inter-satellite ranging signals. A cost effectivemanner to generate these signals is to modify an existing GPS receiver such that it can operate as atransceiver. This could result in a highly miniaturized and accurate ranging device that performs theranging using a one-way signal modulated with a GPS-like pseudo random noise (PRN) code. ThePRN code acts as a noise-like carrier that is used for bandwidth spreading of the signal energy, andfor mitigating the deleterious effects of reflected and interfering signals. The auto-correlation

property of the PRN code, resulting in a sharp peak, allows accurate range measurements. The chiprate of the PRN code will decide the transmission bandwidth. The Coarse/Acquisition (C/A) codewith chip rate of 1.023Mcps and the P(Y) code with chip rate of 10.23Mcps are two types of PRNcode used by GPS.

Several existing inter-satellite transceivers resemble the GNSS paradigm. They include the FFRF

sensor for PRISMA [2] and Proba-3 [3], the Autonomous Formation Flying (AFF) sensor for ST-3[4], the Constellation Communications and Navigation Transceiver (CCNT) for ST-5[5], IRAS forMMS [8], and the Star Ranger for Techsat-21[25].

It is a fact that GPS-like signal can be used for inter-satellite ranging. However, other GNSS codewaveforms exist, such as the signals used for Galileo [26]. The GPS-like signal structure usesBPSK-R (Binary Phase Shift Keying with rectangular spreading symbols) modulation, while theGalileo-like signal uses a Binary Offset Carrier (BOC(a,b)) structure, which offers significantchanges in time and spectral domain. The BOC modulation is a square sub-carrier modulation,where a signal is multiplied by a rectangular sub-carrier of frequency f sc (f sc= a*1.023Mcps ) equalto or higher than the chip rate f c ( f c= b*1.023Mcps ).

Fig. 6 shows the properties of C/A code, P code and BOC code in terms of auto-correlation andamplitude spectrum. The BOC signal has a sharper auto-correlation peak than the C/A code whenthey have the same chip rate. In addition, the C/A and P code place most of the signal power in themiddle frequencies of their bands, while BOC signals split the spectrum into two symmetricallobes. These properties make BOC signals have a better ranging performance in many aspects, suchas ranging accuracy, multipath effect, and interference mitigation effect.

Fig 6. Normalized auto-correlation and amplitude spectrum properties of C/A, P and BOC code



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Ranging Accuracy

Regarding the GNSS-like method, the ranging accuracy is significantly influenced by the selectionof the ranging code in several aspects: chip rate, code length and code modulation. The higher thechip rate, the wider the signal bandwidth (e.g. the spectral main lobe of the P code is ten timeswider than that of the C/A code). The sharper auto-correlation peak results in smaller multipath and

a more accurate estimate of the signal arrival time. Regarding the code length, when we use alonger code, we get a lower cross-correlation and less interference among the signals transmitted bythe various spacecraft [27].

Besides the influence of the signal structure, the ranging accuracy is determined by the type ofrange measurements. Basically, two available kinds of measurements (code/pseudorange and carrier

phase) are biased. The main error contributors are atmospheric errors, clock offset, multipath andreceiver noise (mainly thermal noise). For formation flying missions in LEO, ionospheric patherrors dominate the atmospheric errors, but fortunately ionospheric models and dual-frequencymeasurements can be considered to reduce these errors. Dual one-way ranging can be used forcancellation of clock offset. In addition, differential measurements take advantage of the fact thatthe errors associated with the satellite clock and atmospheric propagation are similar betweenreceivers or time epochs, which can be used to improve ranging accuracy. By combining code andcarrier phase measurements (e.g. carrier-smoothing of the code measurement), the code multipathand receiver noise can also be smoothed to a large extent.

Signal Acquisition and Tracking

The acquisition and tracking for a GNSS-like signal proceeds in two stages. The first stage is aglobal search over frequency and code in a two dimensional search space for approximate values ofDoppler shift and pseudorange. This process, known as signal acquisition, requires long integrationtimes to get sufficiently high carrier to noise ratio ( C/N 0). Therefore this process is time and

processing consuming. Fast Fourier Transform (FFT) can be used to create a one-dimensionalsearch space, but at the cost of having to perform FFT.

The second stage is a local search for accurate estimates of pseudorange and carrier phase. This process is called signal tracking because it is continuous and the estimates are updated as thesatellites move. It is accomplished as a feedback control loop, called a delay lock loop (DLL),which continuously adjusts the replica code to keep it aligned with the received code in theincoming signal. The integration time can be shorter since the C/N 0 does not need to be very highanymore. Another feedback control loop, the phase lock loop (PLL), is used to track the carrier

phase for highly accurate positioning. Doppler shift or delta pseudorange can also be measured viathe PLL.

5. CONCLUSION

This paper presents various inter-satellite communication and ranging technologies. To this end,important communication system design drivers are analysed in the lower three layers of the OSIlayer-based communication protocol, and several inter-satellite ranging methods are summarized.The current analysis is based on the general mission requirements for small satellites. Future workwill follow for a specific mission scenario, especially in the field of autonomous on-boardnavigation with high ranging accuracy requirements. The functionality of the future inter-satellitesensor will be augmented to enable relative navigation and formation control, common time, andcommunication in one package for advanced on-board autonomy.



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