COMMUNICATION SYSTEMS FOR CUBESAT MISSIONS

Anna CaseDepartment of Electrical and Computer Engineering

Missouri University of Science and TechnologyRolla, MO, 65401agc985@mst.edu

Faculty Advisor:Dr. Kurt Kosbar

ABSTRACT

Several design iterations of communication systems at the Missouri S&T Satellite Research Teamreveal that software defined radios (SDR) are viable for low cost, fully functional, and reliablecommunication systems. Recent licensing policy changes have impacted a number of CubeSatmissions, prompting the necessity of bandwidth efficient communication. In searching for solu-tions to minimize spectral congestion, these systems need to minimize power consumption andmaximize data throughput. The flexibility that SDRs provide allows for dynamic link control inorbit. Once completed, the code used to implement this system will be open-sourced for futuremissions use.

INTRODUCTION

Figure 1: Separation of MR SAT & MRS SAT

Missouri-Rolla Satellite (MR SAT) and Missouri-Rolla Second Satellite (MRS SAT) are smallsatellites to be launched in a paired configuration. The primary mission objective is autonomous

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visual proximity operations. This will be achieved through stereoscopic imaging and processing.In this endeavor, MR SAT will act as the inspector and MRS SAT will be an uncooperative residentspace object.

MR SAT will determine “his” relative position and velocity from these images, which are to beverified with guidance, navigation, and control (GNC) data. Post-processing at the base stationwill yield a 3-D reconstruction of MRS SAT. The secondary mission objective is to use a cold gaspropulsion system and verify its efficiency.

The communications system on-board MR SAT is vital to mission success, though the satellite cancomplete its mission completely autonomously. MRS SAT has a simplex Eyestar radio on-boardthat will regularly send “her” GPS data over the Globarstar network. These data are retrievedvia the Internet. MR SAT has on-board a Gomspace SDR that will communicate half-duplexwith the ground station at Missouri S&T. Uplink data may contain telecommand overrides andacknowledgements of mission data or requests for retransmission. Downlink data may includecompressed images, sensor, and GNC data.

CONCEPT OF OPERATIONS

Figure 2 shows the progression of the mission throughout the satellite lifetime. After deploymentfrom the International Space Station (ISS) via the Cyclops robotic arm system, the satellite willperform initialization: charging batteries, turning on boards and sensors in a certain order, per-forming a checkout procedure on the hardware, and executing the GNC algorithm. A small periodof radio silence is required, after which the satellite beacon will begin.

Figure 2: Concept of Operations

Detumble is a mission mode in which the attitude of the satellite is corrected and maintained. GPSprovides the position of the satellite and a magnetometer and Sun sensors provide attitude. The

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satellite is then autonomously oriented so that the stereoscopic imaging cameras are pointed in thegeneral velocity direction using three torque coils and twelve thrusters.

Checkout mode begins after detumble or after any system health check failure. In this mode, torquecoils are throttled to ensure the mode remains power positive. If for some reason the satellite isnot power positive or a thermal sensor trips, safe mode will be entered. Unless detumble needsto reoccur, once battery charge is above 90% the satellite separation will occur. There is only oneopportunity to conduct proximity operations due to the differential drag between MR and MRSSAT, as the two spacecraft will quickly drift apart without immediate application of active control.Thus, if sufficient charge is not maintained, the satellite will reenter checkout mode. Otherwise,the propulsion system and accompanying thermal control will be initialized. Pressure and temper-ature data will be saved for analysis on the ground. The stereoscopic imaging system is initializedand a final health check is performed. Once passed, MRS SAT is ejected from MR SAT.

There are two means of proximity operations in which the cameras on MR SAT will take imagepairs every three seconds. The image processing algorithm is still being tuned, and this may besped up. These processed images will feed data to the GNC algorithm. First, MR SAT will main-tain a ten meter trailing formation for up to forty five minutes, or until a guidance solution hasconverged. Immediately afterwards, MR SAT will begin circumnavigation of MRS SAT. This willcontinue until MR SAT is out of propellant or has acquired sufficient data.

The final task of MR SAT is to downlink all imaging data. Though compressed, this may take themajority of the mission lifetime. Overhead passes are fairly regular, but vary in length. Addition-ally, image sizes and channel conditions are highly dynamic. Once mission critical data have beendownlinked, the satellite will enter end of life procedures to minimize orbital debris. For as longas an orbit is maintained, MR SAT will continue transmitting a beacon signal.

LICENSING

There are a number of coordinating agencies that may be necessary to file applications or ex-emptions with in order to secure a launch. First, about a dozen team members and the principalinvestigator hold their amateur radio license, which gives them access to ground station opera-tions. For this mission, an imaging license was acquired through the Commercial Remote SensingRegulatory Affairs Office of NOAA [1], along with an encryption waiver. As the team aims to belicensed in the amateur space service, only telecommands may be encrypted.

When designed, MR SAT was to be capable of full-duplex over the UHF and VHF bands accessibleto amateurs. However, recent policy changes have eliminated new licensing in the VHF amateursatellite band and restricted coordination in the UHF amateur satellite service [2]. These changesstem from ITU decisions made at WRC-15 and are now adopted by the IARU. Unfortunately,similar policy changes are likely to continue despite best efforts. The number of countries andorganizations launching CubeSats continues to rise, particularly those with commercial endeavors.To date, most amateur satellites use CW or the AX.25 protocol common among HAMs, and send

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at 9600 baud. FSK is the most common modulation [3]. One objective is to implement spectrallyefficient methods and continue to improve upon them.

The path forward will be to acquire a coordination letter through the International Amateur RadioUnion (IARU) by proving Amateur intent: immense educational value and training provided to theMissouri S&T campus related to radio technique. Next, an experimental license is required throughthe FCC. The FCC will not start this process before IARU coordination, nor is it FCC policyto ”directs in writing [to] coordinate a non-amateur satellite.” Next, Advance Public Information(API/C) will be given to the ITU, through the FCC, using the proprietary software SpaceCap. Afterlicensing is acquired, the team needs only to obey relevant rules and follow through with regularauditing.

PHYSICAL LAYER

The ground station is composed of a server, an Ettus B200 SDR running custom C++ code, an an-tenna rotor system accurate to within 5◦, a preamplifier, amplifier, and a RHCP Yagi antenna. Theantenna has eight elements with a boom length of 1.74 m and a beamwidth of 42◦. The antennatracking system is implemented on the ground station server using a custom PyEphem implemen-tation. CubeSats are well known for failed communications after launch. Thus, the functionalityhas been verified via regular tracking of the ISS, which is presented as an outreach opportunity. Acamera is pointed towards the antenna, located on the lab’s roof, for visual confirmation. Unfortu-nately, the noise floor near the system is increased due to the presence of HVAC equipment.

The satellite communications system is composed of a Gomspace AX100U radio, two in-line con-nectors, and a 105◦beamwidth monopole antenna grounded to the satellite structure. It is tunedon campus using a spectrum analyzer. Unfortunately, wiring of the satellite requires a fairly long(nine-quarters of a wavelength) RG-316 cable to connect radio and antenna. This cabling was se-lected due to its low outgassing properties. Further details are shown in table 1.

The satellite is in the process of being licensed, and the team aims to be coordinated in the UHFamateur satellite service, 430-440 MHz. During testing, the center frequency used is 437.5 MHz.The radio link uses Gaussian Minimum Shift Keying (GMSK) modulation, a variant of frequencyshift keying. The data stream is shaped with a Gaussian filter to minimize sidebands. Then, thedata stream goes through a frequency modulator. GSMK has high spectral efficiency and hasbeen recommended by contacts at the IARU. The radio will adaptively calculate the modulationindex, which will be advantageous to the system when Doppler shift affects the signal. As thismay slightly increase the noise floor, the receiver sensitivity will be slightly lowered. A phaselocked loop (PLL) is used for frequency control. As the same frequency will be used for uplinkand downlink, the time to reconfigure the PLL will be minimized. The data rate used during themission will be either 9600 baud or 19.2 kbaud depending on the bandwidth usage, bit error rate,and signal to noise ratio.

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Table 1: System Link BudgetGround Station Uplink Spacecraft Downlink

Power Output 50 W 1 WTransmission Line Losses 1.7 dB 0.7 dB

Antenna Gain 13.3 dBi 2.2 dBiEIRP 28.6 dBW 1.5 dBW

Uplink Path Downlink PathAntenna Pointing Loss 0.2 dB 4.7 dB

Polarization Loss 0.1 dB 0.1 dBPath Loss 148.5 dB 148.5 dB

Atmospheric Loss 1.1 dB 1.1 dBIonospheric Loss 0.4 dB 0.4 dB

Rain Loss 0.0 dB 0.0 dBIsotropic Loss at S/C -121.6 dBW -153.3 dBW

Spacecraft Ground StationAntenna Pointing Loss 4.7 dB 0.2 dB

Antenna Gain 2.2 dBi 13.3 dBiTransmission Line Losses 0.4 dB 0.5 dB

Effective Noise Temperature 137 K 620 KFigure of Merit (G/T) -19.6 dB/K -15.2 dB/K

Eb/No MethodSignal to Noise Power Density 82.7 dBHz 60 dBHz

Data Rate 42.8 dBHz 42.8 dBHzCommand System Eb/No 39.9 dB 17.2 dB

Demodulator Implementation Loss 1 dB 1 dBRequired Eb/No 9.6 dB 9.6 dBEb/No Threshold 10.6 dB 10.6 dB

System Link Margin 29.3 dB 6.6 dBSNR Method

Signal Power at LNA Input -124.5 dBW -140.5 dBWReceiver Bandwidth 25 kHz 25 kHz

Receiver Noise Power -163.3 dBW -156.7 dBWSignal-to-Noise Power Ratio at G/S Receiver 38.7 dB 16 dB

Required S/N 10.6 dB 10.6 dBSystem Link Margin 28.1 dB 5.4 dB

DATA LINK LAYER

The process for data transmission requires that the flight computer streams packets via a socketto a buffer, where they will be kept in a buffer until an overhead pass occurs. This socket thenconnects to the radio using the CubeSat Protocol (CSP) [4]. After framing, the radio implementsseveral measures to protect the integrity of the data, which are implemented on top of the framing.

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Frame assembly compiles a group of packets for transmission. A transmission begins with a shortsynchronization sequence to ensure frequency and phase lock at the ground station. There are alsointer-frame synchronization sequences. ASM (Attached Sync Marker) is the framing protocol ofchoice. It is considered to have a good synchronization, yet is not as sensitive as a Viterbi decoder.The first portion of the frame is a static, four byte ASM word which marks the beginning of aframe. Due to the lengthy synchronization, the MAC layer can use this information in determiningif the receiver is busy. ASM is meant to use NRZ and big-endian. Then follows three bytes ofGOLAY, meant to provide forward error correction to the length byte. GOLAY provides twelvebits for length, eleven bits error correction, and one parity bit. This can correct up to three bit errorsin the GOLAY field, however, some additional processing time is required to verify the GOLAYfield. The last part of the frame is the data field, of variable length. The maximum length of thedata field is 240 bytes.

Figure 3: Amplitude vs. Time; Top: Received Waveform; Bottom: Symbol Recovery

Overhead is slightly increased when more protocols are put in to place. For instance, Reed-Solomon (255,223) FEC performed on the data field costs sixteen bytes per frame. Hash Mes-sage Authentication Code (HMAC) costs ensures only authorized users may issue a telecommandand costs four bytes per frame. The Consultative Committee for Space Data Systems (CCSDS)publishes a number of recommended standards and practices [5]. CCSDS randomization does nothave a byte cost and is widely recommended. Rectangular interleaving will help the system recoverfrom burst errors. It should be noted that all checksums and error corrections are implemented bysoftware. When implementing Reed-Solomon, it is important to virtually fill short frames; thesebits are not transmitted, but are zero padded to the beginning of any short frame. Interestingly,when set to the ASM + GOLAY mode, the AX100 SDR uses an assortment of five types of scram-bling. Additionally, the endianness swaps in every portion of the frame. Determining this through”reverse engineering” was the longest portion of software development.

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DATA BUDGET & PROCESSING

An overhead pass can last anywhere from 10 to 390 seconds when above a 10◦ elevation angle.Use of the AMSAT link budget calculator [6], as well as a simulated communication test at range,support that the satellite will have a suitable margin at this elevation angle. Extrapolating fromthese data, the team has indexed what the expected transmissions will be and used ISS overheadpass data to estimate the length of the downlink phase of the mission. These data are shown intable 2 and table 3.

Table 2: Uplink and Downlink Frame ListFrame Type Length (Bytes) Frame Contents

Downlink Engineering Data 14 Engineering data, described belowDownlink GPS State Vector 161 All parts of the GNC state vector, described below

Downlink Imaging Data 240 Image bits, separated by at intervalsUplink Acknoledgement 1 Instruction for the radio to continue transmission

Uplink End of Life Telecommand 1 Satellite begins end-of-life proceduresUplink Reset Subsystem 1 Resets a subsystem

Uplink Reset Radio 1 Resets radio to safety or start parametersUplink Separation Telecommand 1 Gives the spacecraft the signal to go into separation

Anywhere from 127 to 3850 frames constitute an image, due to the potential variance in croppingthe background of an image. Though image metadata is stripped, some is necessary for identifi-cation purposes. Sequential image packets will be numbered. The number of packets in an imagewill be downlinked, as well as the image number. End of life procedures are meant to minimize or-bital debris. It will be necessary to open all thrusters to expel any remaining propellant. However,this will require ground control to ensure the orbit is not significantly altered.

Table 3: Engineering Data: Basic Satellite TelemetryData Data Type Length (Bytes) Description

Spacecraft Header string 4 Transmitting callsigns and timeBoard Status Boolean 1 2x Cameras, COM, GPS, 2x Sun Sensors, Analog and Digital MCUCamera Store Boolean 0.125 Using angles, has the camera stored data?

MRS SAT Boolean 0.125 Can MR SAT see MRS SAT?Temperature Data Boolean 3.125 25 thermal sensors, detects if overheating

Current Mode uint8 t 1 Current mission modePower Status (DOD) uint8 t 1 Percentage chargePressure Transducer uint8 t 2 Data from the propulsion system’s pressure transducers

Solenoid States Boolean 0.375 On/off for three solenoid (isolation valves)Torque Coil Status Boolean 0.375 Three values received from ADAC MCU

Status of TiNi Boolean 0.75 True for six continuity values from power

Transmissions are acknowledged once per second; this both ensures adequate SNR is achieved andallows an opportunity for the ground station to request retransmission. This process is automated,though a licensed member must be present to issue any telecommand. Once an acknowledgementis received, the radio buffer is cleared. These crucial steps are still being tested in real time.Downlinked data will be stored indefinitely on the encrypted ground station computer. CustomPython code writes the output (demodulated) data to a spreadsheet. The data are then split intoconstituent parts and aggregated into groups; transmitted identifiers (computer, identifier, sensor,

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Table 4: GNC State Vector Table: Internal GNC TelemetryData Data Type Length (Bytes) Description

Filter Health Boolean 0.125 Converged?MR SAT Position Double 24 GNC entries 1-3MR SAT Velocity Double 24 GNC entries 4-6

MR SAT Attitude Quaternion Double 32 Vector first, scalar second. GNC entries 7-10MRS SAT Relative Position Double 24 GNC entries 11-13MRS SAT Relative Velocity Double 24 GNC entries 14-16

MR SAT IMU Accelerometer Double 24 GNC entries 17-19GPS Pseudorange Double 8 GNC entry 20

etc.) will direct to the subsystem that requires the data. The data will then be sent via email tothe subsystem lead. Additionally, team members may request to receive notifications of whenoverhead passes will occur. Currently, the process has been started to join the SatNOGS groundstation network. This will allow the ground station on campus to contribute to data and telemetrycollection for other satellites as well as using the network resources in future missions.

CONCLUSIONS

It is apparent that advances in CubeSat communications are much needed to allow for an increasingnumber of satellites in orbit. Refined controls need to be implemented in order to use the limitedspectrum efficiently. While all of these requirements rely on advancing SDR technology, withoutstandard convergence it is unlikely that these communication systems will be sufficiently flexible.Additionally, the difficulty in doing so should be minimized in cost and integration time.

Lessons learned on the M-SAT team will allow for more cost-effective, evolvable, and flexiblecommunication implementations in the future. By beginning the transition away from COTS radiohardware, the team will have more control over radio functionality and will not be reliant on a sin-gle manufacturer. A significant code base is being built, with plans to open source the majority ofthe implementation for others in the community. In an effort to alleviate this, the team aims to in-spire a collaborative and educational environment. Many universities may turn to the costly optionof using a satellite network for their communication needs, but flexibility and mission criteria mayrequire custom engineering. This mission is fortunate in that the entire communication system hasbeen redesigned in the last year, allowing for a number of beneficial changes to occur and creatinga more robust system. Not only have students at Missouri S&T joined the amateur community andgained valuable experience, but this trend will continue through community outreach.

ACKNOWLEDGEMENT

My thanks goes out to Dr. Kosbar, who advised me on this project. I would also like to thankDr. Pernicka, the Principal Investigator of the Missouri S&T Satellite Research Team and theengineers at Helical Communication Technologies, who continue to mentor students in regards toRF hardware. My acknowledgement goes to Bryan Klofas, whose numerous contributions andpublications on small satellite communications have guided me throughout this process.

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REFERENCES

[1] NOAA, “NOAA CRSRA Licensing Program.” https://www.nesdis.noaa.gov/CRSRA/licenseHome.html, June 2017.

[2] IARU, “IARU Aligns Satellite Coordination Guidelines with ITU WRC-15 De-cisions.” http://www.iaru.org/news–events/iaru-aligns-satellite-coordination-guidelines-with-itu-wrc-15-decisions, June 2017.

[3] B. Klofas, “Cubesat communication systems: 2003-2017.” https://www.klofas.com/comm-table/table.pdf, Apr. 2018.

[4] KubOS, “CubeSat Protocol.” http://docs.kubos.co/0.0.1/libcsp/index.html.

[5] CCSDS, “CCSDS Telemetry Channel Coding.” http://webapp1.dlib.indiana.edu/virtual disklibrary/index.cgi/4278621/FID2960/CCSDS/101x0b4.pdf, Jan. 1999.

[6] J. King, “AMSAT Link Budget Calculator.” http://www.amsatuk.me.uk/iaru/spreadsheet.htm,Jan. 2013.

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