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CubeSat -Sized GNSS Radio Occultation Experiments

CubeSat -Sized GNSS Radio Occultation Experiments. Todd Humphreys, UT Austin Aerospace Dept. MIT Enrichment Lecture | December 1, 2011. Acknowledgements. Radionavigation Lab graduate students Daniel Shepard and Jahshan Bhatti : FOTON receiver development

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CubeSat -Sized GNSS Radio Occultation Experiments

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  1. CubeSat-Sized GNSS Radio OccultationExperiments Todd Humphreys, UT Austin Aerospace Dept. MIT Enrichment Lecture | December 1, 2011

  2. Acknowledgements • Radionavigation Lab graduate students Daniel Shepardand JahshanBhatti: FOTON receiver development • Glenn Lightsey(UT): Director of UT Satellite Design Lab, FOTON collaborator • Steve Powell, Brady O’Hanlon, Mark Psiaki(Cornell): FOTON collaborators • Oliver Montenbruck(DLR): Shared latest results and thinking on COTS GPSRO/POD • Rebecca Bishop (Aerospace Corp.): Shared performance results of CTECS instrument

  3. UT Satellite Design Lab • Space Inspires Students to Pursue STEM Careers • Project Engineering • Complements Coursework • ‘Real-Life Simulation’ • Science and Engineering Applications • Technology Development • Innovation • Relative Cost • Training and Recruiting • Prestige • This is not a watered down program-we want hard problems!

  4. UT Radionavigation Laboratory

  5. Radionavigation Lab Affiliation within UT UT ECE Department UT Aerospace Department

  6. Research Thrust Areas

  7. A Literary Experiment Q: What GPS applications have caught the attention of the broader science community? A: Not many. Consider a search of Science article titles for “Global Positioning System”: Global Positioning System Measurements for Crustal Deformation: Precision and Accuracy Prescott et al., Science 16 June 1989: 1337-1340. Initial Results of Radio Occultation Observations of Earth's Atmosphere Using the Global Positioning System Kursinski et al., Science 23 February 1996: 1107-1110.   Present-Day Crustal Deformation in China Constrained by Global Positioning System Measurements Wang et al., Science 19 October 2001: 574-577. 

  8. Figure credit: International Radio Occultation Working Group

  9. Scientific advance is often driven by instrumentation • Q: How can GNSSRO deliver better science? • A: • Deeper soundings (to surface if possible) • More soundings • Lower latency • Richer measurements • (e.g., finer time resolution, finer resolution of correlation function) • Q: What emerging technologies can be exploited to meet these needs?

  10. Miniaturization Proliferation Modernization Estimation Smaller, cheaper, less power-hungry GPSRO devices enable small-satellite deployment COTS Receiver <200 g, < 1.5 W 120 cm^3 $10-80k CANX-2 (2008) PSSC2 (2011) Pyxis Receiver <2 kg, 12-18 W 12x8x20 cm ~$500k Under development Blackjack/IGOR Receiver 4 kg, 16-22 W, 24x10x20 cm ~$1M SACC, GRACE, CHAMP, COSMIC, TERRA SAR-X

  11. Miniaturization Proliferation Modernization Estimation • Shrinking sensor envelope and cost allows space based sensor networks, e.g., consider a constellation of 10-100 GNSSRO-bearing SVs • Redundancy shifts from sensor to swarm • Challenges posed by large numbers of low-cost GPSRO sensors: • Data rate: ~300 kB per occulation x 300 occultations per day = 90 MB • Occultation capture cannot be orchestrated from the ground  sensors must be autonomous • Low cost implies some radiation hardness sacrifice • Low cost implies less rigorous pre-flight qualification testing of each unit COSMIC: 6 GPSRO spacecraft

  12. Miniaturization Proliferation Modernization Estimation • GPS L2C offers a crucial unencrypted second civil signal • Allows tracking of occultations deeper into troposphere • 9 L2C-capable SVs now in orbit • 20 L2C-capable SVs by 2015 • GPS L1 C/A + L2C most promising signal combination for occultations over next decade • GPS L5 and Galileo signals • Also promising after ~2018 • P(Y) code may be discontinued after 2021 • Software-defined GNSSRO receivers offer complete on-orbit reprogrammability • Reduces operational risk • Enables on-orbit innovation: e.g., add correlator taps as needed to refine resolution of correlatorfcn. • Allows adaptation to science needs/events (Fig. 1 of Wallner et al., "Interference Computations Between GPS and Galileo," Proc. ION GNSS 2005)

  13. Miniaturization Proliferation Modernization Estimation • Challenge: Need good measurement quality despite low-cost and small size of GNSSRO sensors • Climate science requires accurate, consistent measurements • If large, high-gain antennas can’t be accommodated, can sensitivity loss be compensated in signal processing? • Specialized open-loop tracking required to push deep into troposphere • Phasemeasurementsmustbe CDGPS-ready to enable precise orbit determination (Topstar receiver by Alcatel fails this req’t) • Challenge: Atmospheric assimilative models should be modified to ingest raw carrier phase and TEC measurements from occultations • Abel transform an unnecessary step? • Challenge: Toease data downlinkburden, ionospheric scienceparameters such as TEC, S4, tau0, sigmaPhi should be estimated on-orbit

  14. Special GNSS RX Adaptations Needed for GNSSRO • Release ITAR altitude and speed limits • Widen Doppler range to +/- 40 kHz • Suppress clock fixup during occultation • More correlator taps (e.g., 10 vs. 3) • Open-loop tracking: • Excess Doppler modeling • 100-Hz I,Q, and phase • Switching between OL and CL tracking • Data bit prediction (improves reliability of after-the-fact profile processing) • Occultation prediction

  15. CANX-2: First Geodetic RX on a CubeSat • U Calgary (S. Skone); U Toronto • Launched April 2008 • Special modifications to COTS Rx: • None besides releasing ITAR altitude and speed limits! • Performance: • Powered on • Delivered ~30-m-accurate position fixes • Delivered raw dual-frequency measurements • Time to first fix: 2-12 minutes • C/N0 values were ~10 dB lower than expected, probably due to EMI. • Low signal quality • Quirks with tracking channel assignment • Occultation profiles not demonstrated An important step forward despite the problems NovAtel OEM4-G2L

  16. CTECS on PSSC2: First Successful Occultation Profiles on a CubeSat • Aerospace Corp. (P. Straus, R. Bishop) • Launched July 2011 • Special modifications to COTS Rx: • Release ITAR altitude and speed limits • Cooperation with NovAtel for firmware mods • Custom antenna: dual patch antenna with 6-7 dBi gain • Performance: • Obtained clean electron density profiles both night and day • Can identify Appelton anomaly on some occultations • Constrained downlink: A 4-hour TEC data set takes several days to download • Attitude control more challenging than expected, though ultimately successful • CTECS not used for PNT on PSSC2 • C/N0 as high as 45 dB-Hz Demonstrates CubeSat ionospheric sounding NovAtel Rx, Custom Antenna

  17. GNSS Rx on ACES Experiment • DLR, Astrium, GFZ, ESA (O. Montenbruck, A. Helm) • Projected launch ~2013 • Special modifications to COTS Rx: • Close collaboration with Javad engineers • Both hardware and firmware mods • Separate receiver interface board for SEL, SEU protection • Firmware modified to enable open-loop tracking via commands from separate processor • ACES mission required “full-featured” radiation testing: well above cost of some entire CubeSat mission budgets • Performance: • 220 channels: GPS, Galileo/Giove, GLONASS • C/N0 expected to be ~10 dB lower than CHAMP; will limit tropospheric penetration depth • OL functionality will seek to improve tropo penetration Modified Javad Rx Appears to be closing the gap with legacy GPSRO

  18. Since 2008, The University of Texas, Cornell, and ASTRA LLC have been developing a dual-frequency, software-defined, embeddable GPS-based space-weather sensor.

  19. CASES Receiver (2011)

  20. Antarctic Version of CASES • Deployed late 2010 • Remotely reprogrammable via Iridium • Automatically triggers and buffers high-rate data output during intervals of scintillation • Calculates S4, tau0, sigmaPhi, SPR, TEC

  21. CASES Follow-On: FOTON GPSRO • University of Texas, Cornell U. (T. Humphreys, G. Lightsey, S. Powell, M. Psiaki) • Projected launch: Sounding rocket in March 2012, CubeSat in 2013. • Size: 8.3 x 9.6 x 3.8 cm, Mass: 330 g, Power: 4.7 W • All processing downstream of ADC reprogrammable from ground • Dual frequency (L1 C/A, L2C) • Software is tailored for occultation and space weather sensing: • Scintillation triggering • Open-loop tracking • Recording of raw IF data • Data bit wipeoff for improved CL tracking • Commercial provider: Austin Satellite Design FOTON receiver Approaches performance of legacy GPSRO UT Armadillo CubeSat

  22. CubeSat enthusiast’s view: We’ll launch hundreds of CubeSats with low-cost but high-performance GNSSRO sensors! This will usher in a revolution in tropospheric and ionospheric situational awareness and forecasting! But those who know most about occultations (e.g., JPL, UCAR, GFZ, DLR) aren’t targeting CubeSat platforms. Why?

  23. Primary challenge in moving to smaller platforms: less space for high-gain antennas and multiple antennas • CubeSat surface area economy suggests single, wide FOV antenna for both occultations and POD • Loss of 5-10 dB C/N0 for occulting SV compared to CHAMP helical antenna & TerraSAR-X phased array antenna • Degraded C/N0. • Most efficient platform:

  24. C/N0 vs. Tangent Point Altitude Meehan et al., ION GNSS 2008

  25. The challenge of improving occultation C/N0 on CubeSats can be overcome if there is enough of an incentive. But incentive is linked to economics. We may never need enough CubeSat-sized occultation experiments to make it worthwhile economically to develop a high-performance, low-cost, low SWaP GNSSRO sensor.

  26. Key question for CubeSat GNSSRO: Do we really need continually-replenished swarms of 100s of GNSSRO sensors? • Climate science: 6 COSMIC and 12 COSMIC II will be plenty • Tropospheric weather: Utility beyond 12 COSMIC II is questionable except for monitoring cyclones • Space weather: Continuous, low-latency coverage with 100 SVs would be useful

  27. How many GNSSRO needed? Up to 100 GNSSRO SVs would be useful for extreme weather monitoring. But value is linked to deep tropospheric penetration depth. Diminishing returns: horizontal resolution vs. number of GNSSRO SVs Figure credit: T. Yunck

  28. Whereas it is clear that more occultations will benefit space weather observation and prediction (because of the rapid variability of the ionosphere), it is less clear that more “deep troposphere” occultations than those that will be provided by the proposed COSMIC II mission (12 SVs) would significantly improve tropospheric weather forecasting, except possibly in the case of cyclones. Thus, perhaps the emphasis of CubeSat GNSSRO should be limited to ionospheric sounding.

  29. More Information http://radionavlab.ae.utexas.edu

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