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

2. Acknowledgements
Radionavigation Lab graduate students Daniel Shepard and Jahshan Bhatti: 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: 
Initial Results of Radio Occultation Observations of Earth's Atmosphere Using the Global Positioning System
Kursinski et al., Science 23 February 1996:   
Present-Day Crustal Deformation in China Constrained by
Global Positioning System Measurements
Wang et al., Science 19 October 2001:   

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, W
12x8x20 cm
~$500k
Under development
Blackjack/IGOR Receiver
4 kg, W, 24x10x20 cm
~$1M
SACC, GRACE, CHAMP,
COSMIC, TERRA SAR-X

11. Redundancy shifts from sensor to swarm
Miniaturization
Proliferation
Modernization
Estimation
Shrinking sensor envelope and cost allows space based sensor networks, e.g., consider a constellation of 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. GPS L2C offers a crucial unencrypted second civil signal
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 correlator fcn.
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
Phase measurements must be 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: To ease data downlink burden, ionospheric science parameters 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