cubesat -sized gnss radio occultation experiments

Satellite Design LabAerospace Engineering

CubeSat-Sized GNSS Radio OccultationExperiments

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 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

http://www.engr.utexas.edu/


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UT Radionavigation Laboratory




Radionavigation Lab Affiliation within UT

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Research Thrust Areas




A Literary Experiment

Present-Day Crustal Deformation in China Constrained byGlobal Positioning System Measurements Wang et al., Science 19 October 2001: 574-577.

Initial Results of Radio Occultation Observations of Earth's Atmosphere Using the Global Positioning SystemKursinski et al., Science 23 February 1996: 1107-1110.

Global Positioning System Measurements for Crustal Deformation: Precision and AccuracyPrescott et al., Science 16 June 1989: 1337-1340.

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”:



Satellite Design LabAerospace EngineeringFigure credit: International Radio Occultation Working Group


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?

Scientific advance is often driven by instrumentation


Miniaturization Proliferation Modernization Estimation

Smaller, cheaper, less power-hungry GPSRO devices enable small-satellite deployment

Blackjack/IGOR Receiver4 kg, 16-22 W, 24x10x20 cm~$1MSACC, GRACE, CHAMP,COSMIC, TERRA SAR-X

Pyxis Receiver<2 kg, 12-18 W 12x8x20 cm~$500kUnder development

COTS Receiver<200 g, < 1.5 W 120 cm^3$10-80kCANX-2 (2008)PSSC2 (2011)



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



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)



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


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




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

NovAtel OEM4-G2L

An important step forward despite the

problems




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

NovAtel Rx, Custom Antenna

Demonstrates CubeSat ionospheric

sounding




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




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




CASES Receiver (2011)




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


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

UT Armadillo CubeSat

Approaches performance of legacy

GPSRO




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?




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:




C/N0 vs. Tangent Point Altitude

Meehan et al., ION GNSS 2008




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.




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




How many GNSSRO needed?

Figure credit: T. Yunck

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




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.




More Information

http://radionavlab.ae.utexas.edu



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