Presentation on theme: "CubeSat-Sized GNSS Radio Occultation Experiments"— Presentation transcript:
1 CubeSat-Sized GNSS Radio Occultation Experiments Todd Humphreys, UT Austin Aerospace Dept.MIT Enrichment Lecture | December 1, 2011
2 AcknowledgementsRadionavigation Lab graduate students Daniel Shepard and Jahshan Bhatti: FOTON receiver developmentGlenn Lightsey (UT): Director of UT Satellite Design Lab, FOTON collaboratorSteve Powell, Brady O’Hanlon, Mark Psiaki (Cornell): FOTON collaboratorsOliver Montenbruck (DLR): Shared latest results and thinking on COTS GPSRO/PODRebecca Bishop (Aerospace Corp.): Shared performance results of CTECS instrument
3 UT Satellite Design Lab Space Inspires Students toPursue STEM CareersProject EngineeringComplements Coursework‘Real-Life Simulation’Science and Engineering ApplicationsTechnology DevelopmentInnovationRelative CostTraining and RecruitingPrestigeThis is not a watered down program-we want hard problems!
7 A Literary ExperimentQ: 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 CrustalDeformation: Precision and AccuracyPrescott et al., Science 16 June 1989: Initial Results of Radio Occultation Observations of Earth's Atmosphere Using the Global Positioning SystemKursinski et al., Science 23 February 1996: Present-Day Crustal Deformation in China Constrained byGlobal Positioning System MeasurementsWang et al., Science 19 October 2001:
8 Figure credit: International Radio Occultation Working Group
9 Scientific advance is often driven by instrumentationQ: How can GNSSRO deliver better science?A:Deeper soundings (to surface if possible)More soundingsLower latencyRicher measurements(e.g., finer time resolution, finer resolution of correlation function)Q: What emerging technologies can be exploited to meet these needs?
11 Redundancy shifts from sensor to swarm MiniaturizationProliferationModernizationEstimationShrinking sensor envelope and cost allows space based sensor networks, e.g., consider a constellation of GNSSRO-bearing SVsRedundancy shifts from sensor to swarmChallenges posed by large numbers of low-cost GPSRO sensors:Data rate: ~300 kB per occulation x 300 occultations per day = 90 MBOccultation capture cannot be orchestrated from the ground sensors must be autonomousLow cost implies some radiation hardness sacrificeLow cost implies less rigorous pre-flight qualification testing of each unitCOSMIC:6 GPSRO spacecraft
12 GPS L2C offers a crucial unencrypted second civil signal MiniaturizationProliferationModernizationEstimationGPS L2C offers a crucial unencrypted second civil signalAllows tracking of occultations deeper into troposphere9 L2C-capable SVs now in orbit20 L2C-capable SVs by 2015GPS L1 C/A + L2C most promising signal combination for occultations over next decadeGPS L5 and Galileo signalsAlso promising after ~2018P(Y) code may be discontinued after 2021Software-defined GNSSRO receivers offer complete on-orbit reprogrammabilityReduces operational riskEnables 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 MiniaturizationProliferationModernizationEstimationChallenge: Need good measurement quality despite low-cost and small size of GNSSRO sensorsClimate science requires accurate, consistent measurementsIf 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 tropospherePhase 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 occultationsAbel 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 limitsWiden Doppler range to +/- 40 kHzSuppress clock fixup during occultationMore correlator taps (e.g., 10 vs. 3)Open-loop tracking:Excess Doppler modeling100-Hz I,Q, and phaseSwitching between OL and CL trackingData 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 TorontoLaunched April 2008Special modifications to COTS Rx:None besides releasing ITAR altitude and speed limits!Performance:Powered onDelivered ~30-m-accurate position fixesDelivered raw dual-frequency measurementsTime to first fix: 2-12 minutesC/N0 values were ~10 dB lower than expected, probably due to EMI.Low signal qualityQuirks with tracking channel assignmentOccultation profiles not demonstratedAn important step forward despite the problemsNovAtel OEM4-G2L
16 CTECS on PSSC2: First Successful Occultation Profiles on a CubeSat Aerospace Corp. (P. Straus, R. Bishop)Launched July 2011Special modifications to COTS Rx:Release ITAR altitude and speed limitsCooperation with NovAtel for firmware modsCustom antenna: dual patch antenna with 6-7 dBi gainPerformance:Obtained clean electron density profiles both night and dayCan identify Appelton anomaly on some occultationsConstrained downlink: A 4-hour TEC data set takes several days to downloadAttitude control more challenging than expected, though ultimately successfulCTECS not used for PNT on PSSC2C/N0 as high as 45 dB-HzDemonstrates CubeSat ionospheric soundingNovAtel Rx, Custom Antenna
17 GNSS Rx on ACES Experiment DLR, Astrium, GFZ, ESA (O. Montenbruck, A. Helm)Projected launch ~2013Special modifications to COTS Rx:Close collaboration with Javad engineersBoth hardware and firmware modsSeparate receiver interface board for SEL, SEU protectionFirmware modified to enable open-loop tracking via commands from separate processorACES mission required “full-featured” radiation testing: well above cost of some entire CubeSat mission budgetsPerformance:220 channels: GPS, Galileo/Giove, GLONASSC/N0 expected to be ~10 dB lower than CHAMP; will limit tropospheric penetration depthOL functionality will seek to improve tropo penetrationModified Javad RxAppears 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.
20 Antarctic Version of CASES Deployed late 2010Remotely reprogrammable via IridiumAutomatically triggers and buffers high-rate data output during intervals of scintillationCalculates 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 WAll processing downstream of ADC reprogrammable from groundDual frequency (L1 C/A, L2C)Software is tailored for occultation and space weather sensing:Scintillation triggeringOpen-loop trackingRecording of raw IF dataData bit wipeoff for improved CL trackingCommercial provider: Austin Satellite DesignFOTON receiverApproaches performance of legacy GPSROUT 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 antennasCubeSat surface area economy suggests single, wide FOV antenna for both occultations and PODLoss of 5-10 dB C/N0 for occulting SV compared to CHAMP helical antenna & TerraSAR-X phased array antennaDegraded 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 plentyTropospheric weather: Utility beyond 12 COSMIC II is questionable except for monitoring cyclonesSpace 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 deeptropospheric penetration depth.Diminishing returns: horizontal resolution vs. number of GNSSRO SVsFigure 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.
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