FOTON: A Software-Defined, Compact, Low-Cost GPS Radio Occultation Sensor

Satellite Design LabAerospace Engineering

FOTON: A Software-Defined, Compact, Low-Cost GPS Radio Occultation Sensor

Glenn Lightsey and Todd Humphreys, UT Austin Aerospace Dept.

GEOScan Planning Workshop | March 27-30, 2011


Instrument/Sensor Specifications• Mass: 350 g• Power: 4.8 W• Volume: < 1 U• Data rate: 64 kbps (occulation mode), 2.6 kbps (standard)• Flight heritage or stage of development: Under development• Number of satellites required: at least 1• Accommodation requirements: antenna on anti-ram (possibly also

ram) facing surfaces• Expected data products: 100-Hz phase, TEC, S4, sigmaPhi, tau0• Data delivery and distribution: Data posted to central server• Expected results, contribution, broader impact: Prove the promise

of swarms of low-cost GPS occultation sensors for ionospheric and tropospheric science

• Cost: $10k - $50k per unit, depending on number of units

Instrument/Science Team• Main contact: Todd Humphreys, University of Texas at Austin

([email protected])• Collaborators:

• Glenn Lightsey, University of Texas at Austin• Mark Psiaki, Cornell• Steve Powell, Cornell• Chuck Swenson, USU• Chad Fish, SDL

• Sponsors/institutions/individuals with potential interest in funding development of FOTON

• US Air Force under existing SBIR contract• NASA Ames for constellation of cubesats

FOTON Sensor OverviewGrand Challenges

• Responsive, flexible occultation science via software-defined GPSRO sensor

• Exploit emerging technology to maximize science return from GPSRO sensors

• Signals: GPS L1CA and L2C • GPS radio occultation sensors are strongly synergistic with in-situ

electron density sensors, electric field sensors, etc.

Conceptual Design• FOTON• Software-defined space

weather sensor• High-sensitivity

occultation returns• Scintillation triggering• Data-bit wipeoff• Open-loop tracking• Recording of raw IF data


Q: What emerging technologies can be exploited to maximize the science impact of GNSS-based radio occultation over the next decade?

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



Miniaturization Proliferation Modernization Estimation

Smaller, less power-hungry GPSRO devices enable deployment: As hosted payload on larger SVs

(e.g., IridiumNext) On CubeSats

Shrinking Sensor envelope and cost allows ubiquitous space based sensor networks



Low cost enables larger constellations (10-100) of GPSRO-bearing SVs

Redundancy shifts from sensor to swarm Challenges posed by large numbers of

low-cost GPSRO sensors: Data rate (~300 kB per occulation) may

be too high for practical downlink sensors should be smart, do some preliminary processing onboard

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

Like COSMIC but at a fractionof the cost per GPSRO sensor



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 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, must make

up sensitivity 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 appears to be an unnecessary step: does not fully

summarize the information in the data Challenge: To ease data downlink burden, ionospheric

science parameters such as TEC, S4, tau0, sigmaPhi should be estimated on-orbit


Survey of GPSRO Receivers(Flight Qualified or Considered)

Chart adapted from Oliver Montenbruck, 2008; Pictures from Gupta, 2009.

Javad TR-G2T(Javad)

256 1 C1,P1,P2,LA,L2C,L5

1m 1.6 W34 g

? -35 C/+ 75 C

10 k$ ?

COTS receivers




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 Size: 8.3 x 9.6 x 3.8 cm Mass: 350 g Power: 4.8 W Reprogrammable from ground Dual frequency (L1CA, L2C) Software can be tailored for

occultation and space weather sensing: Scintillation triggering Open-loop tracking Recording of raw IF data Data bit wipeoff

Goal: Deliver high-end GPSRO benefits at low-end

Size/Weight/Power and Cost

Prototype FOTON receiverNow undergoing testing




Characteristic NovAtel OEMV-3 FOTON BRE Pyxis-RO

Flight Heritage Precursor OEM4-G2L flown on CanX-2

Plans for 2013-14 flight Precursor IGOR flown on CHAMP, GRACE, COSMIC

Size/Weight/Power 8.2 x 12.5 x 1.3 cm / 75 g / 2.1 W 8.3 x 9.6 x 3.8 / 350 g / 4.8 W 19 x 13.3 x 10 cm / 4.5 kg / 25 W

Cost < $10k $10-50k ~$500k

Signals Tracked/ Num. of channels

GPS L1CA, L2C, L2P(Y), L5 72 channels

GPS L1CA, L2C60 channels

GPS L1CA, L2C, L2P(Y), L548 – 128 channels

Radiation Hardness ~ 6krad ~5-10krad? (can be upgraded) 100 krad?

Time to First Fix 2.25 min. for OEM4-G2L on CanX-2 with aiding scripts

10 seconds with appx. time ~14 min. for IGOR

Precision 0.5 mm carrier phase < 0.5 mm carrier phase < 0.5 mm carrier phase

Antenna Inputs 1 1-2 (2 antenna option increases SWAP)

4

On-orbit Reconfigurable?

Only baseband processor firmware

Completely reconfigurable downstream of ADC

Baseband processor firmware + extra space in FPGA (used to demonstrate L2C on IGOR)

Open-Loop Tracking?

Not natively. May be possible to drive open loop tracking via API.

Yes Yes

Raw L1/L2 IF data capture?

No Yes No

On-board orbit determination

No Yes Yes

Data-bit wipeoff for robust tracking?

No Yes No

On-board Estimation of Space Weather Products?

No S4, TEC, sigmaPhi, tau0, SPR No


Commercialization Path for FOTON Startup Company Created

in Austin for licensing and commercialization of university space technology

Air Force SBIR Phase 1 Awarded (2/11-11/11)

SBIR Phase 2 (if awarded) 2012-2014

FOTON GPSRO CubeSat on-orbit demonstration planned in 2013-2014

FOTON will be ready for selection as a GEOScan payload on IridiumNext




Concern: Our experience with Iridum interference at two Antarctic stations

indicates that this may be a more serious problem for Iridium-hosted GPSRO than earlier

studies suggest.




More Information

http://radionavlab.ae.utexas.edu




Backup Slides




A Closer Look: NovAtel OEMV-3 High-quality device, proven

manufacturer OEM4-G2L flew on CanX-2 CanX-2 adaptations:

Disable altitude and velocity restrictions Upload startup scripts to speed acquisition Set sampling rate to 100 Hz Set elevation mask to -45 deg Reduce carrier phase smoothing of code

measurements

Characteristic ValuePower 2.1 WMass 75 gSize 85x125x13 mmSignals L1, L2,L2C,L5Meas. rate 100 Hz



FOTON: A Software-Defined, Compact, Low-Cost GPS Radio Occultation Sensor

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