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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. FOTON Sensor Overview. Grand Challenges

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foton a software defined compact low cost gps radio occultation sensor

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


FOTON Sensor Overview

  • Grand 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.
  • 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 (
    • 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
  • 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?






  • 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 fraction

of 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
    • 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 appears to be an unnecessary step: does not fully summarize the information in the data
  • Challenge: Toease data downlinkburden, ionospheric scienceparameters such as TEC, S4, tau0, sigmaPhi should be estimated on-orbit
survey of gpsro receivers flight qualified or considered
Survey of GPSRO Receivers(Flight Qualified orConsidered)

COTS receivers

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


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

antarctic version of cases
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

Prototype FOTON receiver

Now undergoing testing

Goal: Deliver high-end GPSRO benefits at low-end Size/Weight/Power and Cost

commercialization path for foton
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
More Information

a closer look novatel oemv 3
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