Foton a software defined compact low cost gps radio occultation sensor
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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

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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

    • 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 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.

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


  • 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

  • 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

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

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