BackForwardInstrument:  GRAS 

Instrument details
Acronym GRAS
Full name GNSS Receiver for Atmospheric Sounding
Purpose Atmospheric temperature/humidity sounding with highest vertical resolution. Also ionospheric total electron content along slant path.
Short description
  • Measuring the phase delay due to refraction during occultation between GPS and LEO
  • Monitoring and calibration information here
Background

Consolidated technology
Scanning Technique

Limb scanning from 80 km altitude to close-to-surface by time sampling – Azimuth: 90° sectors fore- and aft-
Resolution About 300 km horizontal, 0.5 km vertical
Coverage / Cycle 1 GNSS constellation tracked. About 650 soundings/day - Average spacing 880 km - Global coverage (300 km spacing) in 8.5 days
Mass 30 kg Power 30 W Data Rate 27 kbps

 

Providing Agency ESA
Instrument Maturity Flown on operational programme
Utilization Period: 2007 to 2027
Last update: 2024-09-23
Detailed characteristics
Satellites this instrument is flying on

Note: a red tag indicates satellites no longer operational, a green tag indicates operational satellites, a blue tag indicates future satellites

Instrument classification
  • Earth observation instrument
  • Active and radio-occultation sensor
  • GNSS radio-occultation
WIGOS Subcomponents
  • Subcomponent 1
  • GNSS radio occultation (basic constellation)
  • GNSS radio occultation sounder (basic constellation)
Mission objectives
Primary mission objectives
  • Atmospheric temperature
  • Height of the top of PBL
  • Height of the tropopause
  • Ionospheric Total Electron Content (TEC)
  • Specific humidity
  • Temperature of the tropopause
Evaluation of Measurements

The following list indicates which measurements can typically be retrieved from this category of instrument. To see a full Gap Analysis by Variable, click on the respective variable.

Note: table can be sorted by clicking on the column headers
Note: * Primary mission objective.
VariableRelevance for measuring this variableOperational limitationsExplanation
Atmospheric temperature*3 - highInaccurate in low troposphere.Two directional antennas, one GNSS system tracked, covering about 500 soundings/day
Geoid4 - fairHighly indirect.Radio-occultation processing implies precise orbitography. Geoid derived from multi-temporal analysis
Gravity field5 - marginalHighly indirect.Radio-occultation processing implies precise orbitography. Gravity field derived from multi-temporal analysis
Height of the top of PBL*3 - highNo specific limitation.Two directional antennas, one GNSS system tracked, about 500 soundings/day. PBL top measured as discontinuity of the refraction index
Height of the tropopause*3 - highNo specific limitation.Two directional antennas, one GNSS system tracked, about 500 soundings/day. Tropopause height measured as discontinuity of the refraction index
Specific humidity*3 - highInaccurate in high troposphere.Two directional antennas, one GNSS system tracked, about 500 soundings/day
Temperature of the tropopause*3 - highNo specific limitation.Two directional antennas, one GNSS system tracked, about 500 soundings/day
Atmospheric density1 - primaryNo specific limitation.Measuring atmospheric density
Electron Density1 - primaryNo specific limitation.Two directional antennas, one GNSS system tracked, about 500 soundings/day
Ionospheric Total Electron Content (TEC)*3 - highNo specific limitation.Two directional antennas, one GNSS system tracked, about 500 soundings/day
Ionospheric Scintillation4 - fairNo specific limitation.Two directional antennas, one GNSS system tracked, about 500 soundings/day