The main objectives of the GNSS (Global Navigation Satellite System) Receiver for Atmospheric Sounding (GRAS) are to provide stratospheric and tropospheric temperature and humidity profiles that can be assimilated into NWP models.
In addition, GRAS provides navigation solutions of the Metop satellite position along its orbit.
What is GRAS?
The GRAS instrument receives high quality radio signals from GPS navigation satellites, occulting the Earth atmospheric limb, through a tangential path through the Earth's atmosphere. As such, GRAS is not just an instrument, but requires a full system to provide products. GRAS provides a minimum of 500 atmospheric profiles per day by a process of GPS radio occultation.
GRAS needs a GPS constellation of satellites, and a precise orbit determination is required for the Metop satellite.
GRAS tracks the phase of GPS signals at Metop over an occultation interval. An occultation occurs whenever a GPS satellite rises or sets on the Earth limbs, as seen from the Low Earth orbit (LEO) satellite.
The GPS signal is refracted and slowed as it traverses the Earth's atmospheric limb. This causes a phase delay that relates to characteristics of the Earth's atmosphere. GRAS then compares the measured phase with the phase that would be expected in the absence of an atmosphere in order to derive bending angles.
The Doppler shift in the received signals can be processed to obtain vertical profiles (at least from 5 km to 30 km) of atmospheric parameters, such as temperature and pressure, with a high degree of accuracy.
In the stratosphere and upper troposphere, where water vapour density is low, refraction is dominated by the vertical temperature gradients, and an accurate temperature profile can be retrieved.
In the lower troposphere, the water vapour effects are dominant and the combined temperature/water vapour profile can be retrieved. Measurement data from GRAS will be combined with data received in GPS ground based receivers (providing Precise Orbit Determination) to retrieve the final atmospheric profiles.
The nominal GPS constellation consists of 24 satellites distributed in six orbital planes around the globe. Each satellite orbit is circular with an inclination of 55°, a period of 12 h and an altitude of 20,200 km.
GRAS can track up to eight satellites for navigation purposes, plus two satellites for rise and two for set occultation measurements.
GRAS SUMMARY Budget
- Mass — 29.3 kg —
- Power — Average < 30 W, Peak: 46.6 W
- Data rate — Mean: 27 kbps, Max: 60 kbps
The GRAS instrument is developed by the instrument prime contractor RUAG Space AB Space AB in Sweden, supported by Austrian Aerospace, Sener and GMV under contract to ESA/Eumetsat. The GRAS Product Guide, Radio Occultation Meteorology SAF and ESA's GRAS page have further information.