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 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, two additional satellites for rise and two others for set occultation measurements.

GRAS Summary Budget

The GRAS instrument is developed by the instrument prime contractor RUAG Space AB in Sweden, supported by Austrian Aerospace (A), Sener (E) and GMV (E) under contract to ESA/Eumetsat. GRAS flies on Metop-A, and will also fly on Metop-B and Metop-C.

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