The EUMETSAT Polar System (EPS) consists of a series of three meteorological satellites (MetOp) in near-polar orbits, carrying several different instruments for observing weather phenomena and monitoring the Earths climate. To ensure the best possible handling and use of Metop's data eight Satellite Application Facilities (SAF's) were approved. The MetOp Satellite

The Research Division at DMI is leading entity for the GRAS Meteorology SAF project, which concerns data from the GRAS (Global Navigation Satellite System Receiver for Atmospheric Sounding) instrument. The project is international and is financed by EUMETSAT and the participating institutes. Apart from DMI these are ECMWF (European Centre for Medium-Range Weather Forecasts) in Reading, United Kingdom, IEEC (Institut D'Estudis Espacials de Catalunya) in Barcelona, Spain, and UK Met Office in Exeter, United Kingdom.

A major part of the GRAS SAF work includes definition and development of algorithms and a processing system for data from the GRAS instrument, and installing, testing, and operating this system at DMI. A large part of the tasks have a scientific mark as this is a new technique, being used operationally for the first time. Several visiting scientists have been and are employed at the partner institutes. Cooperation with other SAF's is coordinated by EUMETSAT.

The orbits of the satellites as viewed from space

The orbits of the satellites as viewed from space

The GRAS instrument is a receiver for the reception of signals from the GPS (Global Positioning System) which is part of the Global Navigation Satellite Systems (GNSS). Unlike all other instruments on MetOp, GRAS is not viewing the Earth by vertical scanning but instead looks tangentially through the Earths atmosphere. The instrument measures the time delay of the refracted GNSS radio signals as the ray (signal path) skirts the Earths atmosphere on its way from the transmitting GNSS satellite to MetOp (see sketch). By precisely computing position and velocity of MetOp and the GNSS satellite the measured time delay is converted to the bending angle of the ray path, which again is converted to values of temperature, pressure and water vapour content in the atmosphere. The resulting quantities are functions of height above sea level (from ground level up to approximately 50 km), a so-called atmosphere profile. These profiles can be assimilated into weather prediction models for improved global, regional, and local weather predictions. Additionally, the very precise and stable atmosphere profiles are especially suited for monitoring changes in the Earths climate.

The GRAS instrument can theoretically receive signals from each navigation satellite up to 29 times a day. With one instrument in operation, reception of signals from all of the GPS satellites in orbit would result in about 700 atmospheric soundings each day, with an average spacing of some 700 km. Currently GRAS is delivering about 600 profiles per day. The soundings are characterised by high vertical resolution (better than 500 m) and high accuracy (around one degree Kelvin).

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