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The GRAS SAF Project

The EUMETSAT Polar System (EPS) will consist of three meteorological satellites (MetOp) in near-polar orbits, and will carry several different instruments for observing weather phenomena and monitoring the Earths climate. To ensure the best possible handling and use of data 8 Satellite Application Facilities (SAFs) have been approved. The MetOp Satellite

The Atmosphere and Space research division at DMI is heading 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 this system at DMI. A large part of the tasks have a scientific mark as this is a new technique, which has not yet been used operationally. Several visiting scientists have been and are employed at the partner institutes. Cooperation with other SAFs is coordinated by EUMETSAT. A close cooperation between the Climate SAF, the NWP (Numerical Weather Prediction) SAF, and the GRAS SAF is planned.

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 of the other instruments on MetOp, GRAS will not view the Earth system with a vertical scan but will instead look tangentially through the Earths atmosphere. The instrument will measure 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 can be converted to the bending angle of the ray path, which again can be converted to values of temperature, pressure and water vapour content in the atmosphere. The resulting quantities will be functions of height above sea level (from ground level up to approximately 50 km), a so-called atmosphere profile. These profiles will be assimilated in weather prediction models for improved global, regional, and local weather predictions. The very precise and stable temperature profiles are especially suited for monitoring changes in the Earths climate.

The GRAS instrument will receive signals from each navigation satellite up to 29 times The orbits of the satellites as viewed from space a day. Even with only one instrument in operation, reception of signals from all of the GNSS satellites in orbit would result in about 1000 atmospheric soundings each day, with an average spacing of some 700 km. The soundings will be characterised by high vertical resolution (better than 1 km) and high accuracy (around one degree Kelvin).

The EPS/MetOp satellites will be part of an international cooperation with the US for monitoring the Earths atmosphere from polar orbiting satellites. Some of the MetOp instruments will be similar to already existing instruments on NOAA (National Oceanic and Atmospheric Administration) and DMSP (Defense Meteorological Satellite Program) satellites. From 2008 the american satellites will be succeeded by the NPOESS (National Polar Orbiting Operational Environmental Satellite System) satellites. In 1998 DMI participated in the development of the GPSOS (GPS Occultation Sensor) instrument for the NPOESS program. Both GPSOS and GRAS are built by SAAB Ericsson Space, Sweden.

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