The GRAS SAF project

GRAS SAF is an operational project aimed at the operational use of GPS signals for the determination of meteorological parameters. GRAS (Global navigation satellite system Receiver for Atmospheric Sounding) is the name of the receiving instrument. SAF (Satellite Application Facility) is the name of EUMETSAT development consortiums.

The GRAS SAF project is headed by DMI in cooperation with the Met Office of the UK, the Institut d’Estudis Espacials de Catalunya (IEEC) in Spain, and European Centre for Medium-Range Weather Forecasts (ECMWF) in the UK. As one of seven EUMETSAT SAFs the project is the center for development and dissemination of meteorological products from GPS radio occultation measurements.

The aim of the project is to develop and operate a processing center with hardware and software for processing and dissemination of these products. GRAS is onbord EUMETSAT’s first polar orbiting satellite Metop, which was launched in 2006. This new type of meteorological products contributes to improved weather forecasts, to studies of the upper troposphere and stratosphere, and to precise long term time series, important for climate research and monitoring.

The project was kicked off in April 1999 with the definition and development phase, which ended in 2006. Thereafter the GRAS SAF entered the operational phase with automatic dissemination of data products to meteorological institutes and research centers.

During the development phase of the GRAS SAF, the data processing system was developed and tested, and for this purpose data from the german CHAMP satellite were used. Test results showed an accuracy of approximately 1 degree for temperature profiles in the altitude range 5 – 25 km. This was derived by comparison of the measured temperature from the GPS occultations and the temperature predicted by the ECMWF numerical weather model (which is based on accessible measurements from Earth, radio sondes, ships, satellites, etc.). Assimilation of the new radio occultation data in the meteorological models at ECMWF and the Met Office shows substantial improvements of the weather predictions.

Read much more about the GRAS SAF and radio occultations at www.grassaf.org

January 2011