The Operational EPS GRAS Measurement System

GRAS (Global Navigation Satellite System Receiver for Atmospheric Sounding) is an advanced GPS receiver that has been developed by the European Space Agency (ESA) and EUMETSAT for Radio Occultation (RO) measurements in the framework of the EUMETSAT Polar System (EPS). The space segment of the EPS program consists of a series of three Metop satellites, to be flown successively in the years from 2006. The GRAS data products retrieved by the EPS Ground Segment can be assimilated into NWP (Numerical Weather Prediction) models. The GRAS products will potentially have a positive impact on the forecast if they can be delivered within 3 hrs from each observation made by the GRAS instrument with the required accuracy. A positive impact on the forecast by using RO data from the CHAMP mission has already been demonstrated (Healy et al. 2005; Healy and Thepaut 2005). The operational Near Real Time (NRT) processing and dissemination of RO products has not yet been demonstrated and is a challenge, expected to be met for the first time by the EPS/Metop mission. GeoForschungsZentrum Potsdam (GFZ) has shown good product timeliness results for the CHAMP mission by being able to disseminate data within 4 hrs from the measurements (Wickert et al. 2005).