The GRAS instrument (Global navigation satellite system Receiver for Atmospheric Sounding) is a satellite GPS receiver. It was developed by RUAG Space AB (formerly SAAB Ericsson Space) in Sweden, under contract to ESA/Eumetsat and is being used on the series of Metop satellites. GRAS provides a minimum of 500 atmospheric profiles per day through the process of GPS radio occultation, yielding information of the temperature and humidity of the Earth atmosphere. In addition GRAS provides navigation solutions of the Metop satellite position along its orbit.
Relevant links with more information:
ESA's GRAS page
ESA Bulletin 102 article about GRAS
ESA Bulletin 102 article about Metop
GRAS SAF Open Loop Workshop, 6-8 June 2005, Denmark.