GRAS (GNSS Receiver for Atmospheric Sounding)

GRAS is a dual-frequency GNSS radio occultation receiver flown on MetOp-A, MetOp-B, and MetOp-C for atmospheric temperature, humidity, pressure, and ionospheric sounding ^[1][2]. It tracks GPS L1 and L2 limb-occultation signals at 1.57542 GHz and 1.2276 GHz, using passive GNSS L-band measurements rather than an imaging swath ^[3]. The instrument supports the EUMETSAT Polar System MetOp series, giving users a long operational record for radio occultation data continuity across the three MetOp platforms ^[2]. The instrument originated with Saab Ericsson Space and is recorded under RUAG Space AB in current MetOp documentation, so procurement or heritage analysis should treat those names as successive supplier identities rather than separate payload designs ^[1][2].