# GRAS (GNSS Receiver for Atmospheric Sounding)
*sensor . products*

European dual-frequency GNSS radio occultation receiver flown on MetOp-A, MetOp-B, and MetOp-C for atmospheric temperature, humidity, pressure, and ionospheric sounding. GRAS tracks GPS signals through atmospheric limb occultations and has operational heritage across the EUMETSAT Polar System first-generation MetOp series.

## Specifications
- **product status**: operational
- **deployment context**: orbital
- **manufacturer**: ruag-space-ab
- **sensor**: {"instrument_family":"radiometer","measurement_principle":"passive","frequency_bands_ghz":[1.2276,1.57542],"polarisation_modes":["GNSS L-band"],"scan_type":"static","notes":"GNSS radio occultation receiver using GPS L1/L2-band signals; OSCAR classifies GRAS as a GNSS radio-occultation Earth observation instrument. Limb geometry is captured in methodology and edge evidence rather than as imaging swath."}
- **entity type**: sensor
- **last verified date**: 2026-06-14
- **verified by**: agency-doc
- **claim status**: agency-sourced
- **provider**: ruag-space-ab
- **attributes**: {"kind":"sensor","product_status":"operational","deployment_context":"orbital","manufacturer":"ruag-space-ab","description":"European dual-frequency GNSS radio occultation receiver flown on MetOp-A, MetOp-B, and MetOp-C for atmospheric temperature, humidity, pressure, and ionospheric sounding. GRAS tracks GPS signals through atmospheric limb occultations and has operational heritage across the EUMETSAT Polar System first-generation MetOp series.","sensor":"{\"instrument_family\":\"radiometer\",\"measurement_principle\":\"passive\",\"frequency_bands_ghz\":[1.2276,1.57542],\"polarisation_modes\":[\"GNSS L-band\"],\"scan_type\":\"static\",\"notes\":\"GNSS radio occultation receiver using GPS L1/L2-band signals; OSCAR classifies GRAS as a GNSS radio-occultation Earth observation instrument. Limb geometry is captured in methodology and edge evidence rather than as imaging swath.\"}"}
- **relevance**: in-scope

## Editorial
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 [^esa-gras][^eumetsat-metop-instruments]. 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 [^wmo-oscar-gras]. 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 [^eumetsat-metop-instruments]. 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 [^esa-gras][^eumetsat-metop-instruments].

## Compositional position
- metop-a (missions) --[mission_payloads]--> GRAS (GNSS Receiver for Atmospheric Sounding)
- metop-b (missions) --[mission_payloads]--> GRAS (GNSS Receiver for Atmospheric Sounding)
- metop-c (missions) --[mission_payloads]--> GRAS (GNSS Receiver for Atmospheric Sounding)

## Sources
- [esa-gras] | ESA About GRAS | https://www.esa.int/Applications/Observing_the_Earth/Meteorological_missions/MetOp/GRAS2 | tier=agency-doc | accessed=2026-06-14
- [eumetsat-metop-instruments] | EUMETSAT Metop instruments | https://www.eumetsat.int/metop-instruments | tier=operator-engineering | accessed=2026-06-14
- [wmo-oscar-gras] | WMO OSCAR instrument record for GRAS | https://space.oscar.wmo.int/instruments/view/gras | tier=agency-doc | accessed=2026-06-14

---
Source: https://eo-atlas.org/products/sensor/gras
Maintainer: SpectraWorks B.V. (CC-BY 4.0)