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Global Navigation Satellite System Receiver for Atmospheric Sounding Level 1B - The Global Navigation Satellite System (GNSS) Receiver for Atmospheric Sounding (GRAS) is a European GNSS receiver on board MetOp-A that operates as an atmospheric sounder. GRAS uses radio occultation to measure vertical profiles of atmospheric temperature and humidity by tracking signals received from a constellation of GNSS satellites while they are setting or rising behind the Earth's atmosphere. Radio occultation is based on the fact that when radio waves pass through the atmosphere, either during a rise event or during a set event as seen by the receiver, they are refracted along the atmospheric path. The degree of refraction depends on gradients of air density, which in turn depend on temperature and water vapor. Therefore, measurement of the refracted angle contains information about these atmospheric variables. As the measurements are made tangentially to the atmosphere, the profiles will be provided with a resolution within a few hundred meters to 1.5 kilometers, while horizontal coverage of each profile is in the order of a few hundred kilometers. With the nominal number of GNSS satellites, GRAS will provide 500 precise atmospheric profiles every day nearly equally distributed over the Earth's surface. GRAS can track up to eight satellites for navigation purposes, two additional satellites for rise and two others for set occultation measurements. GRAS has on-board GNSS satellite prediction for optimizing the navigation and occultation measurements. GRAS level 1b products contain information on a per occultation basis, along with auxiliary information.