Carried on MetOp, GRAS (Global Navigation Satellite System Receiver for Atmospheric Sounding) uses radio occultation to measure vertical profiles of atmospheric temperature and humidity by tracking signals received from a constellation of GPS navigation satellites while they are setting or rising behind the Earth's atmosphere. By processing the navigation signals received by GRAS, 500 highly accurate profiles of atmospheric temperature, humidity and pressure can be derived every day. These profiles are distributed fairly uniformly over the globe.