The GRAS-2 radio occultation mission

The second generation of the Global navigation satellite system Receiver for Atmospheric Sounding (GRAS-2) is a radio occultation (RO) instrument which is capable of providing 2000 atmospheric profiles per day. The instrument is hosted on all satellites in the MetOp Second Generation (MetOp-SG) series for polar orbit operation. The GRAS-2 instruments provide occultation measurements from the Galileo, GPS, and BeiDou satellites at their common frequencies centred at 1575.42 MHz (L1) and 1176.45 MHz (L5). Using high-gain antennas and an ultra-stable oscillator, neutral bending angles are measured at an unprecedented accuracy of 0.3–0.4 µrad, which is better than the requirement of less than 0.5 µrad. The RO signal will be measured deep into the troposphere using a novel open-loop tracking scheme utilizing multiple correlator outputs for operation with a tailored ground processing algorithm optimized for extracting signals with low amplitudes, approaching the noise floor limitation. Ionosphere measurements to an altitude of 600 km are also acquired.