The GRAS-2 radio occultation mission
2024
J. Rasch | A. Carlström | J. Christensen | T. Liljegren
<p>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 <span class="inline-formula">µrad</span>, which is better than the requirement of less than 0.5 <span class="inline-formula">µrad</span>. 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.</p>
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