Nebula Public Library

Global Navigation Satellite Systems (GNSS) are already used in space missions (e.g. GOCE, Swarm, Sentinel, and MetOp, among others), not only as a navigation sensor (either for autonomous orbit and attitude determination or for relative navigation) but also as a science instrument (e.g. for altimetry, global geodesy, Radio Occultation and GNSS Reflectometry applications). However, its use is usually limited to orbits or mission phases below the orbits of GNSS satellites, since GNSS satellite antennas transmit mainly in the Earth's direction.