Abstract
Global Navigation Satellite Systems (GNSS) play a crucial role in modern day activities. Terrestrial and space-borne receivers at low altitude (up to 3000 km) enjoy seamlessly available GNSS services. However, receivers at higher altitudes operating between Medium Earth Orbit (MEO) and Geostationary Orbit (GEO) depend on GNSS satellites crossing the limb of the Earth. GNSS signals reaching the higher altitudes on the far side of the earth are significantly weaker than those available on the Earth’s surface, due to path loss, gain variation in transmitting antenna pattern, ionospheric loss. Receivers intended for higher altitudes need to have fast signal acquisition capabilities combined with good weak signal tracking performance. This paper assesses the possibility of using combined GNSS services for missions operating between MEO and GEO altitudes. To effectively utilize the available GNSS service, necessary system modifications such as dual antenna design are discussed. Simulations are performed for single constellation services (GPS) to use at higher altitudes and extended to combine GPS/SBAS/QZSS. The simulation results are provided and comparatively analyzed in detail for the above two cases. The results suggest that a single constellation will not be sufficient to cover the entire mission period at higher altitudes, whereas combined GNSS offers better performance for positioning at higher altitudes.
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