Implementation of Software GPS Receiver Algorithm for GEO Satellites

NSS receivers are capable of providing absolute time reference, real-time positioning, and orbit determination for satellites operating in space. For different type of orbits, the GNSS receiver must consider the characteristic of corresponding orbit. For the LEO satellites which move in high speed near the Earth, GNSS receiver must consider the high doppler, doppler rate, and frequent change of visible GNSS satellites. If the GNSS receiver is on the GEO satellite which is higher than the GPS constellation, the speed of satellite becomes slower. However the number of visible GNSS satellites becomes few and even the signal power of received GNSS signal becomes very weak. Therefore the GNSS receiver for GEO satellites must consider these features. In this study, GPS receiver algorithm for GEO satellites is implemented as a software receiver. First of all, some analyses including visibility, doppler, and received power are conducted. Integration time of each correlator is increased to 400ms for acquisition and to 300ms for tracking. The code doppler effect, unknown bit edge positions problem, and unknown data bits problem are resolved using CCMBD algorithm [6]. First of all, to verify implemented software receiver for weak signals, we used software simulation tool produced by Seoul National University GNSS Laboratory, South Korea [8]. For the verification under GEO scenario, GSS8000 hardware simulator is used. GPS L1 signal received on the GEO satellite is generated. The signal acquisition and tracking is successfully performed by implemented software receiver. Pseudorange measurements and PVT solutions are calculated.