GPSEM: A SDR-based LEO satellite in-orbit GPS intermediate frequency emulator

To develop a high performance Low Earth Orbit (LEO) satellite GPS receiver, a flexible and low-cost effective in-orbit GPS emulator is a prerequisite. Catering to this need, a software defined radio based LEO satellite in-orbit GPS Intermediate Frequency (IF) emulator, named GPSEM, was proposed. The GPSEM features a computer software and a USB extension hardware system. The GPSEM computer software can generate precisely combined IF signal samples of all GPS visible space vehicles at the GPS user device. The USB extension hardware system of the GPSEM is based on the most recent Nandflash mass storage technology, which can store and playback the GPS IF samples for more than one LEO orbit cycle time. A key to GPSEM is to calculate GPS space vehicles´ signal transmission time with a given user position and receiver time. Toward this problem, a iterative solution based on Taylor expansion was presented. A test system was built in which the output of the IF emulator is feed into an engineering THU-1 satellite GPS receiver. The positioning results showed that the proposed IF emulator performs well, which made it an effective tool for the development of the next generation LEO satellite GPS receiver.