Next Generation Satellite Communications: Automated Doppler Shift Compensation of PSK-31 Via Software-Defined Radio

Abstract

Abstract : Software-defined radio (SDR) leverages the processing power of computers to create communications systems that run as flexible software applications, where the radio operating parameters can be set or altered by software. SDR satellite communications systems developed today can run on future hardware platforms and update the applications already running on the satellite. GNU Radio, a framework for creating SDR applications, has recently become capable of developing satellite communications systems. One of the major issues with satellite communications is the Doppler shift experienced as the satellite passes overhead. Demodulating signals affected by Doppler shift requires ground stations with circuits dedicated to track and synchronize with the satellite in order to compensate for the Doppler shift. For the PSK-31 waveform, a terrestrial narrowband form of multi-user amateur radio communications for text and simple data messaging, the amount of Doppler shift exhibited by the satellite would prevent communications using standard receivers. This project implemented PSK- 31 in GNU Radio as part of a regenerative satellite repeater. Furthermore, the system estimates and pre-compensates for the Doppler shift generated by an orbiting satellite communicating with a ground station. As a result, the Doppler shift observed at the ground station can be reduced from 10 kHz to less than 20 Hz - a level tolerable by most modern receivers.