Implementation of a Spread Spectrum Receiver and Modem for the Data-Relay System Using Digital Signal Processing

The ESA Data-Relay System (DRS) is a space-borne communications system which provides communications between ground stations and satellites in low Earth orbits (LEOs) with inter-orbit links giving a data-relay facility. The system requires a ground based receiver and modem and this paper describes the implementation of such a receiver relying heavily on Digital Signal Processing Techniques. The spread spectrum receiver and modem is presently under development by Satellites International Limited under contract to the European Space Agency (ESA). The specification requirement for the demodulator is severe, with low signal-to-noise ratios and high Doppler shifts. The demodulator is very flexible in terms of the types of signal it can receive, which include spread spectrum signals. The demodulator is implemented almost completely using digital techniques. Such an approach leads to potentially low implementation losses and allows the use of novel techniques in the receiver's sub-systems.