SGLS COMMAND DATA ENCODING USING DIRECT DIGITAL SYNTHESIS

Abstract

The Space Ground Link Subsystem (SGLS) provides full duplex communications for commanding, tracking, telemetry and ranging between spacecraft and ground stations. The up-link command signal is an S-Band carrier phase modulated with the frequency shift keyed (FSK) command data. The command data format is a ternary (S, 1, 0) signal. Command data rates of 1, 2, and 10 Kbps are used. The method presented uses direct digital synthesis (DDS) to generate the SGLS command data and clock signals. The ternary command data and clock signals are input to the encoder, and an FSK subcarrier with an amplitude modulated clock is digitally generated. The command data rate determines the frequencies of the S, 1, 0 tones. DDS ensures that phase continuity will be maintained, and frequency stability will be determined by the microprocessor crystal accuracy. Frequency resolution can be maintained to within a few Hz from DC to over 2 MHZ. This allows for the generation of the 1 and 2 Kbps command data formats as well as the newer 10 Kbps format. Additional formats could be accommodated through software modifications. The use of digital technology provides for encoder self-testing and more comprehensive error reporting.