The BREMSAT Phase-Modulated Communications Link

BREMSAT is a small scientific satellite being constructed by the west German Zentrum fur angewandte Raumfahrttechnologie und Mikrogravitation (ZARM), at Bremen University. BREMSAT's payload consists of five scientific experiment packages. The satellite is scheduled for Get-Away-Special (GAS) Canister launch during the German D-2 shuttle mission in March 1992. Cynetics Corporation is constructing the TT&C link for the satellite, using its standard CMX9600 modem. This modem is the result of Cynetics' development program for phase-modulated (PM) communications systems for small satellites. The CMX9600 is a 9.6 KC/s, Bi-Phase Shift Keyed (BPSK) modem using a 70-MHz IF and synchronous matched-filter detection. As we reported at the 1989 Small Satellite Conference, many small satellite systems use Frequency-Shift Keying (FSK) for communications. FSK is a sub-optimal modulation which suffers a best-case 3 dB degradation in signal-to-noise ratio when coherently demodulated. When non-coherent demodulation is used, as it often is, additional degradation in signal-to-noise ratio occurs. Antipodal phase modulations, such as Bi-Phase Shift Keying (BPSK) and Quadra-Phase Shift Keying (QPSK) are optimal modulations which do not suffer the signal-to-noise ratio degradations of FSK. Thus, when phase-modulation is used for downlinking, the satellite's transmitter power can be decreased. This saved power is available for payload and bus operations. Or, a smaller satellite with less solar cell area may be used. FSK has been utilized in the past due to the perception that implementing an FSK system is simpler that implementing a phasemodulation system. However, with the advent of off-the-shelf phase-modulation TT&C links, such as the CMX9600, satellite system designers can easily incorporate the more power efficient phase modulation techniques into their satellites.