A Power-Efficient BPSK Communications System for Small Satellites

Many of the small satellites which have been launched or designed to date have used Frequency Shift Keyed (FSK) modulation for the communications link. FSK necessarily suffers a best-case signal to-noise ratio (SNR) loss of 3 dB for coherent demodulation. Many, if not most, of the FSK systems in use today employ non coherent demodulation which suffers additional SNR loss. This means that small satellites using FSK must use two or more times the minimum power required for the communications link. A small satellite using two watts for an FSK communications link could save at least 1 watt by using Bi-Phase Shift Keying (BPSK) or one of the other power-optimal modulations. This saved power would then be available for payloads or for increased data communications. Alternately, a satellite with one-half the solar-cell surface area could be used Cynetics Corporation has tested a commercially available 9.6 Kb/sec communications system which uses asynchronously detected, non-coherent FSK. This system has a measured implementation loss of 23.6 dB, which is roughly 20 dB worse than the 3 dB implementation loss one might expect. When the additional 3 dB FSK loss is considered, this system was 23 dB worse than a simple BPSK system with a 3 dB implementation loss. This means that this FSK system would require two hundred times (23 dB) as much satellite transmitter power as a reasonable BPSK system. Cynetics is completing the development of a 9.6 Kb/sec (BPSK) satellite communications link using synchronous matched-filter data detection. BPSK is one of the optimal pulse modulation methods (in an SNR and power-efficiency sense) which can save substantial power in a satellite transmitter. Cynetics' BPSK system modulates and demodulates at the standard satellite communications IF frequency of 70 MHz. The expressed performance 1S a 10 b1t error rate for -116 dBm (2.5 x 10 watts) received signal power at the input to a 0.5 dB noise figure low-noise amplifier.