Abstract
There is growing interest in networks of small satellites in Low Earth Orbit (LEO) to provide a variety of communications services in conjunction with very small terminals. Communications satellites in LEO offer certain potential advantages when compared with conventional geostationary satellite communications systems, but LEO systems exhibit very different and demanding characteristics. A search of the literature has shown that the practical characteristics of the LEO VHF/UHF communications environment are poorly understood and thus optimised communications techniques have not yet been developed for these new services. This research programme investigates communications techniques suitable for the 'little-LEO' class of small (50kg) low-cost ‘microsatellites’ which provide non-real-time (store-&-forward) narrow-band digital communications services at VHF/UHF. Specifically, the study aims to improve the communications throughput achievable to-and-from the microsatellite, as it transits the groundstation horizon, by optimising the constant-envelope CPFSK/MSK modulation schemes that are imposed by the practical constraints dictated by the use of the microsatellite's non-linear RF systems. In order to provide a highly flexible in-orbit research test-bed, a Digital Signal Processing Experiment (DSPE) payload was designed, built and incorporated into two LEO microsatellites (KITSAT-1 & PoSAT-1) during the course of this study. The PoSAT-1 DSPE payload was first used to investigate and improve the current non-coherent CPFSK communications system through the design and implementation of Nyquist pulse shaping and equalisation filters using a time-domain waveform forcing technique. The use of coherent MSK was then proposed in order to take advantage of the potential 3dB improvement in link BER performance achievable when compared with non-coherent. CPFSK techniques. Coherent MSK, however, has not yet been employed in LEO systems because of the difficulty of practical implementation and the sensitivity of classical (de Buda) coherent demodulators to the effects typical of the LEO communications environment. To overcome these effects, a novel implementation of the robust ‘Hodgart-Massey’ coherent MSK demodulator/decoder was developed and compared rigorously with the ‘de Buda’ demodulator under typical LEO conditions through a sequence of computer simulations. These showed that synchronisation problems cause the main difficulty in the demodulation of coherent MSK. Comprehensive laboratory bench tests of a DSP implementation of the ‘Hodgart-Massey’ coherent MSK demodulator/decoder were carried out and were able to quantify a practical 2dB (Eb/No) improvement compared with noncoherent CPFSK whilst also exhibiting considerable tolerance to both frequency and amplitude variations. In-orbit FM (remote) channel measurements and equalisation using the DSPE identified a previously unknown and severe non-linearity in the PoSAT-1 RF modulator, however the critical synchronisation characteristics of the coherent MSK demodulator/decoder were successfully demonstrated in low Earth orbit. The specific results of this research programme have thus been: i) The design, implementation and test of an advanced DSP communications payload on the KITS AT-1 and PoSAT-1 microsatellites in LEO; ii) The first proof that it is mainly the synchronisation problems restricting applications of coherent MSK and the first demonstration that the ‘Hodgart-Massey’ coherent MSK demodulator solves this problem; iii) The first implementation, characterisation and in-orbit demonstration of a novel and robust ‘Hodgart-Massey’ coherent MSK DSP demodulator/decoder achieving a 2dB improvement in communications performance in both computer simulation and practical laboratory bench tests; iv) An improved method of measuring the communications channel characteristics of a LEO satellite remotely in orbit and optimising the current non-coherent CPFSK communications systems by developing a novel time domain waveform-forcing equalisation technique.