Inter-Satellite Communication Links for Sensor Networks in Low Earth Orbits

Unlike the inter-Satellite communications link (ISL) being used in current space missions, emerging scenarios require point to multi-point links with additional support for autonomous networking amongst multiple satellites. Be it for formation flying or space based wireless sensor networks, these distributed satellite systems require revised communications system design to provide for the various physical topologies, dynamic channel characteristics and user requirements. All three factors being mission specific, that is why, little literature is published, in which physical aspects of the link or the communications system design or the channel is discussed. This thesis determines channel characteristics and performance of OFDM modem for three physical topologies for a proposed mission scenario, bridging the gap in literature. The mission is used as the basis for investigating the impact of relative motion and in-orbit environment on propagating EM waves. The proposed multipoint sensing mission namely the ionosphere monitoring mission (IMM) comprises pico/nano class of satellites in a mother daughter configuration, to take finer easurement of the plasma bulk properties in LEO. Three physical topologies for ISL are identified. Relative motion is studied for (a) free flying CubeSats acting as sensor, (b) a sensor satellite with respect to a relay satellite and (c) several relay satellites in a string-of-pearl configuration. The Doppler shift is nominal tens of Hz for the first and third case, however, the Doppler shift is in kHz for case b. Thus a third order PLL is required in receiver stage. Contrary to what is assumed in literature, the communications channel is identified to exhibit fading. The dynamic fading (fast and slow) can reach up to 20 dB. To cope with the dynamic channel behaviour a reconfigurable OFDM modem is described. This channel is modelled in MATLAB as a Time Delay Line (TDL) structure, and used to evaluate the performance of OFDM modem for data rates up to 1 Mbps. The hardware-in-the-loop simulation methods, assisted in laying a much needed foundation for a testbed for evaluating ISL.