The World's ever increasing demand for communication capacity has been the catalyst for the development of a range of next generation satellite reflectors. This new generation are significantly larger than those currently in orbit. Their dimensions prohibit transportation into space in their operational configuration. This thesis investigates the use of deployable tetrahedral trusses for the reflecting surface support structures of a 50 m diameter Land Mobile Communication System. A deployable structural system was selected ahead of other possible forms, as it satisfied the majority of the criteria of low part count, quick assembly times and an economical packaging volume thereby minimizing transportation and on-orbit fabrication costs. The composite material examined is formed from a polyethersulphone thermoplastic matrix reinforced with high strength, low modulus carbon fibres, C-PES.The modal characteristics of a series of scaled sub-units of the proposed structure were examined. These units, manufactured from the two types of material considered, C-PES and Perspex, were subjected to a range of excitation functions. The C-PES units were thermally cycled under high vacuum to simulate the space environment. The accelerated thermal cycling of the composite structural units revealed that a shift in resonant frequency occurred, together with some surface cracking which could affect the long term stability of the material. The effects of surface pigmentation on the thermal response were also considered. An algorithm has been developed which allows low cost materials to be used to predict the behaviour of geometrically similar units manufactured from the composite material. The interaction between the predicted dynamic structural behaviour and the electrical performance of the satellite is also addressed and suggests that some form of active control system will be required if the maximum defocus parameter is not to be violated.

This thesis describes the definition, implementation, and in-orbit testing of an autonomous navigation unit based upon a GPS receiver for use on board a small satellite in low Earth orbit. It explains the motivation for the use of GPS to provide this function, and describes the practical application and integration of this technology into an existing microsatellite system. Until now, the technology for any satellite to track itself has not existed. Space agencies spend significant funds supporting a network of tracking stations around the world for orbit determination. With the recent realisation of the Global Positioning System and the availability of inexpensive receiver hardware, it has become a practical proposition to include a GPS receiver within the demanding constraints of a small satellite. A GPS receiver on-board a satellite can eliminate the necessity for ground-based tracking by providing an autonomous orbit determination capability. During the course of these studies, the requirements and constraints of a small satellite were identified by the author and matched with the capabilities of a GPS receiver. A GPS Navigation Unit was defined to provide autonomous services available oil demand for the satellite platform and payloads; position and velocity; time synchronisation; orbital elements; payload triggering and GPS data logging (for experimental and research purposes). The GPS Navigation Unit includes a processing facility capable of command and initialisation of the GPS receiver, and data processing to give orbit determination capability. When used on a microsatellite, the additional constraints of low power consumption necessitate the intermittent operation of the GPS receiver. To test the concept of the GPS Navigation Unit, a commercial Trimble TANS II GPS receiver system that had been modified for orbital velocities was integrated into the PoSAT-1 microsatellite which was launched into low Earth orbit in September 1993. A method for orbit determination was developed for use with the output from the GPS receiver, and the GPS Navigation Unit was implemented in software according to the constraints of the PoSAT-1 mission. The significant results from these studies include: The first use of a GPS receiver on a microsatellite, PoSAT-1. The implementation, test and validation of a GPS Navigation Unit in low Earth orbit. The first satellite mission to demonstrate the capability for autonomous orbit determination through the GPS Navigation Unit. The definition of the general-purpose interfaces between a small satellite and a satellite- borne GPS Navigation Unit.

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