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An important concern in spacecraft engineering is attitude determination control and stabilisation (ADCS) - the combination of applied mathematics, classical physics and modern technology which maintains the pointing direction of one or more axes of an Earth-orbiting satellite. This thesis is a detailed study of a particular type of ADCS which exploits the gravity-gradient effect, which is just the weak tendency for an appropriately shaped body to point naturally in preferred directions; reinforced by magnetorquing, which is the active interaction of the geomagnetic field with a switched current passing through coils in the spacecraft body. The advantages of this technology is that it is low-cost, non-consumable and has no moving parts - so constituting no limitation to the satellite's life. The thesis is a detailed study of this form of ADCS with specific application to low Earth polar-orbiting (LEO) satellites, for which it is particularly suitable. The work is also a study in attitude determination based solely on a 3-axis magneometer measurement of the geomagnetic field, which is in principle a simple way, in terms of technology, of determining the attitude of the spacecraft, and from this controlling the attitude, if mediated by an on-board computer implementing appropriate algorithms. The results are for the most part practically based on the author's involvement with two satellites over a six year period with the satellites UOSAT-1 and UOSAT-2, which were designed, built, and continue to be controlled from the University of Surrey. A practical innovation in 2-axis attitude control is described: the active 'delibration' by active damping of a gravity-gradient controlled LEO satellite in an attitude-stabilised state, using a threshold comparison algorithm. A new theory and algorithms are then developed for 3-axis attitude control, based on a complementary use of magnetorquing and gravity gradient. Subject to further development these could alter the general perception of the most effective way of controlling low Earth orbiting satellites.

Control of the inter-satellite distances within a cluster of colocated satellites located in the same GEO window is examined with regards to the close approaches between pairs of satellites. Firstly, the orbital evolution and station keeping control of a single GEO satellite is examined and a new IBM PC based software program capable of performing both these functions autonomously from initial values of the orbital position and date is detailed and validated. Cluster design ideas are then examined in detail and the propagation software is used to generate data for a cluster of four satellites. Two test cases are examined to quantify the frequency of close approaches between individual satellite pairs, each test case using a different orbital element separation strategy but the same station keeping control scheme. The results of the study are then compared with previous research and discussions are presented on the advantages of each method. Finally, a cluster geometry correction manoeuvre, based on Hill's equations of relative motion, is presented which requires only those thrusters used by typical station keeping. This manoeuvre is integrated into the computer software and the two test cases noted previously are again propagated and the close approach results analysed to demonstrate the reduction in the number of close approaches below 5 km.

A novel strategy for automatic satellite constellation design with satellite diversity is proposed. The automatic satellite constellation design means some parameters of satellite constellation design can be determined simultaneously. The total number of satellites, the altitude of satellite, the angle between planes, the angle shift between satellites and the inclination angle are considered for automatic satellite constellation design. Satellite constellation design is modelled using a multiobjective genetic algorithm. This method is applied to LEO, MEO and hybrid constellations. The advantage of this algorithm is automatic satellite constellation design whilst achieving dual satellite diversity statistics. Furthermore a new strategy of dynamic channel allocation is proposed using a genetic algorithm for use in MSS networks. The main idea behind this algorithm is to use minimum cost as a metric to provide optimum channel solutions for specified interference constraints. The frequency reuse condition for all spotbeams is investigated as a function of time. The update interval time and the sampling time are introduced in order to track time valiant coefficients and constraints of the algorithm. The method is demonstrated for S-UMTS based on a MEO satellite constellation. Using this algorithm, it is shown that the proposed model outperforms conventional DCA schemes in terms of capacity of the system and Quality of Service (QoS).We show in the thesis that the genetic algorithm is a robust method for calculation of dynamic variations in satellite constellation design and provides resource allocation improvements over DCA in MSS system networks.

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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