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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.

The future of space development has been examined in the context of the infrastructure necessary support it. It is concluded that the selection of propulsion systems for in-orbit transportation requires the development of general computer codes capable of simulating the use of a wide range of propulsion systems on near-Earth missions. It is also concluded that, even if limited infrastructural development occurs, polar orbiting spacecraft will be an important feature of future space activities. Replacing current single-use polar spacecraft with extended-life serviceable platforms is attractive. However, the very limited manned access polar orbits in the mid-term future suggests that such platforms will only be possible if remote telerobotic/autonomous servicing can be carried out. To this extent polar platforms are considered to provide a useful driver and first testbed for the development of technologies designed to extend human capability in those regimes where direct mediation is not possible. Options for such remote servicing are examined, the concept of performing nodal transfers by enhanced differential nodal drift is introduced and the application of electric propulsion to this discussed. Low-thrust orbital manoeuvres are analyzed in this context and the conditions for optimum nodal transfer defined. Particular service vehicle configurations are then defined against a projected infrastructure and baseline polar platform constellation. A model for the service vehicle is defined and its performance investigated using a number of electric propulsion systems. Simulations of transfer manoeuvres; have been carried out and the effects and relative importance of the various orbit perturbations identified. It is concluded that a service vehicle propelled by a Xenon ion system offers the capability required and two final configurations are identified characterising different servicing mission upload schemes.

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.

The modal characteristics of large skeletal structures (LSS) made from tubes of graphite reinforced Poly (Ether Sulphone) (PES), a high technology thermoplastic composite material, have been investigated. These large skeletal structures are intended for application in land-mobile communications networks and would be positioned at geosynchronous equatorial orbit (GEO). Experimental modal surveys of a number of skeletal configurations have been conducted under simulated unrestrained conditions, and have involved the prior commissioning of the modal survey apparatus used in their examination. The surveys have been performed in tandem with analogous natural frequency extractions from the structures' analytical models using the finite element (FE) method. The skeletal structures were fabricated using either the graphite reinforced PES material or perspex plastic, and formed representative sub-structures of candidate LSS configurations. The structures' geometries ranged in complexity from sparsely configured composite systems to a perspex platform-based bank of reflector arrays, and allowed the evolution of both local and global modal behaviour in these skeletal systems to be observed in detail. It has been found that the modal behaviour of predominantly uniaxially reinforced PES composite, in the state of uniaxial stress to which it will be subjected as a component of multi-bay LSS configurations, can be accurately described using an isotropic approximation for its material characteristics. Additionally, it has been found that the use of perspex plastic as a material for modelling representative multi-bay sub-structures of composite LSS is justified in consideration of the eventual stress environments to which the composite material will be subjected in LSS systems. Following this, a series of analytical parametric studies has been performed on a number of concept composite LSS suitable for use in a data-relay capacity at GEO. It has been found that the modal frequencies developed by these configurations are comparable to the published frequencies of similar skeletal structures intended for use at these orbits. It has also been established that the testing of a reduced modelling of the LSS can be highly instructive as to the general trends in modal behaviour developed by the large skeletal structures in orbit.

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The increase in demand for channel capacity and the limitations in satellite power and frequency spectrum is leading to requirements for advanced multiple beam antenna systems with frequency reuse. The concept of the Olympus satellite evolved from the fact that new technologies and methods are to be explored for future satellites. The role of Olympus can be seen in both the development of earth station networks and at the same time the satellite requirements for the future satellites. It is a step between advanced transparent repeaters and the future intelligent satellite with on-board processing. A review of commercial and experimental satellites employed in the European region is given in the thesis. The services offered by these satellites and the access methods used are discussed with particular emphasis on business services. The new research work reported was done on the experiments related to the Olympus SS-TDMA payload. First a survey of commercial TDMA systems was performed and their capabilities discussed in detail for the SS-TDMA experimental application. The Olympus payload description and link budget analysis was performed in order to identify the size of the earth station required for these networks. The performance of the satellite link was simulated using BOSS software package together with some earlier simulation using the TOPSIM software package. The methods of BER estimation used are investigated and various results compared. These simulations covered most aspects of the RF (degradation due to TWTA non-linearity, fading, carrier spacing, co-channel interference) and some aspects of the baseband circuits. These simulation results have been compared with the experimental results and were found to be in close agreement. Thus giving confidence in the simulation methods used. In SS-TDMA the problem of acquisition and synchronisation is very important. In this respect the design of the acquisition and synchronisation unit (ASU) is discussed in detail. The resulting ASU was interfaced with the reference station of the commercial TDMA system for the first phase of the SS-TDMA experiments. Finally we present results of various network control methods for the SS-TDMA network, including the buffer requirement for the on-board clock control with the sidereal day clock correction. The ASU design, its interface to the reference TDMA terminal and successful operation with the reference station in acquiring and synchronising with the satellite switch provided the real time operation of the SS-TDMA scheme. The operation was conducted experimentally using the BTRL (British Telecom Research Labs) experimental earth station at Martlesham both in the "reference station loopback" and "reference station and traffic station" configuration via the the SS-TDMA switch. Full details of the experiments and comparison with systems simulations are presented in the thesis.

The use of satellites for mobile communication applications has become a global issue. The use of handheld, vehicle mounted and transportable terminals is a key feature of Satellite Personal Communication Networks (S-PCNs). Due to the higher eirp requirements on the Earths surface and also because of their inherent delay, geostationary (GEO) satellites are not considered suitable for such applications. Instead, constellations of satellites at lower altitudes have been proposed for use in what are termed 2nd generation mobile satellite communication systems. Low intensity regions in the Earths surrounding trapped radiation bands, have resulted in two altitude bands of specific interest, resulting in two constellation types - LEO (Low Earth Orbit) constellations at around 1,000 km and MEO (Medium Earth Orbit) constellations at around 10,000 km. A satellite constellation consists of a number of satellites orbiting at the same altitude and inclination and phased in a specific way. The work reported in this thesis proposes a network control architecture for LEO or MEO based S-PCN systems. Air-interface signalling aspects are then considered for mobility management and call setup signalling. LEO and MEO constellation design aspects and properties are initially considered. Important implications on the control network are drawn based on constellation coverage and connectivity properties. Other system constraints such as terrestrial network interworking considerations as well as user, network operator and regulator requirements are also considered. Finally network and more specifically satellite control signalling is examined before a S-PCN architecture is proposed. The reference architecture results in constellation control being distributed globally with individual satellite control, at any one time, being located at a specific earth station. The use of two earth station types allows network administration to be separated from traffic channel carrying aspects. In order to reduce system setup cost and delay, the reuse of network related standards from the GSM terrestrial mobile communication system is envisaged. An equivalence is made between the S-PCN architecture and the GSMs terrestrial architecture. Network implementation aspects are considered for a 14 satellite MEO constellation. Network implications resulting from the use of LEO and MEO constellations are considered. After an examination of S-PCN traffic demand on a MEO constellation, mobility management signalling is considered. A new approach is proposed based on the use of a positioning system. The performance of this approach is examined from a system signalling viewpoint for both LEO and MEO constellations and a method to minimise the required amount of signalling is described. The air interface signalling procedure for location update, based on a modified GSM network layer protocol, is simulated from a delay point of view for both LEO and MEO constellations. User-originated, user-terminated and user-to-user call setup signalling were also simulated and their delay performance examined. The importance of random access channel delay and of user cooperation with the link were highlighted as aspects which have a significant influence on the average signalling delay. Finally, the effect of common and dedicated control channel system signalling on satellite power consumption, based on busy hour call setup and mobility management signalling estimates, was examined for a MEO constellation. From this, conclusions can be made on the signalling power efficiency of S-PCN systems.