This paper highlights research characterizing the distribution and time impacts of spacecraft discrepancies found at the system level of integration and test. Reducing discrepancies will result in cycle time reduction and cost savings, as well as increased product quality and reliability. All of these potential outcomes are indications of successful progress toward becoming a lean organization. / Lean Aerospace Initiative

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First generation digital codecs for speech have been the subject of worldwide studies since the sixties. Speech quality very close to that of the standard 64 Kbit/s PGM, the ability to carry voice band data all at reasonable complexity and cost are the main objectives of candidate coding techniques. This work deals with the subjective performance evaluation of two types of reduced bit-rate encoding techniques operating at 32 Kbit/s, for inclusion into business satellite communication systems. They are CVSDM and ADPCM, which are both capable of doubling the existing transponder capacity available in business satellite systems. The aim has been to determine transmission planning figures for their quality in terms of Richards' Q-factor. The work is also involved with the implementation of different ADPCM algorithms using digital signal processor (DSP) single chips which is fairly new to the field of telecommunications. Throughout the work the replacement of 64 Kbit/s PCM coder by 32 Kbit/s coder in satellite communication systems has been the theme. The achievement of acceptable subjective quality being the major objective.

This thesis is a theoretical investigation into the thermal and structural response of large reflector support trusses subject to the space environment. A review of the elements necessary in a successful spacecraft design is included. The determination of the temperature distributions within large orbiting tetrahedral trusses is described. The thermal environment is shown to be an important factor'' in determining the performance of spacecraft. High quality microwave performance of the communication system depends upon the magnitude of distortions within the reflector support truss, and the thermal expansion or contraction of members makes a significant contribution to this distortion. The possibility of flexural vibration of individual members of the truss arising during entry into, or exit from, the earth's shadow is considered. Individual slender members and the radio frequency reflecting mesh cast shadows within the truss. A method for calculating the duration and position of such shadowing events is discussed. The effect that shadows have upon shadowee temperatures is determined using computer programs written by the author. Situations in which temperature gradients between the truss faces exist are highlighted. Such gradients produce bending deflections of the truss. Classical theories are developed to predict the magnitude of thermally induced stresses in the adhesive layer of tubular lap joints having dissimilar adherends. Tubular joints which contain a transition layer are also examined. An appropriately chosen transition layer will reduce the adhesive shear stresses by a substantial amount. Finally, the development of thermally induced stresses and damage mechanisms in laminated composite tubes is discussed. The designer must be aware of the amount of damage that spacecraft materials will experience during the long life missions envisaged for the near future.

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.

This study investigated the application of fibre reinforced composite materials to spacecraft sandwich structures. In particular, aspects of the manufacture, analysis and design optimisation of components fabricated using the co-cure process were studied. The manufacturing process was developed to ultimately enable a full size thrust tube structure to be built using a single step cure, the design of which was verified by a modal survey test. Techniques for the analysis of stiffness, strength., vibration frequencies and local instability were established and found to correlate well with tests on co-cured sandwich specimens. The current wrinkling theory for composite faced sandwich was extended to the more general case to allow facesheet constitutive matrix coupling and multiaxial loding to be accomodated. The analytical methods were incorporated within simple optimisation schemes, amenable to employment at the preliminary design stage, to allow alternative feasible designs for panel and thrust tube structures to be generated. These illustrated the benefits of the use of composite materials and the co-cure manufacturing technique for spacecraft sandwich components.

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.

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.