The work described in this thesis concerns the application of radar altimetry, collected from the ERS-1 and TOPEX/POSEIDON missions, to precise satellite orbits computed at Aston University. The data is analysed in a long arc fashion to determine range biases, time tag biases, sea surface topographies and to assess the radial accuracy of the generated orbits through crossover analysis. A sea surface variability study is carried out for the North Sea using repeat altimeter profiles from ERS-1 and TOPEX/POSEIDON in order to verify two local U.K. models for ocean tide and storm surge effects. An on-side technique over the English Channel is performed to compute the ERS-1, TOPEX and POSEIDON altimeter range biases by using a combination of altimetry, precise orbits determined by short arc methods, tide gauge data, GPS measurements, geoid, ocean tide and storm surge models. The remaining part of the thesis presents some techniques for the short arc correction of long arc orbits. Validation of this model is achieved by way of comparison with actual SEASAT short arcs. Simulations are performed for the ERS-1 microwave tracking system, PRARE, using the range data to determine time dependent orbit corrections. Finally, a brief chapter is devoted to the recovery of errors in station coordinates by the use of multiple short arcs.

For optimum utilization of satellite-borne instrumentation, it is necessary to know precisely the orbital position of the spacecraft. The aim of this thesis is therefore two-fold - firstly to derive precise orbits with particular emphasis placed on the altimetric satellite SEASAT and secondly, to utilize the precise orbits, to improve upon atmospheric density determinations for satellite drag modelling purposes. Part one of the thesis, on precise orbit determinations, is particularly concerned with the tracking data - satellite laser ranging, altimetry and crossover height differences - and how this data can be used to analyse errors in the orbit, the geoid and sea-surface topography. The outcome of this analysis is the determination of a low degree and order model for sea surface topography. Part two, on the other hand, mainly concentrates on using the laser data to analyse and improve upon current atmospheric density models. In particular, the modelling of density changes associated with geomagnetic disturbances comes under scrutiny in this section. By introducing persistence modelling of a geomagnetic event and solving for certain geomagnetic parameters, a new density model is derived which performs significantly better than the state-of-the-art models over periods of severe geomagnetic storms at SEASAT heights. This is independently verified by application of the derived model to STARLETTE orbit determinations.

Orbit determination from artificial satellite observations is a key process in obtaining information about the Earth and its environment. A study of the perturbations experienced by these satellites enables knowledge to be gained of the upper atmosphere, the gravity field, ocean tides, solid-Earth tides and solar radiation. The gravity field is expressed as a double infinite scries of associated Legendre functions (tesseral harmonics). In contemporary global gravity field models the overall geoid is well determined. An independent check on these gravity field harmonics of a particular order may be made by analysis of satellites that pass through resonance of that order. For such satellites the perturbations of the orbital elements close to resonance are analysed to derive lumped harmonic coefficients. The orbital parameters of 1984-106A have been determined at 43 epochs, during which time the satellite was close to 14th order resonance. Analysis of the inclination and eccentricity yielded 6 lumped harmonic coefficients of order 14 whilst analysis of the mean motion yielded additional pairs of lumped harmonics of orders 14, 28 and 42, with the 14"1 order harmonics superseding those obtained from analysis of the inclination. This thesis concentrates in detail on the theoretical changes of a near-circular satellite orbit perturbed by the Earth's gravity field under the influence of minimal air-drag whilst in resonance with the Earth. The satellite 1984-106A experienced the interesting property of being temporarily trapped with respect to a secondary resonance parameter due to the low air-drag in 1987. This prompted the theoretical investigation of such a phenomenon. Expressions obtained for the resonance parameter led to the determination of 8 lumped harmonic coefficients, coincidental to those already obtained. All the derived lumped harmonic values arc used to test the accuracy of contemporary gravity field models and the underlying theory in this thesis.

The accuracy of altimetrically derived oceanographic and geophysical information is limited by the precision of the radial component of the satellite ephemeris. A non-dynamic technique is proposed as a method of reducing the global radial orbit error of altimetric satellites. This involves the recovery of each coefficient of an analytically derived radial error correction through a refinement of crossover difference residuals. The crossover data is supplemented by absolute height measurements to permit the retrieval of otherwise unobservable geographically correlated and linearly combined parameters. The feasibility of the radial reduction procedure is established upon application to the three day repeat orbit of SEASAT. The concept of arc aggregates is devised as a means of extending the method to incorporate longer durations, such as the 35 day repeat period of ERS-1. A continuous orbit is effectively created by including the radial misclosure between consecutive long arcs as an infallible observation. The arc aggregate procedure is validated using a combination of three successive SEASAT ephemerides. A complete simulation of the 501 revolution per 35 day repeat orbit of ERS-1 is derived and the recovery of the global radial orbit error over the full repeat period is successfully accomplished. The radial reduction is dependent upon the geographical locations of the supplementary direct height data. Investigations into the respective influences of various sites proposed for the tracking of ERS-1 by ground-based transponders are carried out. The potential effectiveness on the radial orbital accuracy of locating future tracking sites in regions of high latitudinal magnitude is demonstrated.

Due to the failure of PRARE the orbital accuracy of ERS-1 is typically 10-15 cm radially as compared to 3-4cm for TOPEX/Poseidon. To gain the most from these simultaneous datasets it is necessary to improve the orbital accuracy of ERS-1 so that it is commensurate with that of TOPEX/Poseidon. For the integration of these two datasets it is also necessary to determine the altimeter and sea state biases for each of the satellites. Several models for the sea state bias of ERS-1 are considered by analysis of the ERS-1 single satellite crossovers. The model adopted consists of the sea state bias as a percentage of the significant wave height, namely 5.95%. The removal of ERS-1 orbit error and recovery of an ERS-1 - TOPEX/Poseidon relative bias are both achieved by analysis of dual crossover residuals. The gravitational field based radial orbit error is modelled by a finite Fourier expansion series with the dominant frequencies determined by analysis of the JGM-2 co-variance matrix. Periodic and secular terms to model the errors due to atmospheric density, solar radiation pressure and initial state vector mis-modelling are also solved for. Validation of the dataset unification consists of comparing the mean sea surface topographies and annual variabilities derived from both the corrected and uncorrected ERS-1 orbits with those derived from TOPEX/Poseidon. The global and regional geographically fixed/variable orbit errors are also analysed pre and post correction, and a significant reduction is noted. Finally the use of dual/single satellite crossovers and repeat pass data, for the calibration of ERS-2 with respect to ERS-1 and TOPEX/Poseidon is shown by calculating the ERS-1/2 sea state and relative biases.

The cost-effectives mall spacecrafht as becomea n enablingt ool in the pursuit of near earth space commerce. Although small spacecraft have typically forgone the complexity and historically high cost of spacecraft propulsion, the inability to cost-effectively reach specific data gathering orbits from secondary launches presents a serious limitation to the small spacecraft industry. A cost-effective propulsion system capable of moving the secondary spacecraft from the launch orbit to the required mission orbit will effectively increase the number of viable secondary launch opportunities and in some cases provide a higher scientific or commercial return. Propulsion will also allow the dispersing of multiple spacecraft from a single launch vehicle and the inherent ability to de-orbit after a useful mission life. While other propulsion alternatives were considered in this research program, the hybrid rocket was identified as having high potential for suiting the established high-performance, lowcost and safety criteria. However, as this research has shown, the conventional hybrid rocket is not well suited to incorporation within small spacecraft; this is primarily due to the required length verses diameter (UD) to achieve high performance in the conventional hybrid. This research program has produced and tested a novel hybrid rocket engine. The all-new engine is significantly different from the conventional hybrid, exhibiting higher performance and with a geometry that drastically reduces hybrid rocket integration and operation issues. In addition, the new hybrid design has been successfully tested at higher volumetric loading factors than the conventionadl esignsi dentifiedi n the literature. The new alternative geometry hybrid rocket employs tangential oxidiser injectors that induce a vortex flow field to the centrally mounted rocket nozzle. The induced flow field has been shown to provide better fuel and oxidiser mixing. In addition, the tangential oxidiser injection provides an inherent film cooling effect for the combustion chamber wall, allowing the chamber to be fabricated of low cost materials. The new hybrid rocket engine was dubbed the Vortex Flow "Pancake" hybrid or "VFP". This researchp rogramr epresentsth e most technologicallya mbitiousp ropulsionr esearch program conducted by the Surrey Space Centre to date as the tools to analyse and design this engine had to be experimentally derived. Although the fundamental process of burning solid fuel remains unchanged, the combustion chamber gas-dynamics - so vital for predicting fuel liberation and performance within the conventional hybrid - are radically changed in the new configuration. Whereas the conventional hybrid has demonstrated a strong correlation with increasing combustion port diameter and fuel liberation, this research has shown that fuel liberation within the VFP does not obey any such relationships. Operationally, this research has shown that the VFP exhibits a higher fuel volumetric loading factor, higher combustion efficiency and less of an O/F (and consequent performance) shift than conventional designs. This research has proven the VFP to be superior to the conventional hybrid design in every aspect tested. However, this is only part of the benefit realised by the new VFP design as the external geometry of the VFP is the primary benefit enabling the technology to be applied to small spacecraft. Conventional hybrids need L/D ratios in excess of 15 to provide adequate performance, the novel VFP design has been regularly tested at UD's less than 1 with combustion efficiency very near 100%. This unique hybrid characteristic allows the VFP to be integrated on the outside of a spacecraft, in or as part of the spacecraft separation system. An externally mounted engine conserves centrally located spacecraft volume (reducing the need for multiple oxidiser tank scenarios). In addition, the external mount also allows waste heat to be radiated to space rather than other (internal) spacecraft components.

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