Sibthorpe, Anthony John; (2006) Precision non-conservative force modelling for Low Earth Orbiting spacecraft. Doctoral thesis , UCL (University College London).

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Abstract

Low Earth Orbiting spacecraft are used in various ways for remote observation and measurement of system Earth some classes of measurements are only useful when modelled in a spatial reference frame. As the position of a satellite at a particular epoch is used to provide a fixed point of reference, it is vital that we know these positions both accurately and precisely. Non-conservative forces, which change the energy state of a spacecraft system, can have a dramatic effect on the estimated position of a satellite if unmodelled or, as is often the case, are modelled only crudely. Downstream Earth observation data can inherit significant errors as a result. As an example of this, it has been recognised that apparent long wavelength signals can be introduced into interferometric synthetic aperture radar (SAR) images by orbit error. Such images are used to monitor surface deformation, and may provide an indication of strain accumulation as a pre-cursor to Earthquake activity. It makes sense therefore to better model these non-conservative forces, thus improving the quality of the Earth observation data. This project develops precise methodologies for modelling of solar radiation pressure / thermal re-radiation / eclipse modelling / Earth radiation pressure / spacecraft internal heat distribution / on-board instrument power output, and applies these techniques to the European Space Agency's ENVISAT satellite. This complicated satellite has necessitated the development of a significant number of new algorithms for dealing with a large number of geometric primitives. A graphical display tool, developed during this research, allows rapid model development and improved error checking. Resultant models are incorporated into the GEODYN II orbit determination software, developed at NASA's Goddard Space Flight Centre. Precise orbits computed using tracking data in combination with the newly developed force models are compared against precise orbits generated using nominal force models to ascertain the improvement this modelling effort affords. The new models are shown to systematically decrease both measurement residuals and empirically estimated once-per-revolutions accelerations. As a result, the ENVISAT model has now also been adopted in the GIPSY-OASIS II orbit determination software developed by NASA's Jet Propulsion Laboratory.

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