Attitude characterisation of space objects using optical light curves

Abstract

This PhD thesis focuses on the relationship between space object attitude state and time-varying brightness signals. The large and increasing number of objects in Earth orbit has motivated improvements to the global Space Situational Awareness (SSA) capability. Radar installations are invaluable for observing space objects as they can operate 24-hours per day regardless of weather and, in the cases of phased array radars, can observe large numbers of objects simultaneously. However, these radar installations are extremely costly to build and operate. Optical telescopes can be built and operated at substantially reduced cost and therefore present an attractive alternative. For this reason, there is considerable interest in the global SSA community to maximise the potential of optical installations. One important area of interest for exploiting optical installations is to understand the relationship between a space object’s brightness and its attitude state. The brightness of an object is, in part, a function of its orientation with respect to the source of illumination and the observer. It is therefore possible to determine information about a space object’s attitude state by inverting light curve data. In this context, a light curve is a high cadence series of brightness measurements, typically in the visual band on the order of a few minutes in duration. This has been performed extensively in the literature for natural objects such as asteroids, and has also been applied to artificial space objects such as active or inactive spacecraft and rocket bodies. The aim of this PhD research was to develop novel methods to derive information on space objects from their light curves, and so augment present day SSA capabilities. The selected approach was to develop a synthetic light curve generation model which was then used to improve understanding of optical light curve data. This synthetic light curve model was applied to a wide range of simulated and real-world light curve examples. First, changes in standard deviation of light curves were used to differentiate active spacecraft from inactive spacecraft and identify possible failure dates. Second, the sensitivity of the synthetic light curve generation model to its various inputs was examined. Third, the relationship between illumination conditions and attitude state was determined to identify those conditions in which attitude states were most distinguishable. Fourth, a technique for determination of the angular velocity vector for rocket bodies was developed. Finally, the synthetic light curve model was used to used to determine reasons for the very bright appearance of Starlink satellites in the night sky and the role of the solar panels in this. These novel applications and results can be used to further improve the value of space object light curve data and their integration into present day SSA capabilities.

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ORCID for Hugh Lewis: orcid.org/0000-0002-3946-8757

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