Taking the Long View: The Impact of Spacecraft Structural Design and High Precision Force Modelling on Long-Term Orbit Evolution

M. Ziebart, S. Adhya, A. Sibthorpe, P. Cross

Abstract:	This paper describes a study undertaken at University College London to assess the impact of GNSS spacecraft structural design and conventionally ignored force effects on the long-term evolution of the orbit. The impact of environmental changes (principally the long period variation in the solar irradiance, based on in-situ measurements by ACRIMSAT) and the degradation of surface material properties are considered. A comparison is made between cylindrical and rectangular bus structures. The modelling is based upon high precision non-conservative force models (radiation pressure and thermal re-radiation effects), high degree and order gravity field forces and planetary third body effects (using the JPL DE405 planetary ephemerides). This latter area is particularly relevant in the light of the current close position of Venus and the up coming nearest approach of Mars for 60,000 years. The non-conservative force modelling uses a newly developed suite of analysis utilities that can incorporate high levels of complexity in the spacecraft design. This modelling technique enables the quantification of secondary intersection effects, whereby radiation reflected from one part of the spacecraft strikes another part prior to being reflected back out into deep space. The thermal response of the spacecraft multi-layered insulation and solar panels is also considered. The utilities have been specifically written to assist in constellation design and orbit prediction studies, and have been tested using GPS Block IIR, GLONASS IIn and the microwave altimetry spacecraft JASON-1. The orbit determination uses an 8th order embedded Runge-Kutta integrator with adaptive step size control. The high order gravity field modelling uses a recursive formulation to generate the required associated Legendre polynomials in a numerically stable fashion. Frame transformations are computed using the International Earth Rotation Service standards. Force modelling during eclipse seasons is augmented by using an oblate Earth model for the occluding body in predicting the umbra/penumbra transitions. The accuracy of this technique in correctly predicting the epoch of the surface boundary transitions has been validated by independent photometric measurements of satellites at the satellite laser ranging facility in the UK. Many subtle effects that are not apparent over several days become significant drivers of the orbit evolution over longer periods. One key systematic difference between satellites using a cylindrical bus compared to those designed with a rectangular bus is the effect of secondary intersections from the spacecraft body onto the solar panels, and this is shown in the paper. The impact of all these effects on constellation design, station keeping and fuel optimisation is quantified and discussed. These considerations can be vital in terms of spacecraft lifetime, reduction of ground segment intervention, planning for thruster maneuvers and the building of value into the constellation performance by the pre-launch analysis of dynamics. The potential for erroneous constellation and space hardware selection decisions derived from the analysis of simple dynamic models is outlined.
Published in:	Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003) September 9 - 12, 2003 Oregon Convention Center Portland, OR
Pages:	1002 - 1008
Cite this article:	Ziebart, M., Adhya, S., Sibthorpe, A., Cross, P., "Taking the Long View: The Impact of Spacecraft Structural Design and High Precision Force Modelling on Long-Term Orbit Evolution," Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003), Portland, OR, September 2003, pp. 1002-1008.
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