Thermal Modelling of GPS Block IIR Satellites for Precise Orbit Prediction

Sima Adhya

Abstract:	The mathematical and physical modelling of non-conservative forces is seen as a limiting factor in prediction of GPS satellite orbits. The accuracy with which their orbits can be determined in turn limits capabilities of applications dependent on GPS, ranging from the orbit determination of LEO spacecraft, through to issues of real-time realization of the global reference frame. One of these non-conservative forces is due to the anisotropic emission of thermal radiation from the GPS satellite. This force perturbs the spacecraft trajectory by a few metres after one 12 hour orbit and thus a thermal analysis is crucial for precise analytical force modelling. This paper discusses the author’s approach to dealing with the thermal force effects that act on GPS Block IIR satellites. The treatment of the spacecraft multilayer insulation, the solar panels and other spacecraft components is described and some attention is given to the author’s work on a novel approach to determining eclipse boundary crossings times for an oblate earth, and how the diminishing flux in penumbral regions is characterized, as both these factors impact upon the satellite’s thermal state. These methods are incorporated into a high precision radiation pressure model developed at UCL which deals with complex spacecraft structures and includes solar radiation pressure and albedo effects. The accuracy of the models are assessed using a numerical integration of the spacecraft dynamic model. This employs an 8th order embedded Runge-Kutta integrator with adaptive step-size control. High order GRACE (Gravity Recovery and Climate Experiment) gravity field coefficients are used and associated Legendre polynomials are generated with a numerically stable recursive formulation. International Earth Rotation Service conventions are used to compute frame transformations. Results are presented for errors in orbit predictions over 12 hours when compared to precise orbits before and after the thermal models are included. Results to demonstrate the effect of including the penumbra flux model are also given. Mean orbit prediction errors for the final model over a 12 hour arc were 0.19m in height, 0.08m across track and 0.59m along track, showing a large and consistent improvement compared to results obtained before inclusion of the thermal models.
Published in:	Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004) September 21 - 24, 2004 Long Beach Convention Center Long Beach, CA
Pages:	1732 - 1739
Cite this article:	Adhya, Sima, "Thermal Modelling of GPS Block IIR Satellites for Precise Orbit Prediction," Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004), Long Beach, CA, September 2004, pp. 1732-1739.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In