Satellite orbit theory for a small computer

This document describes a computer program put onto an LSI-11 microprocessor with 64KB of memory which can provide accurate ephemerides for GPS (Global Positioning System) satellites. The satellite dynamics include averaged orbital element rates due to J2, tesseral resonances, solar radiation pressure and third body perturbations from both the Moon and the Sun. These rates are first integrated up to and across a satellite pass of interest, and a two point Hermitian interpolating polynomial is established for each mean element. Short periodic Fourier coefficients due to J2 and the Moon and Sun are next computed, and three point Lagrangian interpolating polynomials are finally used to provide osculating orbital elements at arbitrary times during the pass. This computer program includes an analytical Lunar/Solar ephemeris so it is self-contained except for input mean orbital elements. Partial derivatives have been implemented which will give the capability to fit observations of the satellites and to consequently obtain the necessary mean elements. The program can be modified quite easily to handle synchronous satellites by modifying the subroutine modules for tesseral resonant perturbations and lunar-solar short-periodics. With the present overlay scheme, considerable expansion of the program is possible to obtain more accuracy and versatility.