Improving the orbit estimates of GPS satellites

The Extended Center for Orbit Determination in Europe (CODE) Orbit Model, an empirical orbit model proposed by Beutler and colleagues in 1994, has been tested extensively since January 1996. Apart from six osculating Keplerian elements, this orbit model consists of nine (instead of the conventional two) parameters to take into account the deterministic part of the force field acting on the satellites. Based on the test results an improved orbit parameterization is proposed. The new orbit parameterization consists of the conventional two parameters plus three additional parameters, a constant and two periodic terms (a cosine and a sine term), in the X-direction to model the effects of the solar radiation pressure. Results based on one full year of routine orbit estimation, using the original and the new orbit parameterization, are presented to demonstrate the superiority of the new approach. Anl improvement of the orbit estimates with at least a factor of two is observed!