Improvements for the CHAMP and GRACE Observation Model

In this chapter the theory and details of the implementation and validation of the GPS carrier phase wind-up correction and the GPS attitude model are presented. It is shown, that a correction of the carrier phase data due to the wind-up effect improves the GPS orbit fit together with the integer ambiguity fixing by 1–2 cm. It is also demonstrated, that changing the definition of the X-axis of the Block IIR satellites (towards Sun as for Block IIA) has no significant influence on orbits and clocks when integer ambiguity fixing is applied, since the difference due to the different axes definitions is completely absorbed by floating L3 ambiguities. The phase wind-up correction was also applied for one week of data for the Low Earth Orbiters CHAMP, GRACE-A and TerraSAR-X. The orbit improvement, measured by Satellite Laser Ranging (SLR) residuals is noticeable and amounts to 3 mm globally. The description of the GPS attitude model, including shadow crossing and post-shadow manoeuvre, noon and midnight turns is given in detail. It is demonstrated, that incorrect attitude modelling can cause yaw angle errors exceeding even the size of one full rotation of the satellite during the shadow crossing.