Error Analysis of the Step Drag-Free System with Respect to Gravity Field Determination

摘要

In the satellite geodesy the gravity potential of the earth is determined from orbit analysis. For this purpose the potential is written as Fourier series along the orbit, periodicity is obtained once the satellite in its orbit and the orbit relative to an earth-fixed frame both have made an integer number of revolutions. The partial derivatives of the gravity potential are coupled with displacements in the satellite's position by means of the Hill equations. The displacements are measured with global positioning system (GPS) positioning. A model can be derived that couples the GPS measurements with the potential coefficients. The satellite is also subject to non-conservative forces, like air-drag. The non-conservative forces are deterministic noise for the (satellite geodesy) model that determines potential coefficients from GPS position measurements. A drag-free system aboard of the STEP satellite measures these disturbances and produces a contraforce by means of helium thrusters. By so doing, most of the drag is cancelled, but the system is not perfect, there remains a residual acceleration. The spectrum of this drag-free noise is continuous. The error spectrum of the potential coefficients is computed from the total of the GPS measurement noise and the drag-free noise and is presented in the thesis.