SEGORD: Sequential Orbit Determination Tool for Geostationary Satellites Operations

Abstract

The orbit determination for geostationary satellites has been traditionally performed using batch processes. These processes are nominally in charge of estimating the satellite state vector, the solar radiation perturbation scale factor and the unknown biases associated to the tracking system. Besides, the manoeuvres performed by the satellite are calibrated as part of the orbit determination process, either within the same batch process of by comparing the orbital solution before and after the manoeuvre. For this purpose, a considerably big set of measurements must be gathered by the tracking retrieval system forcing a considerably high latency to the solutions obtained. GMV has developed SEGORD for performing the orbit determination and manoeuvre calibration in near-real time following a fully automated sequential process based on the well-known SRIF (Square Root Information Filter) algorithm. This tool uses not only the latest collected measurements, but also the whole data history, weighted according to their age. Following this scheme, once the process is converged, the latency of the obtained orbital solution is remarkably reduced as it is regularly updated almost as soon as new measurements arrive from the TT&C stations. The stability of this orbital solution is also significantly enhanced as the amount of data actually used to obtain it increases constantly. SEGORD has been installed and tested at Telenor, the Norwegian leading European satellite-broadcasting operator, for performing the orbit determination operations of their new Thor 5 satellite (formerly Thor II-R) successfully launched on February 11th 2008. SEGORD will allow Telenor to automate the orbit determination operations for their growing fleet as well as to reduce the latency and increase the stability of their orbital solutions, which is nowadays widely required by all satellite operators. This paper describes the near-real time sequential orbit determination and manoeuvre calibration operations driven by the automation tool Autofocus and performed by SEGORD, both part of focusSuite, GMV's advanced off-the-shelf Flight Dynamics solution. The achieved orbit determination and manoeuvre calibration performances in an operational scenario will also be presented. This includes the behaviour when long lack of measurements happens and the required time to reach convergence after the process is started from scratch as well as the overall behaviour of the tool integrated within the operational Flight Dynamics System.