Envisat ASAR Interferometry Baseline Results of the Current Envisat Orbit Maintenance Strategy

Abstract

The biggest and most advanced Earth Observation Satellite in-orbit, developed by the European Space Agency (ESA) and its member states, is Envisat. It was launched on March 1, 2002 by an Ariane V rocket from French Guyana and holds a total of 10 multidisciplinary Earth observation instruments, among which an Advanced Synthetic Aperture Radar (ASAR). The ASAR user community requested the Flight Dynamics division of the European Space Operations Centre (ESOC) to investigate how the orbit control maintenance strategy for Envisat could be changed to optimise ASAR interferometry opportunities overall and in addition support the International Polar Year 2007/2008 initiative. The Polar Regions play a pivotal role in understanding our planet and our impact on it as they are recognized as sensitive barometers of environmental change. One of the main themes of the International Polar Year 2007/2008 is therefore the study of Earth’s changing ice and snow, and its impact on our planet and our lives. Naturally, ESA is supporting this very important initiative by controlling Envisat in such a way ASAR interferometry opportunities are optimised. This paper briefly discusses the orbit control maintenance strategy that has been in place since the launch of Envisat. It also presents the at the start of 2007 adopted orbit control maintenance strategy that aims at improving and increasing the opportunities for Envisat ASAR interferometry, while preserving the fuel on board the spacecraft. The hydrazine on-board Envisat happens to be a precious resource as only approximately 300 kg of it was available at launch, like ERS-2. The difference being that the mass of Envisat is approximately 3.2 times that of ERS-2. The old orbit maintenance strategy effectively resulted in ASAR interferometry baselines of 2000 meters maximum. The new strategy on the other hand performs more regular orbit inclination manoeuvres, which reduces the baselines down to 250 meters over the polar region between certain repeat cycles, making a valuable contribution to the International Polar Year initiative. In addition, the maintenance of the orbit altitude was changed to reduce the baselines of the ASAR interferometry opportunities over the rest of the orbit. The remainder of this paper presents the actual baseline results since the start of 2007 as a consequence of the change in orbit control maintenance to verify its effectiveness. In addition this paper presents an evaluation of the adopted orbit control maintenance strategy in terms of actual fuel consumption, by comparing the actual numbers against the predicted ones that were taken into consideration when selecting the new orbit control maintenance strategy for Envisat.