Navigation and Control of the TanDEM-X Formation

Abstract

Germany is presently preparing the first operational formation flying mission for Synthetic Aperture Radar (SAR) interferometry in low Earth orbit. TanDEM-X comprises two nearly identical satellites (TSX and TDX) that are launched with a two year time shift in 2007 and 2009, respectively. From 2009 onwards, the two satellites will fly in close proximity and collect SAR interferograms for digital elevation model (DEM) generation. The TanDEM-X mission profile is particularly challenging from a flight dynamics point of view and poses new needs for spacecraft navigation and control. These comprise the formation design, the ground-controlled and autonomous formation maintenance, as well the high-precision reconstruction of the interferometric baseline. The paper discusses the geometry of the TanDEM-X formation along with a relative motion model that forms the basis of the formation control concept and the autonomous onboard navigation. Furthermore, the orbit control and precise orbit determination of the primary spacecraft TSX is illustrated using actual flight data from the first 6 months of operations.