In-Flight Performance Validation of the TanDEM-X Autonomous Formation Flying System

Kurzfassung

The TanDEM-X formation flying mission forms currently a unique spaceborne configurable Synthetic Aperture Radar (SAR) interferometer. The primary objective of the mission consists in the generation of a global Digital Elevation Model (DEM) of the Earth with a 2 m height resolution by processing the SAR measurements collected by the two satellites composing the formation. After the completion of the primary objective, the mission will offer the possibility to investigate and demonstrate innovative SAR techniques such as Along-Track Interferometry (ATI) for the measurement of ocean currents. Such a SAR application is much more demanding in terms of formation control performance. Contrary to the coarse 200 m along-track accuracy required for routine global DEM acquisition, the along-track separation desired for ATI oceanography is only 50 m ± 10 m. This is quite challenging for a ground-in-the-loop formation control with typically 30 m RMS along-track control accuracy. Fortunately, the TanDEM-X formation is also equipped with an onboard autonomous formation keeping system able to fulfill such a control requirement. The TanDEM-X Autonomous Formation Flying (TAFF) system is designed to take over the formation maintenance nominally done by the ground segment. For simplicity, it has been decided that TAFF performs only in-plane formation keeping in order to avoid any rotation of the spacecraft whose thrusters are directed only in flight and anti-flight directions. TAFF is implemented as a standalone (relative) Guidance, Navigation and Control (GNC) system. The navigation and control algorithms are based on a special parameterization of the relative motion using relative orbit elements. The main advantage of this parameterization is to allow a quick insight in the geometry of the formation and to provide at the same time a simple criterion to assess the risk of collision. The relative navigation module uses the Earth-fixed navigation solutions coming from the GPS receivers of both spacecraft as measurements and implements a Kalman filter to estimate the six relative orbit elements describing the formation. The filtering is done using a simple dynamical model for the relative motion which considers only the perturbation due to the Earth's equatorial bulge (J2). This streamlined filter provides a continuous real-time onboard relative navigation even in the presence of GPS data gaps. The formation keeping is based on an analytical solution of the relative control problem. A rigorous stepwise validation procedure is required to operate TAFF in such a sensitive scientific mission. It has to be ensured that TAFF will never disturb the mission objectives. After several months dedicated to verify the reliability and the accuracy of the relative navigation, a three-day-long closed-loop functional test was first conducted in March 2011, where the proper behavior of the controller, the pertinence of the internal safety mechanisms and the correct interactions with the ground segment were checked. The successful outcome of this short test gave enough confidence to schedule a longer closed-loop campaign one year later. The paper presents the flight results of this 11-day long campaign conducted in June 2012. The objective of this experiment was to demonstrate the gain of control performance brought by TAFF and to verify its ability to reconfigure autonomously and quickly the formation. Overall the closed-loop campaign demonstrated relative control performance better than 10 m. TAFF was shown to be operationally robust (no anomaly was reported during the campaign), easy to operate and fully predictable, which is of relevance for the mission planning activities. This campaign concluded successfully the series of validation activities for TAFF, paving the way for the support of future ATI experiments.