Autonomous Formation Flying - TanDEM-X, PRISMA and Beyond

Formation flying is commonly identified as the collective usage of two or more cooperative spacecraft to exercise the function of a single monolithic virtual instrument. The distribution of tasks and payloads among fleets of coordinated smaller satellites offers the possibility to overcome the classical limitations of traditional single-satellite systems. The science return is enhanced through observations made with larger, configurable baselines and an improved degree of redundancy can be achieved in the event of failures. Different classes of formation flying missions are currently under discussion within the European engineering and science community: technology demonstration missions (e.g. PRISMA, PROBA-3), synthetic aperture interferometers and gravimeters for Earth observation (e.g. TanDEM-X, postGOCE), dual spacecraft telescopes which aim at the detailed spectral investigation of astronomical sources (e.g., XEUS, SIMBOL-X), multi-spacecraft interferometers in the infrared and visible wavelength regions as a key to new astrophysics discoveries and to the direct search for terrestrial exoplanets (e.g., DARWIN, PEGASE). These missions are characterized by different levels of complexity, mainly dictated by the payload metrology and actuation needs, and require a high level of on-board autonomy to satisfy the continuously increasing demand of relative navigation and control accuracy. In order to respond to this demand the DLR’s German Space Operations Center (GSOC) is pursuing a dedicated autonomous formation flying research and development roadmap since 1998. The research work has largely been motivated by the conviction that only the development, deployment and on-orbit validation of innovative Guidance, Navigation and Control (GNC) techniques can bring formation flying to the forefront and enable the definitive transfer of this revolutionary technology to space. As a result the GSOC’s contributions to TanDEM-X and PRISMA (both launches expected in 2009) will demonstrate, for the first time in Europe, autonomous fuel-efficient formation keeping and reconfiguration on a routine basis, with minimum collision risk. After a comprehensive introduction on the state-of-the-art of the formation flying technology in Low Earth Orbit (LEO), the paper addresses the design, implementation and testing of the DLR/GSOC’s GNC subsystems for TanDEM-X and PRISMA demonstration missions. An overview of the developed subsystems is provided, highlighting communalities and differences of the two parallel developments. Furthermore key results from the validation of the guidance strategy, of the real-time GPS-based navigation and of the impulsive relative orbit control functions are presented. A technological gap clearly exists between the remote sensing LEO formations, yet to be demonstrated, and the planned outer space distributed telescopes in high elliptical orbits or in the vicinity of the Lagrange points. It is not only given by the envisaged three-order-of-magnitude improvement of the required metrology and actuation needs, but is also driven by the necessity of implementing navigation systems at altitudes above the GNSS constellations. The final part of the paper is thus devoted to the identification of the major discrepancies between present and next generation formation flying. An attempt is made to define the way forward and offer an outlook beyond the first European technology demonstration missions