Flight Dynamics Operations of the Tandem-X Formation

Abstract

Since end of 2010 the German TerraSAR-X and TanDEM-X satellites are routinely operated as the first configurable single-pass Synthetic Aperture Radar interferometer in space. The two 1340 kg satellites fly in a 514 km sun-synchronous orbit. In order to collect sufficient measurements for the generation of a global digital elevation model and to demonstrate new interferometric SAR techniques and applications, more than three years of formation flying are foreseen with flexible baselines ranging from 150 m to few kilometers. As a prerequisite for the close formation flight an extensive flight dynamics system was established at DLR/GSOC, which comprises of GPS-based absolute and relative navigation and impulsive orbit and formation control. Daily formation maintenance maneuvers are performed by TanDEM-X to counterbalance natural and artificial disturbances. The paper elaborates on the routine flight dynamics operations and its interactions with mission planning and ground-station network. The navigation and formation control concepts and the achieved control accuracy are briefly outlined. Furthermore, the paper addresses nonroutine operations experienced during formation acquisition, frequent formation reconfiguration, formation maintenance problems and space debris collision avoidance, which is even more challenging than for single-satellite operations. In particular two close approaches of debris are presented, which were experienced in March 2011 and April 2012. Finally, a formation break-up procedure is discussed which could be executed in case of severe onboard failures.

The Cold Gas System on TDX Accurate in-Orbit Evaluation

“…This perturbation is about 100 times larger than maneuver uncertainty or relative drag. Together with relative navigation errors a remarkable formation control accuracy of 5 m (RMS) in cross-track direction and 30 m (RMS) in along-track direction is achieved [5]. …”

“…Furthermore, the slightly different atmospheric drag as well as the relative (along-track) maneuver errors result in a change of the relative semi-major axis, which then yields an along-track drift. An illustration of the TDX-TSX relative motion and control can be found in [5].…”

“…In September 2011 however, after eleven months of formation flight, a switch to branch A had to be made, because of an unusual behavior of the prime branch (cf. [5] for details). Although investigations have shown that branch B is still operational, it was decided to continue the formation flight with branch A as operational prime.…”

Formation Flight in Low-Earth-Orbit at 150 m Distance - AOCS In-Orbit Experience

“…Once per 24 hours one maneuver pair is carried out, that consists of two maneuvers separated by half an orbit in order to correct the relative e-vector [8]. The 24 hour interval may be interrupted in case hydrazine maneuvers are made, which need to be calibrated first before new formation corrections can be planned.…”

Spacecraft Formation Guidance and Control on J2-Perturbed Eccentric Orbits

“…Multiple spacecraft formation flying is rapidly becoming a key space technology, which increases the efficiency, cost effectiveness and flexibility of an operational mission. Examples of spacecraft formation flying missions include GRACE, [1] TanDEM-X, [2] and PRISMA. [3] Despite the success of these past missions, future formation flying missions such as the ESA PROBA-3 mission will have to satisfy increasingly challenging functional and performance requirements in their guidance, navigation, and control systems.…”