Autonomous Navigation and Control of Formation Fliyng Spacecraft on the PRISMA Mission

In this paper in-flight results of the PRISMA on-board GPS based navigation system are presented. The onboard navigation performance is estimated through a comparison with the on-ground POD results and is evaluated in terms of accuracy requirements fulfilment and robustness in critical situations (e.g., attitude and orbit control maneuvers, large GPS data gaps). An overview is also given of the innovative and flexible PRISMA operations concept and the DLR’s PRISMA Experiment Control Center (ECC)

Spaceborne Autonomous and Ground Based Relative Orbit Control for the TerraSAR-X/TanDEM-X Formation

TerraSAR-X (TSX) and TanDEM-X (TDX) are two advanced synthetic aperture radar (SAR) satellites flying in formation. SAR interferometry allows a high resolution imaging of the Earth by processing SAR images obtained from two slightly different orbits. TSX operates as a repeat-pass interferometer in the first phase of its lifetime and will be supplemented after two years by TDX in order to produce digital elevation models (DEM) with unprecedented accuracy. Such a flying formation makes indeed possible a simultaneous interferometric data acquisition characterized by highly flexible baselines with range of variations between a few hundreds meters and several kilometers [1]. TSX has been successfully launched on the 15th of June, 2007. TDX is expected to be launched on the 31st of May, 2009. A safe and robust maintenance of the formation is based on the concept of relative eccentricity/inclination (e/i) vector separation whose efficiency has already been demonstrated during the Gravity Recovery and Climate Experiment (GRACE) [2]. Here, the satellite relative motion is parameterized by mean of relative orbit elements and the key idea is to align the relative eccentricity and inclination vectors to minimize the hazard of a collision. Previous studies have already shown the pertinence of this concept and have described the way of controlling the formation using an impulsive deterministic control law [3]. Despite the completely different relative orbit control requirements, the same approach can be applied to the TSX/TDX formation. The task of TDX is to maintain the close formation configuration by actively controlling its relative motion with respect to TSX, the leader of the formation. TDX must replicate the absolute orbit keeping maneuvers executed by TSX and also compensate the natural deviation of the relative e/i vectors. In fact the relative orbital elements of the formation tend to drift because of the secular non-keplerian perturbations acting on both satellites. The goal of the ground segment is thus to regularly correct this configuration by performing small orbit correction maneuvers on TDX. The ground station contacts are limited due to the geographic position of the station and the costs for contact time. Only with a polar ground station a contact visibility is possible every orbit for LEO satellites. TSX and TDX use only the Weilheim ground station (in the southern part of Germany) during routine operations. This station allows two scheduled contact per day for the nominal orbit configuration, meaning that the satellite conditions can be checked with an interval of 12 hours. While this limitation is usually not critical for single satellite operations, the visibility constraints drive the achievable orbit control accuracy for a LEO formation if a ground based approach is chosen. Along-track position uncertainties and maneuver execution errors affect the relative motion and can be compensated only after a ground station contact

A numerical approach to the problem of angles-only initial relative orbit determination in low earth orbit

A practical and effective numerical method is presented, aiming at solving the problem of initial relative orbit determination using solely line-of-sight measurements. The proposed approach exploits the small discrepancies which can be observed between a linear and a more advanced relative motion model. The method consists in systematically performing a series of least-squares adjustments at varying intersatellite distances in the vicinity of a family of collinear solutions coming from the linear theory. The solution presenting the smallest fitting residuals is then selected. The investigations specifically focus on the rendezvous in low Earth near-circular orbit with a noncooperative target. The objective is to determine the relative state of the formation using only bearing observations when the spacecraft are separated by a few dozen kilometers without any a priori additional information. The method is validated with flight data coming from the ARGON (2012) and AVANTI (2016) experiments. Both cases demonstrate that an observation time span of a few maneuver-free orbits is enough to compute a solution which can compete with Two-Line Elements in terms of accuracy

Navigation of Formation Flying Spacecraft using GPS: the PRISMA Technology Demonstration

A fundamental need of spacecraft autonomous formation flying is the determination of the relative motion between individual satellites in near real-time. For formation-flying in low Earth orbit (LEO), differential GPS represents an ideal sensor which can be used to directly measure the relative positions and velocities to a high level of accuracy with low costs. This paper addresses the design, implementation and validation of the GPS-based on-board navigation system for the PRISMA technology demonstration mission. The objective of the navigation system is to provide in real-time absolute and relative orbit information for the PRISMA space segment which consists of two satellites flying in formation in LEO. The key drivers for the design of the navigation system are the accuracy requirements on the absolute and relative orbit determination and prediction. Furthermore, a high level of robustness and flexibility imposed by the numerous formation flying scenarios and related thrust activities is required during the mission lifetime of about eight months. The paper focuses on the description of the navigation software architecture and algorithms. In contrast to earlier approaches that typically separate the GPS-based navigation task into the independent reconstruction of absolute and relative states, here a single reduced-dynamic Kalman filter has been developed which processes pseudorange and carrier-phase data from both spacecraft in order to exploit the full GPS measurements information at all times. Emphasis is given to key functional and performance test results of the GPS hardware/software navigation system once integrated into the PRISMA spacecraft in its flight configuration

Spaceborne Autonomous Formation Flying Experiment on the PRISMA Mission

The Prototype Research Instruments and Space Mission Technology Advancement (PRISMA) represents the first European technology demonstration of formation-flying and on-orbit-servicing techniques. Several hardware and software experiments, either at subsystem or system levels, have been successfully conducted since the launch of the dual-satellite mission in June 2010. This paper describes the guidance, navigation, and control functionalities and presents key flight results from the so-called Spaceborne Autonomous Formation-Flying Experiment (SAFE) executed in September 2010 and March 2011 as one of the primary PRISMA mission objectives. SAFE is intended to demonstrate autonomous acquisition, keeping, and reconfiguration of passive relative orbits for advanced remote sensing and rendezvous applications. As shown in the paper, the onboard Global Positioning System navigation system provides relative orbit information in real time with an accuracy better than 10 cm and 1 mm/s (threedimensional, root mean square) in position and velocity, respectively. The impulsive formation control achieves accuracies better than 10m (three-dimensional, root mean square) for separations below 2 km with minimum usage of thrusters, ensuring high predictability for simplified mission operations and minimum collision risk for increased safet

Generalized Multi-Impulsive Maneuvers for Optimum Spacecraft Rendezvous

This work describes the design of an impulsive maneuvers’ planner meant for on-board autonomous optimum formation flying reconfigurations. The whole variation of the relative orbit is stepwise achieved through intermediate configurations, so that passive safety and delta-v consumption minimization are pursued. The description of the relative motion is accomplished in terms of relative orbital elements and the reconfiguration plan takes into account mean effects due to the Earth oblateness coefficient and differential drag. Maneuvers consist of sets of triple tangential impulses and a single out-of-plane burn to establish each intermediate configuration. They are scheduled in time intervals compliant with the user-defined permissible time control windows

Navigation and Control of the TanDEM-X Formation

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

Autonomous Navigation and Control of Formation Flying Spacecraft on the PRISMA Mission

PRISMA is a small-satellite formation flying mission created by the Swedish National Space Board (SNSB) with the Swedish Space Corporation (SSC) as prime contractor and additional contributions from the German Aerospace Center (DLR), the French Space Agency (CNES), and the Technical University of Denmark (DTU). This mission will serve as a test platform for autonomous formation flying and rendezvous of spacecraft. PRISMA comprises a fully maneuverable small-satellite (MANGO) as well as a smaller sub-satellite (TANGO) which have been launched together in a clamped configuration on June 15th 2010 and separated in orbit after completion of all checkout operations. The mission schedule foresees a targeted lifetime of at least eight months. Through PRISMA, novel approaches in the areas of formation flying guidance, GPS based relative navigation, impulsive relative orbit control and space mission operations will have an in-flight validation. DLR’s key contributions comprise the on-board GPSbased absolute and relative navigation system, the Spaceborne Autonomous Formation Flying Experiment (SAFE), the Autonomous Orbit Keeping (AOK) experiment as well as the on-ground Precise Orbit Determination (POD) layer. In this paper in-flight results of the PRISMA on-board GPS based navigation system are presented. The onboard navigation performance is estimated through a comparison with the on-ground POD results and is evaluated in terms of accuracy requirements fulfilment and robustness in critical situations (e.g., attitude and orbit control maneuvers, large GPS data gaps). An overview is also given of the innovative and flexible PRISMA operations concept and the DLR’s PRISMA Experiment Control Center (ECC)

Autonomous Formation Flying Based on GPS - PRISMA Flight Results

This paper presents flight results from the early harvest of the Spaceborne Autonomous Formation Flying Experiment (SAFE) conducted in the frame of the PRISMA mission. SAFE represents one of the first demonstrations in low Earth orbit of an advanced guidance, navigation and control system for dual-spacecraft formations. Innovative techniques based on differential GPS-based navigation and relative orbital elements control are validated and tuned in orbit to fulfill the typical requirements of future distributed scientific instruments for remote sensin