Martin Wermuth

GOCE Data Analysis: From Calibrated Measurements to the Global Earth Gravity Field

The goal of this chapter is to describe an in-situ approach to determine a global Earth gravity model and its variance/covariance information on the basis of calibrated measurements from the GOCE mission. As the main characteristics of this procedure, the GOCE data are processed sequentially on a parallel computer system, iteratively via application of the method of preconditioned conjugate gradient multiple adjustment (PCGMA), and in situ via development of the functionals at the actual location and orientation of the gradiometer. We will further explain the adaption of the unknown stochastic model, determined by estimating decorrelation filters and variance components with respect to the GOCE observation types (i.e. SST, SGG, and regularizing prior information).

Safe Picosatellite Release from a Small Satellite Carrier

The Berlin Infrared Optical System satellite, which is scheduled for launch in 2016, will carry onboard a picosatellite and release it through a spring mechanism. After separation, it will perform proximity maneuvers in formation with the picosatellite solely based on optical navigation. Therefore, it is necessary to keep the distance of the two spacecraft within certain boundaries. This is especially challenging because the employed standard spring mechanism is designed to impart a separation velocity to the picosatellite. A maneuver strategy is developed in the framework of relative orbital elements. The goal is to prevent loss of formation while mitigating collision risk. The main design driver is the performance uncertainty of the release mechanism. The analyzed strategy consists of two maneuvers: the separation itself, and a drift-reduction maneuver of the Berlin Infrared Optical System satellite after 1.5 revolutions. The selected maneuver parameters are validated in a Monte Carlo simulation. It is ...

Flight Dynamics Operations of the TanDEM-X Formation

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 non-routine 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.

Long-Term Validation of TerraSAR-X and TanDEM-X Orbit Solutions with Laser and Radar Measurements

Precise orbit determination solutions for the two spacecrafts TerraSAR-X (TSX) and TanDEM-X (TDX) are operationally computed at the German Space Operations Center (GSOC/DLR). This publication makes use of 6 years of TSX and TDX orbit solutions for a detailed orbit validation. The validation compares the standard orbit products with newly determined enhanced orbit solutions, which additionally consider GPS ambiguity fixing and utilize a macro model for modeling non-gravitational forces. The technique of satellite laser ranging (SLR) serves as a key measure for validating the derived orbit solutions. In addition, the synthetic aperture radar (SAR) instruments on-board both spacecrafts are for the first time employed for orbit validation. Both the microwave instrument and the optical laser approach are compared and assessed. The average SLR residuals, obtained from the TSX and TDX enhanced orbit solutions within the analysis period, are at 1.6 ± 11.4 mm ( 1 σ ) and 1.2 ± 12.5 mm, respectively. Compared to the standard orbit products, this is an improvement of 33 % in standard deviation. The corresponding radar range biases are in the same order and amount to − 3.5 ± 12.5 mm and 4.5 ± 14.9 mm. Along with the millimeter level position offsets in radial, along-track and cross-track inferred from the SLR data on a monthly basis, the results confirm the advantage of the enhanced orbit solutions over the standard orbit products.

Operational precise baseline determination for TanDEM-X DEM processing

This paper evaluates the quality of the operational TanDEM-X baseline products. Three critical components that affect the baseline quality, namely maneuver handling, time synchronization and eclipse handling, are identified and analyzed in the context of long-term baseline comparison. Based on the analysis, a baseline comparison between three official baseline products through the bi-static period are performed. The comparison shows a good match between the baselines, with a sub-millimeter standard deviation of the baseline difference and a very stable bias track.

Challenging the Precision - Impact and Comparison of Non-Gravitational Force Models on Sentinel-3A Orbit Determination

Since the beginning of satellite altimetry missions, the ocean surface topography community requires precise and accurate satellite orbits. With start of the satellite Sentinel-3A on February 16, 2016, two radio- and one altimeter onboard the satellite accomplish the Copernicus program with a ocean- and land monitoring mission, planned for a nominal mission lifetime of 7 years. The satellite is orbiting the Earth on a polar, Sun-synchronous trajectory at an altitude of 815 km. For the purpose of precise orbit determination, the satellite is equipped with a geodetic-grade dual-frequency Global Positioning System (GPS) receiver. The GPS measurements are employed together with a set of gravitational and non-gravitational models in a Reduced-Dynamic Orbit Determination (RDOD) approach, which combines the advantages of a dynamic and a kinematic positioning for deriving precise satellite orbits. However, especially the non-gravitational force models require sophisticated modeling techniques. Therefore, a satellite macro model is introduced, which allows a proper modeling of accelerations due to Solar Radiation Pressure (SRP), Earth Radiation Pressure (ERP), and atmospheric drag. Especially the Sun-synchronous orbit, and the huge solar array, which is loosely coupled to the satellite body, makes the precise orbit determination challenging. As members of the Copernicus Quality Working Group, the Astronomical Institute of the University of Bern (AIUB), and the German Aerospace Center (DLR) are, among others, responsible for the orbit validation of Sentinel-3A. Both groups make use of a satellite macro model within a reduced-dynamic approach but differ in the employed software solutions and the pseudo-stochastic modeling. Basically, the pseudo-stochastic parameters allow to compensate potential deficits in the employed force models. Within this poster, the modeling aspects are briefly introduced, followed by comparing the results of both groups. The results include the estimated empirical accelerations, and the estimated satellite orbits of Sentinel-3A. Furthermore, the orbit quality is assessed by Satellite Laser Ranging (SLR), an external and independent tool for orbit validation.

Two years of TanDEM-X baseline determination

The TanDEM-X mission is a German dual satellite formation with the task to acquire a global digital elevation model (DEM) by bistatic interferometric synthetic aperture radar (SAR) data takes. In order to reach the intended DEM accuracy, the two satellites are kept in formation with a distance of less than 500 m and the baseline vector between the two spacecraft needs to be determined with an accuracy of 1 mm. The baseline determination uses observations from the onboard GPS receivers. The baseline vector between the satellites is determined by three independent software packages as there is no independent instrument onboard to verify the GPS results. In studies prior to the mission, comparisons between independent software packages showed biases of a few millimetres. In order to ensure the highest accuracy, a baseline calibration and combination process has been installed. The baseline products are validated by dedicated baseline calibration data takes over test sites, where the DEM is well known. Finally, the different solutions are merged in order to further reduce the errors.