Potential of global SAR positioning for geodetic applications - Lessons learned from TerraSAR-X and Sentinel-1

With our implementation of geodetic techniques for data processing and data corrections, spaceborne Synthetic Aperture Radar (SAR) has attained the possibility of fixing global positions of dedicated radar points at the low centimeter accuracy level. Such points can be created by passive radar corner reflectors, and the positioning method relies on the inherent ranging capabilities of SAR sensors. Thus, we may refer to the method as SAR imaging geodesy or geodetic SAR. Determining accurate long-term global positions of objects on the Earth’s surface is typically associated with Global Navigation Satellite Systems (GNSS) and one of the core elements of modern space geodesy. In order to do so, high-grade geodetic equipment with constant power supply, as well as the possibility for data transfer are required, limiting dense application on a large scale and poses difficulties for very remote areas with little or no infrastructure. Whereas certain regions like Japan or the San Andreas Fault are densely covered by GNSS such coverage may not be achievable everywhere on the globe. To improve the situation, we present a concept of jointly using SAR and GNSS for expanding geodetic positioning to applications requiring long-term coordinate monitoring. In future, the use of cost-effective passive reflectors in X-band SAR or low-cost battery-powered active transponders, which are currently in development for C-band SAR, could provide global coordinates anywhere where SAR imagery is acquired under multiple incidence angles. The main requirements are precise orbit determination, processing of the SAR imagery omitting geometric approximations, as well as the rigorous correction of perturbations caused by atmospheric path delay and signals of the dynamic Earth. If a reflector or transponder already has known reference coordinates, e.g. from co-location with GNSS, the perturbing signals can be mitigated for the surrounding radar points by applying differential SAR positioning techniques similar to differential GNSS, provided that all the points are included in the same radar image. In this contribution we discuss the geodetic SAR methods with respect to our experiences gained with the TerraSAR-X mission, and present first results of experiments carried out with Sentinel-1 data

Cross-calibration of the TRIG and PODRIX GNSS receivers onboard Sentinel-6A

The Sentinel 6A (S6A) satellite ("Michael Freilich") hosts a unique complement of sensors for precise orbit determination (POD). Aside from a Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) and a laser-retroreflector array (LRA) for satellite laser ranging (SLR), the spacecraft is equipped with three Global Navigation Satellite Systems (GNSS) receivers. These include a redundant pair of PODRIX receivers supporting GPS and Galileo tracking as well as a TRIG receiver supporting GPS tracking and radio occultation measurements. All three receivers are connected to high-performance GNSS antennas based on a patch-excited cup (PEC) design with choke rings for enhanced multipath mitigation. Simultaneous operations of individual receivers or receiver pairs of S6A enables determination of the relative antenna phase centers through ambiguity resolved differential carrier phase observations. The results demonstrate a good consistency of the PODRIX-PODRIX antenna baseline with nominal coordinates provided by the manufacturer, but systematic deviations for the TRIG-PODRIX baselines in all axes. While offsets in boresight direction may be expected due to minor differences in the antenna design and a limited availability of phase pattern calibration data, offsets of 9 mm and 14 mm w.r.t. to the design values may be noted in the direction of the longitudinal and lateral s/c axes, respectively. Other than antenna offset calibrations in single-receiver POD, the calibrations of relative antenna positions is unaffected by nongravitational force modeling uncertainties and can be obtained with good confidence from purely kinematic GNSS measurements. Concerning baseline inconsistencies in longitudinal, i.e. flight direction, timing offsets in GNSS receivers have earlier been identified as a cause of along-track position errors in GNSS receivers. By way of example, a 1 micro-second error in the latching of carrier phase measurements will cause a 7 mm along-track position error. To disentangle timing errors from geometric antenna position errors, measurements collected in limited phases of a reversed flight orientation were used. These suggest that the antenna baseline in longitudinal (+x) direction matches the design values, leaving a 1.2 micro-second inconsistency between PODRIX and TRIG receivers as the main cause of the apparent along-track baseline error. Independent validation of TRIG and PODRIX POD results through SLR observations suggest that the timing offset can largely be attributed to the TRIG receiver, while the PODRIX timing appears consistent with the SLR data. Dedicated GNSS signal simulator tests are recommended for the preflight validation of the next Sentinel-6B spacecraft to further consolidate these findings and a thorough review of spacecraft design information is encouraged to resolve the baseline inconsistency in cross-track direction

Sentinel-6A precise orbit determination using a combined GPS/Galileo receiver

The Sentinel-6 (or Jason-CS) altimetry mission provides a long-term extension of the Topex and Jason-1/2/3 missions for ocean surface topography monitoring. Analysis of altimeter data relies on highly-accurate knowledge of the orbital position and requires radial RMS orbit errors of less than 1.5 cm. For precise orbit determination (POD), the Sentinel-6A spacecraft is equipped with a dual-constellation GNSS receiver. We present the results of Sentinel-6A POD solutions for the first 6months since launch and demonstrate a 1-cm consistency of ambiguity-fixed GPS-only and Galileo-only solutions with the dual-constellation product. A similar performance (1.3 cm 3D RMS) is achieved in the comparison of kinematic and reduced dynamic orbits. While Galileo measurements exhibit 30-50% smaller RMS errors than those of GPS, the POD benefits most from the availability of an increased number of satellites in the combined dual-frequency solution. Considering obvious uncertainties in the pre-mission calibration of the GNSS receiver antenna, an independent inflight calibration of the phase centers for GPS and Galileo signal frequencies is required. As such, Galileo observations cannot provide independent scale information and the estimated orbital height is ultimately driven by the employed forces models and knowledge of the center of-mass location within the spacecraft. Using satellite laser ranging (SLR) from selected high-performance stations, a better than 1 cm RMS consistency of SLR normal points with the GNSS-based orbits is obtained, which further improves to 6mm RMS when adjusting site-specific corrections to station positions and ranging biases. For the radial orbit component, a bias of less than 1mm is found from the SLR analysis relative to the mean height of 13 high-performance SLR stations. Overall, the reduced-dynamic orbit determination based on GPS and Galileo tracking is considered to readily meet the altimetry-related Sentinel-6 mission needs for RMS height errors of less than 1.5 cm

Flight Dynamics Experience on Galileo Station-Acquisition Operations

December 5 2021 marked the start of Galileo’s eleventh launch, L11, from the Guiana Space Centre in Kourou with a Soyuz carrying the latest two Galileo spacecraft, GSAT0223 and GSAT0224. For the first time, the Flight Dynamics operations was under the full responsibility of DLR GfR - in close cooperation with DLR German Space Operations Centre’s Flight Dynamics team - conducted from within the Galileo Control Centre in Oberpfaffenhofen, Germany. The preparation and execution of the Galileo station-acquisition operations are described, focusing on the close collaboration between both Flight Dynamics teams. The paper explains the mission analysis to define a manoeuvre strategy of three drift-start manoeuvres, three drift-stop manoeuvres and up to six fine-positioning manoeuvres after separating from the Soyuz launcher. While the target acquisition method was laid out and the Flight Dynamics teams were trained and prepared for mission execution, sources of dispersion were introduced during L11 operations, causing the operational manoeuvre strategy timeline to diverge from the nominal timeline originating from the mission analysis. By investigating the divergence between station-acquisition manoeuvre plan and manoeuvre execution, this paper shows an assessment on the robustness of the mission planning and operation procedures of the Flight Dynamics teams. Outlining the refinements that needed to be introduced during operations - in order to react to these sources of dispersion - are an important aspect of this paper. The main sources of dispersion mentioned in this paper are: (1) four launch delays, which is more than covered by the ESA-required mission analysis including two delays; (2) injection assessment and separation; (3) orbit determination and propagation; and (4) thruster activity early in the spacecraft’s life. Analysing the effect of these sources of dispersion led to valuable insights and lessons learned for upcoming launches. An example recommendation is to extend the time in between fine-positioning manoeuvres in order to improve the orbit determination process. In its turn, it allows for a better assessment in the decision-making process whether to execute an additional fine-positioning manoeuvre to reach the target slot. Ultimately, the successful L11 stems from an efficient collaboration between both Flight Dynamics teams so that GSAT0223 reached its target slot B03 after 10 manoeuvres: three drift-start, three drift-stop and four additional fine-positioning manoeuvres. GSAT0224 needed one additional fine-positioning manoeuvre to reach its target slot B15

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

Implementation and evaluation of a multi-level mental health promotion intervention for the workplace (MENTUPP): study protocol for a cluster randomised controlled trial

Background Well-organised and managed workplaces can be a source of wellbeing. The construction, healthcare and information and communication technology sectors are characterised by work-related stressors (e.g. high workloads, tight deadlines) which are associated with poorer mental health and wellbeing. The MENTUPP intervention is a flexibly delivered, multi-level approach to supporting small- and medium-sized enterprises (SMEs) in creating mentally healthy workplaces. The online intervention is tailored to each sector and designed to support employees and leaders dealing with mental health difficulties (e.g. stress), clinical level anxiety and depression, and combatting mental health-related stigma. This paper presents the protocol for the cluster randomised controlled trial (cRCT) of the MENTUPP intervention in eight European countries and Australia. Methods Each intervention country will aim to recruit at least two SMEs in each of the three sectors. The design of the cRCT is based on the experiences of a pilot study and guided by a Theory of Change process that describes how the intervention is assumed to work. SMEs will be randomly assigned to the intervention or control conditions. The aim of the cRCT is to assess whether the MENTUPP intervention is effective in improving mental health and wellbeing (primary outcome) and reducing stigma, depression and suicidal behaviour (secondary outcome) in employees. The study will also involve a process and economic evaluation. Conclusions At present, there is no known multi-level, tailored, flexible and accessible workplace-based intervention for the prevention of non-clinical and clinical symptoms of depression, anxiety and burnout, and the promotion of mental wellbeing. The results of this study will provide a comprehensive overview of the implementation and effectiveness of such an intervention in a variety of contexts, languages and cultures leading to the overall goal of delivering an evidence-based intervention for mental health in the workplace

Erstellung und Validierung eines FORMOSAT Imports für die automatische Prozesskette CATENA

Erstellung und Validierung eines FORMOSAT Imports für die automatische Prozesskette CATEN

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

Refinement of Reduced-Dynamic Orbit Determination for Low Earth Satellites

Precise orbit information for Earth observation satellites gains ever more importance as the request for high quality remote sensing products increases. Global PositioningSystem (GPS)-based reduced-dynamic orbit determination has evolved as the state-of-the-art for high-precision orbit estimation of low altitude spacecraft. It combines a priori models of the spacecraft dynamics with varying levels of empirical parameters to best exploit the high precision of the available GPS observations. The optimum trade-off between the quality of dynamical models, the required level of stochastic parameters, and the geometric strength of the GPS observations is a matter of ongoing research and focus of this work. Given the high quality of orbit determination solutions that has been demonstrated even without any non-gravitational force models in missions such as CHAllenging Minisatellite Payload (CHAMP), Gravity Recovery And Climate Experiment (GRACE), Gravity field and steady-state Ocean Circulation Explorer (GOCE), and Swarm, the present thesis aims to answer the question whether and to what extent refined dynamical models and refined GPS processing techniques can contribute to further improve the achievable orbit determination accuracy. The study is based on a comprehensive set of current Earth observation missions in low Earth orbit that are equipped with geodetic-grade GPS receivers. This comprises the Sentinel-1A, Swarm-C, and TerraSAR-X/TanDEM-X missions with altitude of 450 km to 693 km, where notable perturbations due to atmospheric forces affect the satellite motion. For each of these missions, dedicated macro models have been established and used for the description of atmospheric drag and lift forces, solar radiation pressure, and Earth radiation pressure with targeted modeling accuracies at the one nm/s2 level. The benefit of using such models is assessed through different performance metrics including self-consistency tests, satellite laser ranging (SLR), and radar ranging as an external validation technique. Use of advanced atmospheric density models and spacecraft macro models for atmospheric forces is found to slightly reduce the associated empirical accelerations but does not allow to entirely waive the estimation of such parameters. With respect to radiation pressure, the macro model appear to be essential for a realistic description of Earth radiation pressure (ERP). In particular, it benefits the modeling of the radial ERP acceleration, which directly impacts the height leveling of the resulting orbit, and is such of key relevance for altimetry missions. With respect to GPS observation modeling, the use of ambiguity fixing is shown as a key technique to achieve improved orbit determination accuracy for all orbit geometries. Its benefit largely outweigh that of non-gravitational force models and contribute to major improvements in the horizontal (along-track/cross-track) position knowledge. On the other hand, the reduced vertical dilution of precision still makes the resulting orbit solutions sensitive to geometric orbit modeling errors in radial direction and justifies the use of refined radial acceleration models. Overall, ambiguity fixing can offer 33 % improvement in orbit determination accuracy and allows to reach a one-cm level (1-D) performance, as evidenced by the analysis of SLR residuals for the aforementioned missions. Radar-ranging is shown to enable independent validation of precise orbit determination solutions in Synthetic Aperture Radar (SAR) missions. However, it is not yet fully competitive with SLR in terms of both accuracy and coverage. In particular, it is confined to high resolution SAR imaging, and a global network of corner cube reflectors would be required for a wider use