The Copernicus program, and particularly Sentinel-1, are among the largest Earth Observation SAR data providers, serving an ever-increasing number of services, users, and applications. A key aspect of the program is the constant provision of quality data, which requires long term engagement to carefully monitor, preserve, and even improve the system performances. These tasks are mainly carried out within the SAR Mission Performance Cluster (SAR-MPC), an international consortium of SAR experts in charge of the continuous monitoring of the Sentinel-1 instruments status and of the L1 and L2 products quality. This paper provides an update on the monitoring and the actions implemented by the SAR-MPC during 2022 and 2023. The analysis regards the monitoring of the instrument status, the evaluation of the instrument radiometric and geometric accuracy, and the updates of the S-1 Instrument Processing Facility.

The Extended Timing Annotation Dataset (ETAD) product provides timing corrections for Sentinel-1 (S-1) single-look complex (SLC) data for improved geometric accuracy. Given the close relationship between path delay and radar signal phase, it also has the potential to serve as the basis for phase corrections in SAR interferometry (InSAR) applications with Sentinel-1. The provided timing corrections may be converted to phase offsets, which can be used to derive interferometric phase screens between pairs of repeat-pass acquisitions. In this work we explore the capabilities and limitations of ETAD to provide accurate interferometric phase corrections and introduce new features currently under investigation in the context of S1-ETAD scientific evolution study.

This paper introduces the extended timing annotation dataset (ETAD) product for Sentinel-1 (S-1) which was developed in a joint effort of German Aerospace Center (DLR) and the European Space Agency (ESA). It allows to correct range and azimuth timing of S-1 images for geophysical effects as well as for inaccuracies in synthetic aperture radar (SAR) image focusing. In combination with the precise orbit solution, these effects determine the absolute geolocation accuracy of S-1 SAR images and the relative collocation accuracy of repeat pass image stacks. ETAD contains the gridded timing corrections for the tropospheric and ionospheric path delays, the tidal-based surface displacements, and the SAR processing effects, all of which are computed for each data take using standard models from geodesy and auxiliary atmospheric data. The ETAD product helps S-1 users to significantly improve the geolocation accuracy of the S-1 SAR products to better than 0.2 m and offers a potential solution for correcting large scale interferometric phase variations. The product layout and the product generation are described schematically. The paper also reports first results for different SAR techniques: first, the improvement in geolocation accuracy down to a few centimeters by verification of accurately surveyed corner reflector positions in the range-azimuth plane; second, the well-established offset-tracking technique, that is used for systematic ice velocity monitoring of ice sheets and glaciers, where ETAD can reduce velocity biases down to sub-centimetric values; and third, the correction of atmospheric phase contributions in wide-area interferograms used for national and European ground motion services. These early results proof the added value of the ETAD corrections and that the product design is well suited to be integrated into the processing flows of established SAR applications such as absolute ranging of targets, speckle/feature tracking and interferometry.

This article shows how the array of corner reflectors (CRs) in Queensland, Australia, together with highly accurate geodetic synthetic aperture radar (SAR) techniques-also called imaging geodesy-can be used to measure the absolute and relative geometric fidelity of SAR missions. We describe, in detail, the end-to-end methodology and apply it to TerraSAR-X Stripmap (SM) and ScanSAR (SC) data and to Sentinel-1 interferometric wide swath (IW) data. Geometric distortions within images that are caused by commonly used SAR processor approximations are explained, and we show how to correct them during postprocessing. Our results, supported by the analysis of 140 images across the different SAR modes and using the 40 reflectors of the array, confirm our methodology and achieve the limits predicted by theory for both Sentinel-1 and TerraSAR-X. After our corrections, the Sentinel-1 residual errors are 6 cm in range and 26 cm in azimuth, including all error sources. The findings are confirmed by the mutual independent processing carried out at University of Zurich (UZH) and German Aerospace Center (DLR). This represents an improvement of the geolocation accuracy by approximately a factor of four in range and a factor of two in azimuth compared with the standard Sentinel-1 products. The TerraSAR-X results are even better. The achieved geolocation accuracy now approaches that of the global navigation satellite system (GNSS)-based survey of the CRs positions, which highlights the potential of the end-to-end SAR methodology for imaging geodesy.

We report our first results with Sentinel-1 Interferometric Wide Swath (IW) data using novel off-the-shelf electronic corner reflectors (ECRs) for geometric measurements with C-band Synthetic Aperture Radar (SAR). At the German Aerospace Center (DLR) campus in Oberpfaffenhofen, we set up an arrangement consisting of two trihedral corner reflector and two active ECRs. We describe the practical aspects of such ECRs as well as first radiometric characteristics. Moreover, we present geometric accuracy as derived from imaging geodesy, i.e. absolute radargrammetric positioning in 2D and 3D, as well as interferometric phase measurements.

Synthetic Aperture Radar (SAR) satellites observe range and azimuth geometric accuracies at the low centimeter level. The accuracy of geolocation is driven by several aspects, e.g. orbit determination, SAR image processing, or atmospheric error correction. Our paper concentrates on the Sentinel-1 mission and the compensation of the platform motion effects in the geolocation, which were found to limit the best possible geolocation capabilities of Sentinel-1. The key to advance the geolocation results is the rigorous compensation of the bistatic effect in azimuth, and the correction of the Doppler-induced shifts in range. First results for Sentinel-1 at the Australian reflector array consisting of 40 Corner Reflector (CR) show consistent improvement in the geolocation$(1\sigma)$to 6 cm in range and 28 cm in azimuth for both spacecrafts when using the Interferometric Wide swath (IW) product.

Geodetic stereo synthetic aperture radar (SAR) is capable of absolute 3-D localization of natural persistent scatterers, which allows for ground control point (GCP) generation using only SAR data. The prerequisite for the method to achieve high-precision results is the correct detection of common scatterers in SAR images acquired from different viewing geometries. In this contribution, we describe three strategies for automatic detection of identical targets in SAR images of urban areas taken from different orbit tracks. Moreover, a complete workflow for automatic generation of large number of GCPs using SAR data is presented and its applicability is shown by exploiting TerraSAR-X high-resolution spotlight images over the city of Oulu, Finland, and a test site in Berlin, Germany.

Geodetic stereo SAR is capable of absolute 3-D localization of natural persistent scatterers (PS)s which allows for Ground Control Point (GCP) generation using only SAR data. The prerequisite for the method to achieve high precision results is the correct detection of common scatterers in SAR images acquired from different viewing geometries. In this contribution, we describe three strategies for automatic detection of identical point targets in SAR images of urban areas taken from different orbit tracks. Moreover, a complete work-flow for automatic generation of large number of GCPs using SAR data is presented and its applicability is shown by exploiting TerraSAR-X high resolution spotlight images over the city of Oulu, Finland and a test site in Berlin, Germany.

The eldercare sector is a promising deployment area for robotics where robots can support staff and help to bridge the predicted staff-shortage. A requirement analysis showed that one field of robot-deployment could be supporting physical therapy of older adults with advanced dementia. To explore this possibility, a long-term autonomous robot was deployed as a walking group assistant at a care site for the first time. The robot accompanied two weekly walking groups for a month, offering visual and acoustic stimulation. Therapists' experience, the robot's influence on the dynamic of the group and the therapists' estimation of the robot's utility were assessed by a mixed methods design consisting of observations, interviews and rating scales. Findings suggest that a robot has the potential to enhance motivation, group coherence and also mood within the walking group. Furthermore, older adults show curiosity and openness towards the robot. However, robustness and reliability of the system must be high, otherwise technical problems quickly turn the robot from a useful assistant into a source of additional workload and exhaustion for therapists.

Robots are an increasingly discussed solution for assistance of seniors. Importance of testing natural interaction therefore becomes crucial. This paper presents first results of a study with an autonomous mobile social service robot prototype that was deployed in 18 private households of senior adults aged 75 years and older for a total of 371 days. Findings show that utility met the users' expectations. However, the robot was rather seen as a toy instead of being supportive for independent living. Furthermore, despite of an emergency function of the robot, perceived safety did not increase. Reasons for this might be the good health conditions of our users, a lack of technological robustness and slow performance of the prototype. However, users believed that a market ready version of the robot would be vital for supporting people who are more fragile and more socially isolated.

Only until recently, it has been demonstrated that absolute localization with centimeter accuracy can be achieved for manually matched Persistent Scatterer (PS)s from TerraSAR-X images acquired from cross-heading geometries [1]. This paper describes an automatic algorithm for absolute localization of natural PSs in SAR images, where the detection of potential PSs is aided by high resolution optical data. As the focus of the study is on urban area, the target detection part relies on identification of lamp posts using template matching. These targets are, most probably, the only ones visible in SAR images acquired from both ascending and descending orbits. Thus, the methodology includes identification of lamp posts from high resolution optical data and retrieves the precise absolute three-dimensional coordinates of the points from corrected TerraSAR-X timing measurements using the stereo SAR method [1]. Preliminary results for a test site in the city of Berlin acquired from TerraSAR-X high resolution spotlight mode are shown.

This paper evaluates the applicability and achievable SAR accuracy for octahedrons - a combination of eight corner reflectors with a common phase centre. In an experiment at the observatory in Wettzell from July to November 2015 these cost-efficient and mobile radar targets were measured with TerraSAR-X Staring Spotlight and High-Resolution Spotlight. Applying the geodetic stereo SAR concept, octahedrons are very robust for absolute 3D positioning through their backscattering in multiple directions. Using octahedrons with as size of 47 cm, we achieve 3σ standard deviations of about 3 cm for east, north and height components. For individual measurements in Staring Spotlight the standard deviation shows 1.4 cm in range and 3.2 cm in azimuth.

In this paper, we propose a framework referred to as “geodetic synthetic aperture radar (SAR) tomography” that fuses the SAR imaging geodesy and tomographic SAR inversion (TomoSAR) approaches to obtain absolute 3-D positions of a large amount of natural scatterers. The methodology is applied on four very high resolution TerraSAR-X spotlight image stacks acquired over the city of Berlin. Since all the TomoSAR estimates are relative to the same reference point object whose absolute 3-D positions are retrieved by means of stereo SAR, the point clouds reconstructed using data acquired from different viewing angles can be geodetically fused. To assess the accuracy of the position estimates, the resulting absolute shadow-free 3-D TomoSAR point clouds are compared with a digital surface model obtained by airborne LiDAR. It is demonstrated that an absolute positioning accuracy of around 20 cm and a meter-order relative positioning accuracy can be achieved by the proposed framework using TerraSAR-X data.

This paper presents preliminary results of a workflow for generating facade reference data as a basis for evaluating the localization capability of 3D SAR algorithms (tomographic or radargrammetric). In this context, the required facade model is provided based on photogrammetric reconstruction and terrestrial surveys with cm-accuracy. The SAR simulator RaySAR is used to provide the phase center positions of prominent scatterers in the world coordinate system. Simulation results in 2D and 3D are presented for a facade located at the TUM campus in Munich to indicate the necessary level-of-detail for the modeling step.

In this paper we propose a new SAR data product that shall allow for geometric corrections down to millimeter level. We propose methods to generate and architecture to represent a new type of geodetic SAR product. The product concept can easily be applied to different type of sensors as a kind geometric correction layer.

The paper describes the direct retrieval of global coordinates of persistent scatterers (PS) including their secular displacement by plate tectonics from Synthetic Aperture Radar (SAR). For this purpose we combine stereo SAR methods, least squares parameter estimation, and geodetic observation corrections. The procedure is based on our previous research with TerraSAR-X and now applied for PS situated on the facade of a building of Technische Universität München (TUM). In order to verify the PS-based solution, we have created an accurate building model with global coordinates suitable for SAR simulations, and we use the results of permanent Global Navigation Satellite Systems (GNSS) to validate the displacement rates. Our preliminary results for the simulated phase centers and stereo SAR already indicate similar phase centers on the building facade, and the displacement rates could be retrieved with mm/year accuracy.

The grammar of facade structures is often related to regularly distributed signature patterns in high-resolution synthetic aperture radar (SAR) images. Given those patterns in the imagery, they should be used as a source of information for identifying changes related to the facade. This paper presents a method for characterizing the layover area pertinent to regularly arranged facade structures, formulated on a general basis for single azimuth/range SAR images and geocoded SAR images. The analysis follows assumptions on the intensity distribution, the linear arrangement, and the regularity of point-like signatures. Two case studies on facades are presented, which confirm the applicability of the method for different building types. Based on that, the potentials and limitations of the algorithm are discussed with respect to applications such as change detection and persistent scatterer interferometry.

This paper reports on the absolute 3-D localization of radar corner reflectors and persistent scatterers (PSs) by stereo synthetic aperture radar (SAR) carried out with TerraSAR-X and TanDEM-X. The novel combination of rigorously linearized range-Doppler equations with thorough sensor modeling, consideration of atmospheric delays, geodynamic signals, satellite dynamics, and geometrical calibration allows a direct target localization at the centimeter level. Therefore, there is no need for the application of geocoding since our approach delivers 3-D positions directly in the International Terrestrial Reference Frame (ITRF) 2008. Four radar corner reflectors located at the Wettzell Geodetic Observatory, Metsähovi Geodetic Observatory, and German Antarctic Receiving Station O'Higgins were localized in 3-D with a precision better than 4 cm. By introducing an updated geometrical calibration of TerraSAR-X and TanDEM-X, the absolute accuracy of the same level becomes possible, which was demonstrated by externally validating the reflector positions at Metsähovi and Wettzell against results of terrestrial geodetic surveying. Furthermore, PSs located in Berlin were retrieved with a precision of 10 cm, making the method a suitable tool for radar “positioning” in urban environments. Potential fields of application for our approach are the joint analysis with phase-based methods, geometrical calibration of radar satellites, and direct integration of SAR observations into Global Navigation Satellite System networks.

Due to the indirect measurement principle of radar on base of signal travel time, a very precise calibration of the sensor's internal clock with regard to clock rate and potential time offsets relative to the Coordinated Universal Time is an essential prerequisite for accurate pixel localization in Synthetic Aperture Radar. Giving special considerations on this aspect, we developed two algorithms that improve the already high localization accuracy of TerraSAR-X: The either one very precisely determines the true oscillator clock rate, while the other one effectively refines the accuracy of time annotation. Experimental analyses evaluate the obtained improvements in localization accuracy from both techniques.

The German satellites TerraSAR-X/TanDEM-X are high resolution imaging SARs with a resolution in the meter and even sub-meter range. The original product specification document stated an absolute geometric pixel localization accuracy in the same order of magnitude i.e. 1 meter. It was shown by several teams that this accuracy requirement is easily met and even surpassed [1] by the operational products. During the last years the remaining residual error sources could be further studied and attributed mainly to tropospheric delay variations [2] geodynamic effects such as solid earth tides [3] and plate tectonics to refraction in the ionosphere and to technical limitations in the satellite [4]. Our team investigated all these contributions developed correction and calibration methods and validated them in experiments with corner reflectors at different locations. Furthermore we explore novel applications of this high geolocation accuracy that range from classical earth surface displacement measurements to the localization of scatterers in 3D space with a precision comparable to GNSS. This paper provides a summary of latest measurement results of our globally distributed corner reflectors which show consistently a high accuracy better than 2 cm in range and azimuth. Furthermore we report on new developments concerning tropospheric correction using ECMWF and numerical weather models (WRF). Using the best available compensation techniques we can demonstrate accurate 3D localization of corner reflectors and of other objects. Current investigations aim to further improve the orbital accuracy to sub-centimeter level by improved modelling of air-drag and solar radiation pressure on the spacecraft.

In this paper, we propose a framework referred to as “geodetic TomoSAR“ that fuses the SAR image geodesy and TomoSAR approaches to obtain absolute 3D positions of a large amount of natural scatterers. The methodology is applied on four Very High Resolution (VHR) TerraSAR-X spotlight image stacks acquired over the city of Berlin. Since the TomoSAR estimates are referred to the identical reference point whose absolute 3D positions are retrieved by means of Stereo-SAR, the point clouds from ascending and descending orbits are automatically fused. To assess the accuracy of the position estimates, the resulting absolute shadow-free 3D TomoSAR point clouds are compared to a DSM obtained by airborne LiDAR.

Previous studies have shown the unprecedented absolute pixel localization accuracy of the German SAR (Synthetic Aperture Radar) satellites TerraSAR-X and TanDEM-X. Now, by thoroughly correcting all signal path delays and geodynamic effects like tides, loadings and plate movements, range accuracies of about 1 centimeter are demonstrated to be attainable. While Global Navigation Satellite System (GNSS) data provide local correction values for the atmospheric delays, correction values for the geodynamic effects are based on the IERS (International Earth Rotation and Reference Systems Service) conventions. Our recent measurements are based on a corner reflector with very precisely known ground position which we installed at Wettzell, Germany, close to the local GNSS reference stations. Further comparable high precision test sites in the world are in progress and shall prove the worldwide reproducibility of the achieved results.

This paper presents a new algorithm for recognizing patterns pertinent to the layover of façades in very high resolution SAR images, serving the increasing demand for monitoring individual buildings. The grouping of signatures is based on the combination of a weighted Hough transform and the analysis of spectrum peaks. First results of pattern extraction are shown for an office building located in the Munich city area.