Taking advantage of Capella’s ability to dwell on a target for an extended period of time (nominally 30s) in its spotlight (SP) mode, an unsupervised methodology for detecting moving targets in this data is presented in this paper. By colourizing short segments (sub-apertures) of the total imaging time, a colourised sub-aperture image (CSI) can be formed. This can be used in conjunction with well-established computer vision techniques to detect moving targets and track them in the SP image. In essence, the moving target detection problem is transformed from temporal image stack identification to colour segmentation in a single image. The presented detection and tracking are wholly unsupervised. Additionally, computer-vision-based tracking algorithms are demonstrated on detected movers and qualitatively assessed for accuracy of tracking.

Capella's first commercial Synthetic Aperture Radar (SAR) satellite was launched in August 2020. After more than one year of successful operations, Capella plans a continuous growth of both number of satellites and satellite capabilities. New tasking patterns allow the collection of pairs and time series of very high resolution (VHR) SAR images. A novel repeat tasking request pattern will enable SAR applications such as change detection and the exploitation of interferometric SAR (InSAR) techniques. The combination of on-board GPUs and dedicated on-board processing algorithms will be used for rapid target detection and minimized satellite downlink. On-board processing, applicable in many quasi real-time operational scenarios, can also be used for tipping and cueing other satellites to immediately capture a high resolution imagery. Finally, Capella Space Open Data program started in 2021 with new images added each quarter.

This letter proposes the exploitation of the overlapping areas between bursts of Sentinel-1 interferometric wide (IW) swath data for the estimation of ground azimuth deformation velocities from time series, which provides high sensitivity to the North–South direction. The availability of two separated Doppler spectral looks over these areas allows one to obtain high accurate estimates, overcoming the limited performance of classical correlation techniques with coarse resolution modes. Since the estimation is restricted to the (sparsely distributed) burst overlaps, its applicability is limited to geophysical phenomena characterized by a slow spatial variability, e.g., seismic scenarios. We demonstrate the suitability of this approach to time series over a region in Pakistan affected by post-seismic deformation. The estimated mean azimuth velocity performance from real data indicates an accuracy better than 7 mm/year over high long-term coherent areas for a three-year data stack.

The BIOMASS DEM Product Prototype Processor (BIO-DEMPP) is being developed in the frame of ESA's Earth Explorer BIOMASS mission. The prototype includes a complete interferometric SAR chain, from the stack co-registration until the mosaicking of the derived height products (Digital Elevation and Digital Terrain Models). This paper presents an overview of the BIODEMPP architectural design and its validation strategy, as well as first results obtained with simulated BIOMASS-like data.

Flooding is a hazard that impacts more people than any other type of disaster, and is increasing in frequency and magnitude due to climate and demographic change. Synthetic Aperture Radar (SAR) satellites are an excellent remote sensing data source for monitoring flood events, thanks to their ability to acquire data over the land surface in all-weather conditions. The Capella high-resolution X-Band SAR system will provide an unprecedented temporal revisit when its entire constellation is deployed. This contribution presents results with Capella data covering the inundation that occurred in Windsor, Australia, in March 2021. A qualitative comparison employing Sentinel-1 for the same event is as well provided.

Future spaceborne SAR satellites will achieve both high resolution and wide swath with their multiple receivers. In order to suppress range ambiguities, cyclic up and down chirp and azimuth phase coding have been widely applied. A multiple receiver system has lower pulse repetition frequency to increase the swath width and thus, the effect of azimuth phase coding to mitigate range ambiguities is decreased. In this paper, we reveal the possible combination of the two methods to overcome this drawback.

This contribution will present the investigations carried out with stacked time-series of TOPS acquisitions to retrieve azimuth displacements. The exploitation of the burst overlapping areas allows to retrieve highly accurate ground displacement estimates in that direction, mitigating the limited performance of classical correlation techniques, due to the low resolution of the mode. The focus of this contribution is the application to time-series of Sentinel-1 data, where slow azimuthal motion is expected. Results with Sentinel-1 data on the proposed methodology are provided for the region of Balochistan, in Pakistan.

Burst-mode acquisition schemes achieve wide coverage at the expense of a degraded azimuth resolution, reducing, therefore, the performance on the retrieval of ground displacements in the azimuth direction, when interferometric acquisitions are combined. Moreover, the azimuth varying line of sight can induce discontinuities in the interferometric phase when local azimuth displacements are present, e.g., due to ground deformation. In this contribution, we propose the interferometric 2-look terrain observation by progressive scans (TOPS) mode, a sustaining innovation, which records bursts of radar echoes of two separated slices of the Doppler spectrum. The spectral separation allows to exploit spectral diversity techniques, achieving sensitivities to azimuth displacements better than with StripMap and eliminating discontinuities in the interferometric phase. Moreover, some limitations of the TOPS mode to compensate ionospheric perturbations, in terms of data gaps or restricted sensitivity to azimuth shifts, are overcome. The design of 2-look TOPS acquisitions will be provided, taking the TerraSAR-X system as reference to derive achievable performances. The methodology for the retrieval of the azimuth displacement is exposed for the case of using pairs of images, as well as for the calculation of mean azimuth velocities when working with stacks. We include results with experimental TerraSAR-X acquisitions demonstrating its applicability for both scenarios.

The Terrain Observation by Progressive Scans (TOPS) SAR acquisition mode achieves wide coverage area by employing subapertures. This way multiple subswaths can be acquired. It overcomes the problems of scalloping and azimuth-varying ambiguities inherent to the ScanSAR mode by introducing a steering of the beam in the along-track direction. The drawback is that, due to the burst operation mode, the azimuth resolution is strongly worsened with respect to the conventional StripMap mode. This reduces the performance on the retrieval of the ground displacement in the azimuth direction. In this contribution we provide an update on the last developments with our proposed 2-looks TOPS mode. This mode has been devised to mitigate the degraded sensitivity on the retrieval of ground deformation azimuth shifts of 1-look TOPS acquisitions. We will present the last improvements to the mode as well as updated results with time-series of experimental TerraSAR-X data.

In this contribution, we propose the use of 2-looks Synthetic Aperture Radar data to retrieve azimuth velocities of moving targets with a single platform. The established technique to retrieve velocities of moving targets is Along-track Inter-ferometry (ATI), which provides a measurement of the velocity in the radar line of sight direction by employing two or more phase centers separated in the along-track direction. The use of 2-looks data allows to observe targets at two different instants of time, with a Doppler separation, enabling the retrieval of the target velocity in the azimuth direction. We introduce the 2-looks Terrain Observation by Progressive Scans (TOPS) mode, presenting the performance that can be achieved with the TerraSAR-X system. Moreover, we present experimental results with real data acquired with TerraSAR-X over coastal areas to retrieve velocities of vessels. A validation of the results with Automatic Identification System (AIS) data (ground truth) provides accuracies below 1 m/s.

This paper addresses the performance in the retrieval of 3-D mean deformation maps by exploiting simultaneous or quasi-simultaneous squinted synthetic aperture radar (SAR) interferometric acquisitions in a repeat-pass scenario. In multisatellite or multibeam low earth observation missions, the availability of two (or more) lines of sight (LOSs) allows the simultaneous acquisition of SAR images with different squint angles, hence improving the sensitivity to the north-south component of the deformation. Due to the simultaneity of the acquisitions, the troposphere will be highly correlated and, therefore, will tend to cancel out when performing the differential measurement between the interferograms obtained with the different LOSs, hence resulting in a practically troposphere-free estimation of the along-track deformation measurement. In practice, however, the atmospheric noise in the differential measurement will increase for increasing angular separations. This paper expounds the mathematical framework to derive the performance by properly considering the correlation of the atmospheric delays between the simultaneous acquisitions. To that aim, the hybrid Cramér-Rao bound is exploited making use of the autocorrelation function of the troposphere. Some performance examples are presented in the frame of future spaceborne SAR missions at C and L band.

The contribution focuses on the current status of the ESA study entitled “InSARAP Sentinel-1 Constellation Study”, which investigates the interferometric performance of the S1A/S1B units. General aspects like the interferometric compatibility in terms of common range and Doppler bandwidth and the burst synchronization are addressed. Besides the first interferometric results with both units, time series results over the pilot sites combining both satellites are also shown, as well as some investigations with fast moving (i.e., glaciers) scenarios.

The contribution focuses on the technical aspects related to the focusing and interferometric processing of bistatic data acquired by companion satellite (CS) SAR missions. In particular, the processing aspects related to the large along-track baseline configuration will be addressed, for the processing needs to properly consider a potential high squint angle. The technical challenges encompass synchronization, image formation and coregistration. On the other hand, the availability of a large squint angle has some advantages in terms of the sensitivity to the north-south component of the deformation for interferometric applications. The theoretical derivations are complemented with point-target simulations and Monte-Carlo simulations.

The coregistration of synthetic aperture radar images is of fundamental importance for the generation of interferograms. The high azimuth coregistration requirements imposed by the TOPS acquisition mode imply that an advanced approach for the coregistration of stacked time series images is needed due to temporal decorrelation effects. In some scenarios, the conventional approach of estimating the shifts pairwise with respect to the same master might result insufficient. Therefore, a joint estimation is proposed here, which exploits jointly all interferograms in order to retrieve more accurate results. Simulated data and Sentinel-1A images acquired in IW mode are used to validate this procedure, demonstrating the better performance of the joint approach when compared to the standard single-master approach.

This paper presents results of the Sentinel-1 sensor in the interferometric wide-swath (IW) mode encompassing the first two years of operation of the mission. The paper focuses on persistent scatterer interferometric results and their validation. Further applications and investigations are also addressed, e.g., earthquakes, volcanoes and tomography.

This contribution presents a set of experiments performed with the TerraSAR-X (TSX) and TanDEM-X (TDX) satellites, whose aim is to estimate the scene motion in the along-track direction in repeat-pass scenarios with an accuracy better than the one given by the stripmap azimuth resolution. The achievement of an enhanced performance is possible by exploiting the angular diversity of the following modes: (i) dual-beam stripmap modes, (ii) π-shifted (or staggered) TOPS, (iii) two-looks ScanSAR and (iv) two-looks TOPS. Interferometric results with these experimental modes are provided.

Sentinel-1 (S-1) has an unparalleled mapping capacity. In interferometric wide swath (IW) mode, three subswaths imaged in the novel Terrain Observation by Progressive Scans (TOPS) SAR mode result in a total swath width of 250 km. S-1 has become the European workhorse for large area mapping and interferometric monitoring at medium resolution. The interferometric processing of TOPS data however requires special consideration of the signal properties, resulting from the ScanSAR-type burst imaging and the antenna beam steering in azimuth. The high Doppler rate in azimuth sets very stringent coregistration requirements, making the use of enhanced spectral diversity (ESD) necessary to obtain the required fine azimuth coregistration accuracy. Other unique aspects of processing IW data, such as azimuth spectral filtering, image resampling, and data deramping and reramping, are reviewed, giving a recipe-like description that enables the user community to use S-1 IW mode repeat-pass SAR data. Interferometric results from S-1A are provided, demonstrating the mapping capacity of the S-1 system and its interferometric suitability for geophysical applications. An interferometric evaluation of a coherent interferometric pair over Salar de Uyuni, Bolivia, is provided, where several aspects related to coregistration, deramping, and synchronization are analyzed. Additionally, a spatiotemporal evaluation of the along-track shifts, which are directly related to the orbital/instrument timing error, measured from the SAR data is shown, which justifies the necessity to refine the azimuth shifts with ESD. The spatial evaluation indicates high stability of the azimuth shifts for several slices of a datatake.

This contribution presents two experiments performed with the TerraSAR-X (TSX) and TanDEM-X (TDX) satellites working in the pursuit monostatic configuration. Their objective is to estimate the along-track component of the motion in the scene in repeat-pass scenarios with an accuracy better than the one given by the stripmap azimuth resolution. Such performance is possible by exploiting the angular diversity of the bidirectional (BiDi) SAR mode and the π-shifted (or staggered) TOPS.

This paper presents a robust mis-registration estimator based on ESD method for TOPSAR interferometry. For the in-terferometric applications of TOPSAR mode data, a master and slave SLC pair requires a co-registration accuracy in the milli-pixel order due to the large Doppler centroid frequency variation. Although ESD method estimates the mis-registration with the required sensitivity, a prior fine co-registration step is a prerequisite. This paper presents an advanced co-registration method that relaxes the fine co-registration requirement of the ESD and improves the estimation sensitivity. Experimental results with Sentinel-1A data are shown to validate the proposed method.

This paper addresses the November 2013 Balochistan Earthquake. A co-seismic TerraSAR-X pair acquired in wide-swath ScanSAR mode has been used to derive two-dimensional deformation measurements (radar line-of-sight and azimuth direction) of the eastern part of the main M7.7 earthquake and the large M6.8 aftershock by correlating SAR amplitude images. Atmospheric and solid Earth tide corrections have been considered to achieve accuracy in the order of several centimeters. Correlation measurements from Landsat-8 images have been additionally estimated. The intention is to isolate vertical and horizontal components in order to obtain three-dimensional deformation measurements. Interferometric processing issues of ScanSAR data for non-stationary scenarios, specifically co-registration, are additionally discussed.

The aim of this work is to show two possible applications for Alternating Bistatic TanDEM-X data. The first application is a tomographic processing while the second is a 2D ship velocity estimation. Results over real data are presented for both applications.

This paper presents the processing results of the dual-baseline phase unwrapping correction chain of the Integrated TanDEM-X Processor. Second year data are operationally processed since August 2012 with the help of supporting CoSSCs. They are shifted of half a swath width with respect to the first year data. Supporting CoSSCs are the first year scenes. The first and second year scenes have different height of ambiguity but are taken from the same look angle. The chain has to deal with huge amount of data of different quality and from every part of the world landmass. A success rate of more than 97% is achieved overall.

The german TerraSAR-X and TanDEM-X satellites are able to acquire images operationally in ScanSAR mode and experimentally in TOPS mode, the future Sentinel-1 Interferometric Widesath and Extra Wideswath modes. This paper gives an overview of the interferometric processing steps of burst-mode acquisitions and present the current status in preparation for future algorithm development towards PSI. Interferometric results of TerraSAR-X repeat-pass images in ScanSAR and TOPS mode are shown.

Japan was struck by an M9.0 megathrust earthquake on March 11, 2011. Synthetic aperture radar (SAR) images from the TerraSAR-X satellite have been used to generate a ground motion map by means of correlation techniques. Geophysical corrections due to solid Earth tide and atmospheric path delay effects have been applied. These corrections can reach up to 20 cm in the radar line of sight. Using this approach, absolute displacements in the radar line of sight and in the satellite flight direction are determined. This letter shows the potential of correlation techniques for ground motion monitoring and a comparison with the interferometric technique. Using multiple scenes, a wide area displacement map is generated and quantitatively compared with GPS data, showing a divergence of about 15 cm.

We report on results from spaceborne velocity and topography measurements of the Drygalski glacier, Antarctic Peninsula. For our analysis we use a time series of TerraSAR-X data acquired between 2007 and 2010 which enables us to perform an error analysis and to determine higher order velocity parameters such as acceleration. Our estimated accuracy for velocity estimation is on the order of 2 cm/day and we find a weak slow down at the terminus of the glacier. Very first TanDEM-X single pass interferometric data sets acquired in 2010 allow us to reconstruct the 3D-glacier topography with unprecedented accuracy, delivering both, surface topography and roughness.

Since July 2010, TerraSAR-X and TanDEM-X jointly acquire interferometric data. Starting their common commissioning phase with a so called pursuit monostatic configuration with 3 seconds time lag between the two passes, they were later put in a close formation in October 2010, acting since then as the first freely configurable bistatic SAR interferometer in space. All operational acquisitions were processed from instrument raw data to DEMs from day one of the data taking on by one single processing system: the Integrated TanDEM-X Processor (ITP) (see [1],[2]). Data take analysis, common parameter calculation, synchronization, bistatic focusing, filtering, co-registration, phase unwrapping and geocoding are all performed in one sequence inside this processor. This approach allows a high precision processing by passing all applied corrections and determined parameters from one step to the next. Specifically the geometric & phase accuracy and stability of the instruments, the processor and the auxiliary data (i.e. the millimetric precision of the baseline products) provide an unprecedented level of relative and absolute geometric accuracy in the bistatic operation. While many challenges of bistatic processing of the TanDEM-X data are encountered, the benefits of this single pass acquisition mode can be used to derive additional information from the data itself for further processing and calibration. In this paper, we will outline the bistatic interferometric processing steps of the ITP and focus on the aspects of geometric accuracy in DEM generation.

On the 12thof January 2010, Haiti was struck by a M7.0 earthquake; the hypocenter (18.443°N, 72.57ΓW, Depth 13 km) was located very close to the capital Port-au-Prince, causing considerable damage to buildings and infrastructure in the city and its conurbation and a high number of human fatalities. For evaluation of the co-seismic surface shift caused by this earthquake, especially at the Enriquillo Plantain Garden fault zone (EPGF), TerraSAR-X Stripmap data from ascending and descending orbits were acquired and processed using InSAR and incoherent cross-correlation techniques. The results, azimuth and range offset maps, were then geocoded and corrected for Solid Earth Tides (SET) and atmospheric propagation delays, as described in Eineder et al [1] . Each data-set indicated strong surface deformations. By combining the different viewing geometries and performing a 3D-Inversion (ascending & descending crossing directions), 3D-motion vectors, describing the surface displacements caused by the earthquake could be generated. As a reference, pre-seismic and post-seismic interferometric pairs were processed accordingly. The results showed no indications for strong deformations and thus validated the results.

On June 21st, 2010, the German radar satellite TanDEM-X was launched and successfully placed in an orbit approaching the TerraSAR-X satellite until both systems will fly in close formation and will establish the only available bi-static interferometer in space. The primary TanDEM-X mission goal is to generate a global Digital Elevation Model (DEM) with a relative point-to-point height accuracy of 2 meters for moderate terrain at 12 m posting. For that purpose interferometric SAR data will be acquired over a period of 3 years in parallel to the operational running TerraSAR-X mission. Systematic processing of SAR raw data to so-called Raw-DEMs is performed by one single processing system, the Integrated TanDEM Processor (ITP). The final global DEM is then calibrated and mosaicked by a second system, the Calibration and Mosaicking Processor (MCP). The scope of this paper is to present an overview of ITP functionalities and to summarize the first processing results.

An incoherent correlation approach is used to derive ice motion fields for outlet glaciers through the Transantarctic Mountains, Antarctica. High-resolution repeat-pass, left-looking-mode TerraSAR-X data from 2009 were analyzed. Detailed ice velocity patterns on the Nimrod glacier basin and Starshot glacier are presented.

The purpose of this paper is to provide an algorithmic overview of the interferometric processing embedded in the Integrated TanDEM-X Processor (ITP), settled to the generation of the raw digital elevation model (DEM). The main processing blocks are described, with a focus on the spectral matching of the azimuth spectra, the high-precision coregistration, the dual-baseline phase unwrapping and the geocoding of the products. The robustness of the algorithms is demonstrated through a dual-pass TerraSAR-X scenario.

In 2009, the German radar satellite TerraSAR-X will be supplemented with the TanDEM-X satellite to form the first bi-static single pass interferometer in space. TanDEM-X will fly close to TerraSAR-X in a controlled helix configuration for 3 years to jointly acquire interferometric SAR data in bistatic mode. The primary TanDEM-X mission goal is to generate a global Digital Elevation Model (DEM) with a relative point-to-point height accuracy of 2 meters for moderate terrain at 12 m posting. This paper outlines the SAR data workflow from quality check screening through to bistatic focusing and interferometric processing to raw DEM generation.

An incoherent amplitude correlation approach is used to derive ice motion fields of three major outlet glaciers of the Patagonia Icefields. High resolution repeat pass TerraSAR-X data of 2008 and 2009 were analyzed. The strong gradients in ice velocity of the terminus of San Rafael glacier, ranging from 2 to 16 m d-1, were captured well. Significant acceleration of the ice flow and losses in mass were observed for Upsala glacier.

We present a comparison of 1997 RADARSAT Antarctic Mapping Project (RAMP) SAR data with 2008-09 TerraSAR-X observations of a tributary glacier that is part of the Recovery Glacier drainage network in Coates Land Antarctica. The Recovery Glacier system is of scientific interest because of its role in discharging East Antarctic ice to the sea and because it has been subsequently learned that the flow of the glacier is likely controlled by the presence of subglacial lakes near the onset of faster glacier flow.

Recently, synthetic aperture radar (SAR) data with 1-m resolution acquired by satellites in spotlight mode became available to the public. In this paper, we elucidate the differences between interferometric processing of strip map and of spotlight SAR data, and we outline adequate algorithms for key processing steps such as azimuth Doppler filtering. We further present first TerraSAR-X spotlight interferograms, together with an evaluation of the paraeters that are critical for interferometry. Our results indicate a very good geometric accuracy, stability, and phase fidelity of the TerraSAR-X sensor and its products. From the interferograms, we are able to determine the heights of larger buildings and millimeter-scale structural deformation in several examples. The high detail level of imaged buildings and the good temporal phase coherence of urban areas in X-band make spotlight interferometry an exciting processing technique that enables new applications such as surveying individual buildings.

Since the launch of TerraSAR-X on June 15, 2007, the required precise orbit products have been provided by the German Space Operations Center to support operational spaceborne synthetic aperture radar (SAR) and interferometric SAR image processing. The TerraSAR-X precise trajectory is reconstructed solely based on the Global Positioning System (GPS) measurements from a geodetic-grade dual-frequency Integrated Geodetic and Occultation Receiver (IGOR) onboard the spacecraft. The GPS-based precise orbit determination (POD) strategy used in the estimation of the precise TerraSAR-X orbit and its performance will be fully described in this paper. Five-month statistics from the internal and external orbit assessment indicate a root-mean-squared 3-D orbit accuracy of better than 10 and 20 cm for the precise science orbit and precise rapid orbit (PRO) products, respectively. The POD performance of the backup single-frequency MosaicGNSS receiver to support operational PRO product generation in case of IGOR tracking failure or interruptions is described as well.

The German radar satellite TerraSAR-X was launched in June 2007. Radar interferometry (InSAR) is supposed to be one of the major applications of this sensor. It allows the three-dimensional mapping of the Earth's surface (InSAR) or even its displacement effects using differential interferometry (D-InSAR) or the more advanced persistent scatterer interferometry (PSI). All InSAR-applications are well-supported by the sensor, the SAR-processor and the mission design. DLRs operational interferometric system PSI-GENESIS has been adapted to cope with the new sensor and the innovative acquisition modes. Interferometric example applications could be shown already in the early stage of the six months commissioning phase. In this paper, recent processing examples and an update on the interferometric characteristic are presented.

TerraSAR-X data are operationally available to the public since January 2008. This paper shows selected TerraSAR-X interferometric examples with an emphasis on challenges introduced by the high resolution in spotlight mode and with 300 MHz.