Radio frequency interference (RFI) in synthetic aperture radar (SAR) is a daunting challenge, affecting both sensing reliability and image quality. To ensure that SAR remains a powerful tool for Earth observation, this letter presents a 2-D variable attenuation space (azimuth)-frequency filtration (VASFF) method. This framework leverages the time-frequency characteristics of Level-0 SAR data, the RFI power profile, estimated RFI signal parameters, and the SAR antenna pattern to design a novel variable filter. Signal power localization estimates the interference source’s relative position, facilitating filter application. Simulated results, obtained using our open-source emulator, SEMUS, to generate both clean and interference-contaminated raw SAR data, demonstrate that the proposed filter achieves a 2 dB improvement over traditional notch filtering. The framework is further tested on real-life interference events on TerraSAR-X revealing previously obscured image details, validating the framework’s effectiveness.

Maritime surveillance using synthetic aperture radar (SAR) calls for both wide swaths and high resolution. This enables monitoring of wide areas with high detection probabilities and low false alarm rates at short time intervals. Ambiguous SAR modes have proven effective for ship monitoring in remote offshore regions. They deliver high-resolution SAR images over wide swaths that can be effectively exploited for ship detection, even though they are corrupted by significant ambiguities of ships and sea clutter. Building upon the successful demonstration of the staggered ambiguous mode using TerraSAR-X, this letter presents the results of a further experimental acquisition, carried out near the Dutch coast, where an ultrawide ground swath of 160 km is imaged at an azimuth resolution of 2.2 m by introducing in the staggered ambiguous mode alternation of up- and down-chirp waveforms; 79 small ships were detected in the dataset, some of which even near the coast and despite the presence of first-order range ambiguities caused by land scatterers, as these ambiguities are very blurred and manifest as noise like disturbances. These results are of fundamental importance for incorporating the ambiguous modes into the existing and future SAR systems as an efficient additional mode for ship monitoring, suitable for both open sea and coastal surveillance.

The recently proposed frequency scanning (F-Scan) technique, together with the new International Telecommunication Union (ITU) allocation of 1200 MHz in X-band, enables the improvement of important performance parameters of synthetic aperture radar (SAR) acquisitions, such as the swath width, the signal-to-noise ratio, and the range ambiguity-to-signal ratio. The concurrent imaging technique, in turn, increases the flexibility of the radar system by allowing the simultaneous imaging of two or more areas with independent imaging modes. The integration of both techniques, therefore, enables the already valuable concurrent mode to achieve much better performance. Furthermore, motivated by the high-resolution wide-swath (HRWS) mission proposal in X-band, the displaced phase center antenna (DPCA) technique is considered to improve the azimuth resolution by using multiple receive (Rx) channels in azimuth. In this article, we discuss the design and performance of a new concurrent imaging mode that is enhanced by the F-Scan and DPCA techniques to make simultaneous imaging more flexible and powerful. Special attention is given to timing, range ambiguities, and availability aspects. The capability to simultaneously acquire two high-quality images with noteworthy flexibility is innovative and of great value, not only in daily operational applications but especially in extraordinary and crisis situations.

The German SAR satellites TerraSAR-X and TanDEM-X are in orbit since 2007 and 2010, respectively. This paper gives an overview about the challenges faced by such long-enduring missions, recent measures taken to preserve the satellites and to continue the mission, and major lessons learned for future SAR missions.

Maritime surveillance using synthetic aperture radar (SAR) calls for both wide swath and high resolution. This allows frequent monitoring of large areas with high detection probability and low false alarm rate. Conventional SAR modes are, however, limited in that a wide swath can only be imaged at the expense of a reduced azimuth resolution. Ambiguous SAR modes, based on low pulse repetition frequency or continuous variation of short pulse repetition intervals (staggered ambiguous mode), overcome this limitation and allow imaging a wide swath with high resolution for specific ship monitoring applications without the need for digital beamforming or multiple receive apertures. This paper reports on the demonstration of the staggered ambiguous mode via an experimental acquisition with the TerraSAR-X satellite over the North Sea. In spite of technical limitations in the SAR instrument, a ground range swath of 110 km was imaged with an azimuth resolution of 2.2 m, i.e., with a resolution improvement of a factor of eight with respect to TerraSAR-X ScanSAR mode. Despite the higher disturbance level resulting from the presence of range ambiguities of the sea clutter a detection probability higher than 0.8 was achieved for small ships of 21 m × 6 m size. Range ambiguities of the ships were furthermore identified based on their position and signature. The detected ships were validated using maritime positioning data from their automatic identification system. These results motivate the adoption of ambiguous SAR modes in existing and future SAR systems and missions.

The paper provides an up-to-date overview of the German TanDEM-X satellite mission status and its ongoing science activities. The global digital elevation model (DEM) of TanDEM-X became available in 2016 and surpassed all expectations: It has 99.9% coverage, 12-m posting, absolute height accuracy of approximately 1 m and a relative height error (standard deviation) of 0.8 m. This unique data set has been available since then for commercial and scientific applications. In addition, a low-resolution version of the global digital elevation model with 90-m posting has been released for free download for scientific applications. Since 2017 new interferometric acquisitions have been started with the goal to generate a global change layer which will register all the changes of the Earth topography occurred since the first acquisitions of TanDEM-X. The acquisition for the global change layer was completed by mid-2020. Since then a new science phase of TanDEM-X with a focus on interferometric acquisitions over forest, ice sheets and permafrost regions are ongoing.

Synthetic aperture radar (SAR) provides high-resolution images for remote-sensing applications regardless of sunlight and weather conditions. The pulsed operation of SAR may lead to an occurrence of nadir echoes in SAR images that significantly affect the image quality in case the pulse repetition frequency (PRF), which is not properly constrained within the SAR system design. As an alternative, pulse-to-pulse variation of the transmitted waveform and dual-focus postprocessing can be exploited to remove the nadir echo and alleviate the PRF constraints (also in ScanSAR operation). This work provides a demonstration of the latter concept through an experimental acquisition of the TerraSAR-X satellite. The experiment is designed by selecting the scene and the acquisition parameters in order to have the nadir echo appearing in the SAR image. The waveform variation is achieved by alternating up- and down-chirps on transmit. The analysis of the results shows the effectiveness of dual-focus postprocessing for nadir echo suppression.

The concurrent imaging technique enables parallel acquisitions with different beams or modes, e.g., a wide area Stripmap mode with a High-Resolution Spotlight mode. Such a concurrent Stripmap/Spotlight imaging technique is investigated for TerraSAR-X. This technique employs a pulse-to-pulse interleaving scheme to acquire two acquisitions-even of disjunctive areas-at the same time, offering products with different resolution and coverage portfolios. This capability is especially interesting for customers interested in an overview of a larger area but at the same time observing an area of interest with higher resolution, e.g., for infrastructure monitoring or reconnaissance applications. The basic concepts, as well as the driving system parameters, are discussed in detail, together with a coverage analysis revealing the high availability rate of the mode combinations on a global scale. A processing approach re-using a substantial part of the existing infrastructure is described and exemplary acquisitions are shown, together with a detailed performance analysis with respect to resolution and ambiguities.

The paper provides an up to date overview of the German TanDEM-X satellite mission status and its ongoing science activities. In June 2020 TanDEM-X reached his 10 years of successful operation - a view forward will be presented in the final paper. The global digital elevation model (DEM) of TanDEM-X became available in 2016 and surpassed all expectations: It has 99.9% coverage, 12-m posting, absolute height accuracy of approximately 1 m and a relative height error (standard deviation) of 0.8 m. This unique data set has been available since then for commercial and scientific applications. Last year, a low-resolution version of the global digital elevation model with 90-m posting has been released for free download for scientific applications. Since 2017 new interferometric acquisitions have been started with the goal to generate a global change layer which will register all the changes of the Earth topography occurred since the first acquisitions of TanDEM-X. The data collection for the global change layer will be ready by the end of 2020. Currently, a new science phase of TanDEM-X is running with a focus on interferometric acquisitions over forest, ice sheets and permafrost regions.

The paper provides an up to date overview of the German TanDEM-X satellite mission status and its ongoing science activities. In June 2020 TanDEM-X will reach his 10 years of successful operation - a view back and forward will be presented in the final paper. The global digital elevation model (DEM) of TanDEM-X became available in 2016 and surpassed all expectations: It has 99.9% coverage, 12-m posting, absolute height accuracy of approximately 1 m and a relative height error (standard deviation) of 0.8 m. This unique data set has been available since then for commercial and scientific applications. Last year, a low-resolution version of the global digital elevation model with 90-m posting has been released for free download for scientific applications. Since 2017 new interferometric acquisitions have been started with the goal to generate a global change layer which will register all the changes of the Earth topography occurred since the first acquisitions of TanDEM-X. The data collection for the global change layer will be ready by the end of 2020. By that time, it is planned to start a new science phase of TanDEM-X with a focus on interferometric acquisitions over forest, ice sheets and permafrost regions.

The paper provides an up to date overview of the German TanDEM-X satellite mission status and its ongoing science activities. The global digital elevation model (DEM) of TanDEM-X became available in 2016 and surpassed all expectations: It has 99.9% coverage, 12-m posting, absolute height accuracy of approximately 1 m and a relative height error (standard deviation) of 0.8 m. This unique data set has been available since then for commercial and scientific applications. Last year, a low-resolution version of the global digital elevation model with 90-m posting has been released for free download for scientific applications. Since 2017 new interferometric acquisitions have been started with the goal to generate a global change layer which will register all the changes of the Earth topography occurred since the first acquisitions of TanDEM-X. The global change layer will be available by 2020. By that time, it is planned to start a new science phase of TanDEM-X with a focus on interferometric acquisitions over forest, ice sheets and permafrost regions.

Multistatic or distributed satellite systems offer new and unique capabilities necessary for Earth observation with high spatial and temporal resolution. This letter describes a multistatic synthetic aperture radar (SAR) experiment employing the satellites TerraSAR-X and TanDEM-X. In order to demonstrate distributed SAR imaging from space, special data acquisitions with a dedicated geometry were performed. The data evaluation approach is outlined and azimuth profiles over a region with high-contrast backscatter are used to evaluate the azimuth signal reconstruction performance. The results are verified with simulations performed with a flexible SAR simulator reproducing the acquired scene. Finally, the effect of target motion on the reconstruction is analyzed and discussed based on the experimental data.

Remote sensing of Antarctic sea ice is required to characterize properties of the vast sea ice cover to understand its long-term increase in contrast to the decrease of Arctic sea ice. For this objective, the OIB/TanDEM-X Coordinated Science Campaign (OTASC) was successfully conducted in 2017 to obtain contemporaneous and collocated remote sensing data from NASA's Operation IceBridge (OIB) and the German Aerospace Center (DLR) TanDEM-X Synthetic Aperture Radar (SAR) system at X band together with Sentinel-1 and RADARSAT-2 SARs at C band in conjunction with WorldView satellite spectral sensors, surface measurements, and field observations. The Weddell Sea and the Ross Sea were two primary regions while SAR data were also collected over six other regions in the Southern Ocean. Satellite SAR data included both polarimetric and interferometric capabilities to infer snow and sea ice information in three dimensions (3D), while OIB/P-3 aircraft data include snow radar together with altimeter data for snow and sea ice observations in 3D over the Weddell Sea. Across the Ross Sea, IcePOD and AntNZ/York-University flights were carried out together with satellite SAR data acquisitions.

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.

This paper proposes the wrapped staring spotlight (WSS) SAR imaging mode, which is a new method to extend the azimuth steering capability for phased array SAR to achieve either an improved azimuth geometric or radiometric resolution. It investigates the utility of steering directions with main lobe gains that are smaller than that of the grating lobes and exposes how these directions can be exploited. Furthermore, two methods are proposed to reduce the speckle and the image noise at once, i.e., the Look-Normalized Pattern Correction and the Ω-weighting. Based on two example TerraSAR-X WSS acquisitions, the image performance of extended and point targets is discussed.

TanDEM-X is a spaceborne mission consisting in two satellites that are operated simultaneously for bistatic SAR acquisitions. The flexibility offered by both SAR instruments allows the acquisition of full polarimetric data by activating the experimental dual-receive antenna (DRA) mode. For the first time on the TanDEM-X mission, it is possible to systematically command quad polarization acquisitions. We have estimated the quality of such full polarimetric products by a first interferometric performance analysis. The influence of different instrument parameters on the interferometric performance, such as chirp bandwidth or block adaptive quantization, have been investigated. In this paper first results are presented and recommendations for the optimization of the TanDEM-X quad polarization products are given.

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.

TanDEM-X is a spaceborne mission consisting in two satellites that are operated simultaneously for bistatic SAR acquisitions. The main objective of the mission is the systematic acquisition of a global and homogeneous digital elevation model (DEM) in bistatic stripmap mode. The close formation of the satellites makes the system very flexible and allows the commanding of a diversity of challenging experimental modes as bistatic spotlight or alternating bistatic stripmap modes. This paper gives an overview of the TanDEM-X experimental modes, focused on the analysis of already executed scientific orders, giving an overview of the different possibilities in commanding and image acquisition geometries. Also a first image characterization of the modes is included, in terms of image quality assessment and performance compliance.

TanDEM-X is an interferometric SAR (InSAR) mission acquiring bistatic images with two satellites. Systematic mapping of the Earth's land masses will provide individual interferometric data sets which will be mosaicked and calibrated into a global Digital Elevation Model (DEM). The concept of InSAR and DEM quality monitoring throughout the acquisition and processing phase is presented in this paper.

Examining the growth rate of pleural thickenings in consecutive 3D-CT images requires the matching of identical thickenings in lung images acquired at two different points in time. The thickenings can be subject to strong deformations caused by their growth. This implies that position information should play a major role in finding correspondences. Here, a MGRF approach is presented to determine a rigid transformation. It aligns the lung volumes by maximizing the probability of the regarded lung tissue to fit an offline trained model. To ensure a symmetrical matching of lung surfaces this probability is calculated reciprocally. Using precalculation, strong sub-sampling and a multiscale approach, the required time can be reduced by a factor of about 80, depending on the image resolution. Due to this speed-up, online follow-up assessment is feasible. We show that this approach results in precise registrations which can be used for a reliable matching of lung thickenings.