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.

TanDEM-X is a formation flying interferometer consisting of two SAR satellites. Together they acquire bistatic SAR data of the Earth in order to generate a global Digital Elevation Model and individual scientific interferometric data sets on user request. This paper gives an overview about ten years of acquisition and formation planning for the TanDEM-X mission. This planning on the one hand includes the derivation of the formation, i.e. the 3D distance between both satellites in orbit. On the other hand it needs to consider the regions of interest and the mission, satellite, and instrument constraints in order to effectively plan the bistatic acquisitions. During the ten years in operation TanDEM-X successfully completed different mission goals during different mission phases. All these phases, their characteristics and the scientific and commercial outcome are described in detail.

Introduction: SARS-CoV-2 infection results in varying disease severity, ranging from asymptomatic infection to severe illness. A detailed understanding of the immune response to SARS-CoV-2 is critical to unravel the causative factors underlying differences in disease severity and to develop optimal vaccines against new SARS-CoV-2 variants.Methods: We combined single-cell RNA and T cell receptor sequencing with CITE-seq antibodies to characterize the CD8⁺ T cell response to SARS-CoV-2 infection at high resolution and compared responses between mild and severe COVID-19.Results: We observed increased CD8⁺ T cell exhaustion in severe SARS-CoV-2 infection and identified a population of NK-like, terminally differentiated CD8⁺ effector T cells characterized by expression of FCGR3A (encoding CD16). Further characterization of NK-like CD8⁺ T cells revealed heterogeneity among CD16⁺ NK-like CD8⁺ T cells and profound differences in cytotoxicity, exhaustion, and NK-like differentiation between mild and severe disease conditions.Discussion: We propose a model in which differences in the surrounding inflammatory milieu lead to crucial differences in NK-like differentiation of CD8⁺ effector T cells, ultimately resulting in the appearance of NK-like CD8⁺ T cell populations of different functionality and pathogenicity. Our in-depth characterization of the CD8⁺ T cell-mediated response to SARS-CoV-2 infection provides a basis for further investigation of the importance of NK-like CD8⁺ T cells in COVID-19 severity.

The TanDEM-X mission, formed by the TanDEM-X satellite (TDX) in cooperation with its almost identical twin TerraSAR-X (TSX), has mainly been designed to acquire bistatic Synthetic Aperture Radar (SAR) images of the Earth. Initiated in 2010, the primary mission objectives were to generate a global Digital Elevation Model (DEM), to perform scientific measurements and to explore novel SAR techniques. Up to the present day all these objectives have been completed with outstanding results. In addition, the satellites maintained a stable image quality from the start throughout their entire lifetime. The ground segment, which was tailored to the capabilities of the space segment, as well as an excellent design and manufacturing of the satellites were the key elements to achieve such a performance. In addition, the elaborate calibration concept and a careful treatment of the on-board resources enabled full operability up to the present day and allowed additional mission objectives such as the generation of a Change-DEM layer as an update to the global DEM. The stable bus and SAR payload performance as revealed by long-term system monitoring provide the basis for further extension of the TanDEM-X mission for several more years. In the following paper these long-term system monitoring results are presented to provide an overview of the evolution of the radar system. The results and experience gained with more than a decade of TanDEM-X mission operation are a great value for both the scientific and commercial community and serve as a forerunner for the development of future SAR missions.

The main goal of the TerraSAR-X Add-On for Digital Elevation Measurements (TanDEM-X) mission is the generation of a global digital elevation model (DEM) of unprecedented accuracy and coverage. The global TanDEM-X DEM product became available in 2016, surpassed all expectations, and became a reference for a wide range of Earth science, commercial, and geospatial applications. In addition, new information products, such as DEM change maps (DCMs), have been developed and are available to the geoscience and remote sensing community. Beyond the operational products, new science applications have been demonstrated and are summarized in this article, along with experimental data acquisitions. This article also aims to provide an overview of science activities with TanDEM-X data and science data acquisitions planned for the coming years.

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.