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 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.

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.

Synthetic aperture radar (SAR) concepts based on digital beamforming (DBF) in elevation and multiple receive beams, for example, staggered SAR, allow high-resolution imaging of wide swaths and are, therefore, well-suited for the systematic observation of dynamic processes on the Earth's surface. While these concepts are compatible with quad-polarimetric (quad-pol) operation, due to severe ambiguity constraints, the achievable swath in the quad-pol mode is narrower than in the single- (or dual-) pol mode. Therefore, either the single- (or dual-) pol data over a wider swath or the quad-pol data over a narrower swath can be acquired. This article proposes a novel SAR acquisition mode, based on pulse-to-pulse alternation of the antenna pattern on transmit, which delivers at the same time single- (or dual-) pol data over a wider swath and quad-pol data over a narrower swath at the expense of an acceptable degradation of the ambiguity performance. This technique yields remarkable benefits for the design and operation of future SAR systems and represents an effective solution to manage and resolve conflicting user requirements.

Synthetic aperture radar (SAR) concepts based on digital beamforming (DBF) in elevation and either multiple azimuth channels or multiple receive beams, e.g., staggered SAR, allow high-resolution imaging of wide swaths and are therefore well suited for the systematic observation of dynamic processes on the Earth's surface. While these concepts are compatible with quad-polarimetric (quad-pol) operation, due to severe ambiguity constraints, the achievable swath in quad-pol mode is narrower than in single- (or dual-) pol mode. Therefore, either single- (or dual-) pol data over a wider swath or quad-pol data over a narrower swath can be acquired. This paper proposes a novel SAR acquisition mode, based on pulse-to-pulse alternation of the antenna pattern on transmit, that delivers at the same time single- (or dual-) pol data over a wider swath and quad-pol data over a narrower swath at the expense of an acceptable degradation of the ambiguity performance. This technique yields remarkable benefits for the design and operation of future SAR systems and represents an effective solution to manage and resolve conflicting user requirements.

Tandem-L is a proposal for an innovative L-band SAR mission for the systematic observation of dynamic processes on the Earth's surface. The mission concept is based on two SAR satellites flying in close formation featuring latest digital beamforming techniques in combination with a large deployable reflector for increasing the swath width and imaging resolution. This enables innovative operation modes such as polarimetric SAR tomography for determining the vertical structure of vegetation and ice. With novel imaging and processing techniques and the vast recording capacity of up to 8 Terabyte/day, it will provide vital information for solving pressing scientific questions in the biosphere, geosphere, cryosphere, and hydrosphere and will make an essential contribution for a better understanding of the Earth system and its dynamics. This paper gives an overview of the Tandem-L mission project which has successfully passed the intermediate system requirements review of Phase B1.

Tandem-L is a highly innovative SAR satellite mission for the global observation of dynamic processes on the Earth's surface with hitherto unknown quality and resolution. Thanks to its novel imaging techniques and its unprecedented acquisition capacity, Tandem-L will deliver urgently needed information for the solution of pressing scientific questions in the areas of the biosphere, geosphere, cryosphere and hydrosphere. The feasibility of Tandem-L has been analyzed and confirmed in the scope of a phase A study, which has been conducted in close cooperation between the German Aerospace Center (DLR) and the German space industry. This paper provides an overview of the Tandem-L mission concept and summarizes the actual development status.

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.

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 possibility of combining data acquired with different platforms to generate a single SAR image results in an advantageous relaxation of the timing and sampling requirements of the system which can be used to improve in different ways its performance. We report about a generalised approach for distributed SAR image formation which is based on a platform dedicated compensation of the observation geometry followed by a differential calibration of the data. The validity of the approach on simulated data is presented. Further tests with actual data from a TanDEM-X dedicated campaign are being conducted.

Launched in June 2010, TanDEM-X is an interferometric mission with the main goal of providing a high-resolution, global and unprecedentedly accurate digital elevation model (DEM) of the Earth by means of single-pass X-band SAR interferometry. Despite its usual quasi-monostatic configuration, TanDEM-X is the first genuinely bistatic SAR system in space. During its monostatic commissioning phase, the system was operated in pursuit monostatic mode. During that time, some pioneering bistatic SAR experiments with both satellites commanded in non-nominal modes were conducted with the main purpose of testing the performance of both space and ground segments in very demanding scenarios. In particular, this article includes results of the first bistatic acquisition and the first single-pass interferometric (mono/bistatic) acquisition with TanDEM-X, addressing their innovative aspects and focussing on the analysis of the experimental results. Even in the absence of essential synchronisation and calibration information, bistatic images and interferogramswith similar quality to pursuit monostatic have been obtained. Some months later, with TanDEM-X already in its operational DEM-acquisition phase a further challenging bistatic acquisition carried out in cooperation with DLR's airborne radar F-SAR has been carried out. The objective was to acquire fully polarimetric interferometric data with a high range of available bistatic angles. A dedicated commanding of both TanDEM-X and F-SAR was required, including irregular sampling schemes, partially missing bistatic echo reception and bistatic synchronisation, which pose a number of technological challenges in SAR data processing before the calibrated bistatic SAR images are obtained. This article reports about these two set of experiments.

TanDEM-X (TerraSAR-X Add-on for Digital Elevation Measurements) is a high-resolution interferometric mission with the main goal of providing a global and unprecedentedly accurate digital elevation model of the Earth surface by means of single-pass X-band synthetic aperture radar (SAR) interferometry. Despite its usual quasi-monostatic configuration, TanDEM-X is the first genuinely bistatic SAR system in space. During its monostatic commissioning phase, the system has been mainly operated in pursuit monostatic mode. However, some pioneering bistatic SAR experiments with both satellites commanded in nonnominal modes have been conducted with the main purpose of validating the performance of both space and ground segments in very demanding scenarios. In particular, this letter reports about the first bistatic acquisition and the first single-pass interferometric (mono-/bistatic) acquisition with TanDEM-X, addressing their innovative aspects and focusing on the analysis of the experimental results. Even in the absence of essential synchronization and calibration information, bistatic images and interferograms with similar quality to pursuit monostatic have been obtained.

This paper reports on several experiments performed with the TerraSAR-X (TSX) and the TanDEM-X (TDX) satellites. The experiments consist in the generation of improved image products by coherently combining the images acquired individually by both satellites, hence the name distributed imaging. In the literature it has already been suggested the use of two or more satellites in close formation not only to have interferometric capabilities, but also to improve the azimuthal or range resolutions by performing the acquisitions simultaneously (no temporal decorrelation). This idea can be carried out by acquiring different portions of the spectrum and adding them coherently afterwards. Another possibility is to synthesize quad-pol acquisitions using dual-pol ones, an idea already carried out with airborne systems. Such experiments have been performed with TSX and TDX and are described in this paper.

TanDEM-X is a high-resolution interferometric mission with the main goal of providing a global and unprecedentedly accurate digital elevation model (DEM) of the Earth surface by means of single-pass X-band SAR interferometry. De spite its usual quasi-monostatic configuration, TanDEM-X is the first genuinely bistatic SAR system in space. During its monostatic commissioning phase, the system has been mainly operated in pursuit monostatic mode. However, some pioneering bistatic SAR experiments with both satellites commanded in non-nominal modes have been conducted with the main purpose of testing the performance of both space and ground segments in very demanding scenarios. In particular, this paper reports about the first bistatic acquisition and the first single-pass interferometric (mono/bistatic) acquisition with TanDEM-X. Even in the absence of essential synchronisation and calibration information, bistatic images and interferograms with similar quality to pursuit monostatic have been obtained.

This paper recalls the essential elements of the joint TerraSAR-X and TanDEM-X ground segment. It elaborates on some topics which are usually not in the primary focus from a pure SAR technical point of view, e.g. the flight formation. Both commissioning and early routine phase results from operating the joint TerraSAR-X and TanDEM-X ground segment are given.

In this paper we describe new achievements of surface current measurements obtained by space-borne Along-Track Interferometry (ATI). We show how tidal currents can be mapped using the TerraSAR-X satellite and adequate dual-channel SAR data processing techniques. We present results from tidal currents at the Orkney Islands that clearly demonstrate the potential and suitability of the method.

The detection of ships and the retrieval of parameters like vessel velocity, heading and size are important techniques for ocean and shore surveillance systems. Space borne SAR along-track interferometry (ATI) can detect ships for large areas and measure their velocity at the same time by means of interferometric phase. In this paper we report about first results of automatic ship detection using TerraSAR-X ATI.

The TerraSAR-X satellite is a high-resolution synthetic aperture radar (SAR) system launched in June 2007 which provides the option to split the antenna in along-track direction and sample two physical channels separately. Modern SARs are equipped with active phased array antennas and multiple channels. In order to keep costs low, TerraSAR-X uses the redundant receiver unit for the second channel such that fore and aft channel signals are combined by a hybrid coupler to form sum and difference channel data. The dual receive antenna (DRA) mode can either be used to acquire along-track interferometric data or to acquire signals with different polarizations at the same time (Quad-Pol). Fore and aft channel reconstruction is necessary if ground moving target indication (GMTI) algorithms such as the displaced phase center antenna technique or along-track interferometry shall be applied, and in order to separate the horizontally and vertically polarized received signal components. The proposed approach uses internal calibration pulses from different calibration beams in order to estimate and compensate the hardware impact. The theoretical framework together with the results from the experimental data evaluation for the fore and aft channel reconstruction of the TerraSAR-X DRA mode are presented. The impact of the receive hardware transformation matrix estimation accuracy on errors in the reconstructed fore and aft channel image data is studied, and first examples on the GMTI capability of the TerraSAR-X DRA mode are given.

This paper reports about the performed investigations for the implementation of the wide-swath TOPS (Terrain Observation by Progressive Scan) imaging mode with TerraSAR-X (TSX). The TOPS mode overcomes the limitations imposed by the ScanSAR mode by steering the antenna along track during the acquisition of a burst. In this way, all targets are illuminated with the complete azimuth antenna pattern, and, thus, scalloping is circumvented, and an azimuth dependence of signal-to-noise ratio and distributed target ambiguity ratio (DTAR) is avoided. However, the use of electronically steered antennas leads to a quantization of the steering law and a nonideal pattern for squinted angles (grating lobes and main lobe reduction). The former provokes spurious peaks, while the latter introduces slight scalloping and DTAR deterioration. These effects are analyzed and quantified for TSX, and a TOPS system design approach is presented. Next, the requirements concerning interferometry are investigated. Finally, several results are shown with the TSX data, including a comparison between the TOPS and the ScanSAR modes and the reporting of the first TOPS interferometric results.

We present the first analysis of surface current fields derived from TerraSAR-X along-track interferometric synthetic aperture radar (along-track InSAR, ATI) data. The images were acquired over the mouth of the Elbe river (Germany) during six satellite overpasses in spring and summer 2008, using the experimental "aperture switching" mode of TerraSAR-X. In this mode, the phased-array synthetic aperture radar (SAR) antenna is split into two halves for receiving, but in contrast to the "dual receive antenna" mode, which uses two independent receivers in parallel, a single receiver is multiplexed to process signals from the two antenna halves in an alternating manner at a doubled pulse repetition frequency. The effective ATI baseline is on the order of 0.8 m. The SAR/ATI raw data processing is described in another paper in this issue. This paper focuses on the conversion of the basic interferograms into line-of-sight surface current fields, which includes an elimination of ship signatures, identification, and correction (as far as possible) of imaging artifacts, additional filtering and smoothing, and a subtraction of contributions of wave motions to detected velocities according to a theoretical model. We evaluate the quality of the results by comparison with current fields from a numerical flow model and with availablein situdata. The ATI performance of TerraSAR-X is found to be basically consistent with theoretical expectations. After applying the same data processing algorithms to all six images, mean differences between TerraSAR-X-derived currents and reference currents in our main test area range from -0.11 to + 0.08 m/s in five of the six cases with one outlier at +0.42 m/s. The spatial current variations within the TerraSAR-X-derived current fields are consistent with the model in three cases, but unrealistically strong variations across the images are found in the other three cases. We attribute this to shortcomings of our preliminary raw data processing algorithms, which can probably be fixed after some more detailed analysis and testing. The results obtained so far encourage us to believe that our internal performance goal of a typical current measuring accuracy of 0.1 m/s at an effective spatial resolution better than 1 km can be met.

This paper presents the TerraSAR-X instrument operations embedded into the Instrument Operations and Calibration Segment. Special focus is on the data-take (DT) command generation. The command generation for standard DTs is discussed, and the nonnominal DT commanding is described for several examples which demonstrate the flexibility of both the TerraSAR-X instrument and the instrument operations system on-ground.

Spaceborne synthetic aperture radar (SAR) offers great potential for the measurement of ground traffic flows. A SAR with multiple receiving apertures aligned in flight direction repeatedly images the same ground area with a short time lag. This allows for an effective detection of moving ground objects, whose range variation translates into an interferometric phase signal between the receiving channels. The high-resolution German SAR satellite TerraSAR-X offers several ways to create multiple along-track apertures. We exploit this to demonstrate satellite-based traffic-flow measurements using along-track interferometry (ATI) and Displaced Phase Center Array techniques. In this paper, we address the usage of different TerraSAR-X ATI modes for data acquisition and describe an automatic near-real-time processing chain for the extraction of traffic information. The performance of this TerraSAR-X traffic processor is significantly driven by incorporatinga prioriknowledge of road networks. We present examples of automatic traffic detection as well as empirical evaluations thereof using different kind of reference data.

TerraSAR-X is a high resolution synthetic aperture radar (SAR) satellite which provides the option to split the antenna in along-track direction and sample two physical channels separately. This so-called dual receive antenna (DRA) mode is implemented by use of a Magic-T hybrid junction which provides the sum and difference channel data instead of the fore and aft channel data. One way to exploit this spatial degree of freedom is to reconstruct the fore and aft channel data in the processing. The critical issue with the fore and aft channel reconstruction is the fact, that for space-based radar systems very high precision calibration is required. This is even more difficult since the wide bandwidth pulses employed by the radar makes the receive hardware transformation frequency dependent. In this paper a fore and aft channel reconstruction approach is described based on an estimation of the receive hardware transformation matrix by use of internal calibration pulses and calibration beams. The quality gain of the fore and aft channel reconstruction is demonstrated with experimental data and results on the ground moving target indication (GMTI) capability are shown.

The potential of the along-track interferometric SAR (along-track InSAR, ATI) modes of TerraSAR-X for high-resolution imaging of surface current fields has been discussed in several publications. In this paper we show first results based on actual TerraSAR-X ATI data, explain our data processing procedure, and give an update on the status of the instrument and on upcoming developments.

TerraSAR-X offers different possibilities for Along-track Interferometry (ATI) SAR data acquisition. This enables to test new SAR applications for the detection and measurement of moving ground objects from space. In this paper we demonstrate space-borne traffic flow measurements using TerraSAR-X ATI data and an automatic traffic data extraction processor. The obtained results evidence the potential of space-borne SAR for this particular application. The traffic information can be derived in near-real time, which underlines the practicability of the method. Another application of TerraSAR-X ATI is the measurement of water surface currents, of which we also show a first example.

The paper presents selected results from the TerraSAR-X Commissioning Phase from instrument performance, SAR system performance and command generation.

Following an original proposal by the authors to the TerraSAR-X (TSX) scientific coordination board, a spaceborne-airborne bistatic experiment was successfully performed early November 2007. TSX was used as transmitter and DLR's new airborne radar system, F-SAR, as receiver; due to the capability of the latter to acquire data quasi-continuously, no echo window synchronisation is needed. Monostatic data were also recorded during the acquisition. This paper includes description and results of the spaceborne-airborne bistatic experiment, with special focus on data processing and image comparison. Given the acquisition scenario, with two-channel sampling and transmitter and receiver clocks operating independently, data processing must necessarily follow a three-step strategy: 1) channel balancing, 2) data synchronisation and 3) bistatic SAR processing. Since neither absolute range nor Doppler references are available in the bistatic data set, synchronisation is done with the help of calibration targets on ground and based on the analysis of the acquired data compared to expected data. Due to the variant nature of the bistatic acquisition and the required precision for the processing, data are processed using a bistatic backprojection approach.

The paper reports about investigations on the implementation of the recently proposed TOPSAR acquisition mode with TerraSAR-X. Differently from ScanSAR,TOPSAR mode allows a wide swath coverage with nearly uniform signal to noise ratio (SNR), therefore avoiding scalloping and azimuth dependent SNR. To achieve this goal the antenna beam is steered within the different swaths in the along-track direction. TerraSAR-X can electronically steer the azimuth beam; however, angle quantization is introduced because of the limited number of available azimuth beams per data take. The effects of the TerraSAR-X angle quantization of the antenna steering are analyzed resulting in a negligible distortion. The maximum azimuth steering angle of TerraSAR-X allows the implementation of TOPSAR with the same coverage and resolution as the nominal TerraSAR-X ScanSAR mode, four range subswaths with 16 m azimuth resolution and a total swath of 100 km. Finally,TerraSAR-X TOPSAR interferometry capability and the possibility of using an inverse TOPSAR configuration are discussed.

Repeat-pass interferometry data were acquired during the first and second SIR-C/X-SAR missions in April and October 1994. This paper presents the first results from X-SAR interferometry at four different sites. The temporal separations were one day and six months. At two sites the coherence requirements were met, resulting in high quality interferograms. A digital elevation model in ground range geometry has been derived. The limitations of the X-SAR interferometry are discussed.>