PERTEO is a mission proposal intended to provide real-time EO services to reduce Natural Disaster impact by proposing a heterogeneous small satellite constellation and performing in-orbit processing. For that aim, this mission takes advantage of the enhanced capabilities of AI edge processing by enabling on-demand user services through the "satellite as a service" concept. The design of the constellation includes three types of sensors: SAR, Hyperspectral and Multispectral imager VHR combining their capabilities to achieve almost real-time due to their in-orbit distances. Six spacecrafts are considered in each orbit organized in pairs (180° phased platforms) mounting the same instrument. The proposed solution is responsive achieving almost real-time latencies ( Ex: ≲ 1 min, down to seconds is achieved in the first observation product). Persistence is achieved with a low revisit time (≲ 1 hour any payload; ≲ 3 hour s any specific payload choice).

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.

Satellite remote sensing acquisitions are usually processed after downlink to a ground station. The satellite travel time to the ground station adds to the total latency, increasing the time until a user can obtain the processing results. Performing the processing and information extraction onboard of the satellite can significantly reduce this time. In this study, synthetic aperture radar (SAR) image formation as well as ship detection and extreme weather detection were implemented in a multiprocessor system on a chip (MPSoC). Processing steps with high computational complexity were ported to run on the programmable logic (PL), achieving significant speed-up by implementing a high degree of parallelization and pipelining as well as efficient memory accesses. Steps with lower complexity run on the processing system (PS), allowing for higher flexibility and reducing the need for resources in the PL. The achieved processing times for an area covering 375 km2were approximately 4 s for image formation, 16 s for ship detection, and 31 s for extreme weather detection. These developments combined with new downlink concepts for low-rate information data streams show that the provision of satellite remote sensing results to end users in less than 5 min after acquisition is possible using an adequately equipped satellite.

This paper describes the concept and prototype implementation of a low latency spaceborne onboard Synthetic Aperture Radar (SAR) processor runing on a Multi-Processor-System-On-Chip (MPSoC) computing device combining an ARM processor and a Field-Programmable-Gate-Array (FPGA). The SAR processor is designed to generate SAR imagery from TerraSAR-X stripmap data for subsequent ship detection and sea state determination. Low latency data processing is a key development goal. Currently, a raw data block of$8\mathrm{k}\times 32\mathrm{k}$samples, covering 375 km2to 500 km2, is focused on the hardware within 4 s. Together with an attached level-2 ship detection, wind, and sea state processor, running on the same device, a SAR data processing chain for generation of maritime alerts is formed. This chain is part of a larger prototype system being developed in the frame of the H2020 EO-ALERT project which further comprises an optical data chain, data compression/encryption, and scheduling on multiple reconfigurable MPSoC boards.

In modern society, the ever-increasing demand for Earth Observation products in a large variety of sectors is exposing the limitations of traditional satellite data chain architectures. The European Union Horizon 2020 EO-ALERT project aims at overcoming the existing bottlenecks by leveraging the performance of state-of-the-art commercial off-the-shelf devices to move the critical elements of data processing on the flight segment without sacrificing processing performance. This paper introduces the architecture of the EO-ALERT CPU Scheduling, Compression, Encryption and Data Handling Subsystem, responsible for coordinating the onboard optical and Synthetic Aperture Radar data chains, as well as providing data compression, encryption, and storage services. The performance obtained by a reference implementation of the proposed architecture is also presented, showing an extremely low contribution to the overall system latency that allows real-time Earth Observation product delivery to the end user in less than 5 min.

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.

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.

In the past years we have exploited the TanDEM-X elevation model for impact crater research. This is the first high-resolution global DEM which permits accessing all confirmed impact structures. We demonstrate that the high horizontal and vertical accuracies, coupled with the dense pixel grid, allow studying the morphologies of simple and complex craters over a wide diameter range.

Synthetic aperture radar (SAR) is a well-established technique for observing the Earth on a global scale. As applications become more demanding, it is desirable to overcome the limitations imposed by the SAR principle, one of which is the tradeoff between the swath width and the instantaneous azimuth bandwidth, determining the resolution. Recombination of multiple channels with displaced phase centers has been proposed as a convenient way to create high resolution wide-swath images. We analyze various approximations made in the channel transfer functions and their impact on the reconstruction result using examples inspired by current imaging modes of the TerraSAR-X and TanDEM-X missions. In order to do so, we introduce an efficient method to assess the quality of reconstruction filters for an arbitrary number of channels without the need of full time-domain simulations.

The work presented exploits the long Synthetic Aperture Radar (SAR) of a single TerraSAR-X Staring Spotlight (ST) acquisition to derive absolute heights. Here, the slight azimuth defocussing effect due to height mismatch between true height and the height assumed in SAR focusing is analyzed. The impact is almost negligible for most of acquisition modes. In contrast, spaceborne modes with very long aperture, such as TerraSAR-X ST acquisition mode, present sensibility that can be used to retrieve absolute heights. The accuracy depends on incidence angle, orbit type and mainly on Signal to Clutter Ratio (SCR). A result over real data is presented to demonstrate that absolute heights can be retrieved with an accuracy of few meters using a single TerraSAR-X ST acquisition.

The work presented in this letter exploits the long synthetic aperture radar (SAR) of a single TerraSAR-X Staring Spotlight (ST) acquisition to derive absolute heights. Here, the slight azimuth defocusing effect due to height mismatch between the true height and the height assumed in SAR focusing is analyzed. The impact is almost negligible for most of acquisition modes. In contrast, spaceborne modes with very long aperture, such as the TerraSAR-X ST acquisition mode, present sensibility that can be used to retrieve absolute heights. The accuracy depends on incidence angle, orbit type, and mainly on signal-to-clutter ratio. Two different results are presented to demonstrate that absolute heights can be retrieved with accuracy of few meters using a single TerraSAR-X ST acquisition.

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.

InSAR Processing and especially phase unwrapping is a crucial step in the realization of the global TanDEM-X DEM. After three years of operation, the InSAR chain has delivered more than 350,000 RawDEMs, which mostly fulfill already the high accuracy requirements. At the same time, it has been also optimized and improved to cope with input data of various quality. Special efforts have been made in the dual-baseline interferometric chain to deal with CoSSCs with larger ranges of height of ambiguity and time spans. This paper presents the key elements of the InSAR chain and analyses the dual-baseline phase unwrapping results.

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.

The German SAR (synthetic aperture radar) satellites TerraSAR-X (TSX-1) and TanDEM-X (TDX-1), launched in June 2007 and June 2010 respectively, provide an unprecedented geometric accuracy. Previous studies showed an absolute pixel localization for both sensors at the centimeter level [4] [5] [6]. However, recent measurements show that in range, under extraordinary good conditions, a location accuracy of even a few millimeters seems to be attainable. While on a long-term scale, we observed a slow variation of subsequent measurements; on a short-term scale, they coincided to within a few millimeters. The measurement series will be continued. The cause of the long-term variation is the subject of current investigation.

Our paper reports about temporal changes of the Earth's topography found in first samples of TanDEM-X data when compared to SRTM Digital Elevation Models collected ten years earlier. We give an accuracy prediction and explain the methods to avoid systematic errors from biases and phase unwrapping ambiguities. Our paper provides examples from different Earth surface domains such as glaciers, volcanoes, mining activity and urban areas. We demonstrate and conclude that the higher the resolution of a DEM gets, the more frequent updates are required to keep the DEM valid with time.

Started in June 2010, the TanDEM-X satellite joined the TerraSAR-X satellite in space to perform the conjoint interferometric TanDEM-X mission to acquire a truly global Digital Elevation Model (DEM) of unprecedented accuracy [1]. Since the very first interferometric acquisitions, the Integrated TanDEM-X Processor (ITP) delivered operationally “Raw”-DEMs and complex products of mono- and bistatic data. The RawDEMs are scenes of about 50 km × 30 km, generated for a dedicated DEM Mosaicking and Calibration Processor (MCP) which produces the final DEM. The so-called Coregistered Single-look Slant-range Complex (CoSSC) products are provided for each of these scenes in different flavors for production internal purposes and system performance monitoring as well as for scientific use. The capabilities of the ITP go far beyond the primary mission objective of DEM generation alone: it also provides the operationally available end-user products from different experimental modes as e.g. pursuit monostatic, dual polarization bistatic data, alternating bistatic in single and dual polarization and different bistatic and alternating bistatic spotlight modes. This paper focuses on the accuracy of the generated products, the ITPs contribution to the achieved accuracy of the data and the direct effect of it on the use and interpretation of RawDEM heights for temporal change detection. Also the basic characteristics of the operational experimental products are introduced..

During the second operational year of the TanDEM-X mission, a second coverage of the whole land mass is acquired in order to produce a high accurate and high resolution DEM from a combination of both data sets. This paper presents the dual-baseline interferometric processing chain. Its main steps consist of coregistering the different interferograms (having different baselines), of unwrapping the phases and of comparing them to eliminate the possible unwrapping errors.

The German SAR (synthetic aperture radar) satellites TSX-1 and TDX-1 outperform former civilian space-borne radar sensors in terms of geometric accuracy. Recently, both satellites constitute the first bistatic SAR system in space: TanDEM-X. With the high geometric accuracy of both satellites, the geolocation of ground targets has to accurately consider signal propagation effects and geodynamic effects which formerly were negligible. The prediction of expected radar positions is additionally complicated by the bistatic acquisition geometry of TanDEM-X. Here, we present our approach to cope with these demanding requirements. With the application of these techniques, measurements of the geometric accuracy of TSX-1 and TDX-1 reveal that their pixel localization accuracy is significantly better than in previous studies.

The German TanDEM-X mission flies the two satellites TerraSAR-X (TSX) and TanDEM-X (TDX) in a close orbit formation establishing a bistatic interferometer in space. A major challenge in bistatic SAR data processing is the synchronization of the satellites' oscillators. The paper at hand summarizes the synchronization concept, provides in-orbit verification and performance figures, describes the compensation approach within the SAR processing workflow and finally quantifies the synchronization performance in terms of achieved DEM accuracy.

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 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.

The purpose of this paper is to investigate the estimation of surface currents directly on stripmap TerraSAR-X data, as an alternative to Along Track Interferometry. The algorithm relies on efficient baseband magnitude-based Doppler estimation, preventing the estimate from possible biases like azimuth ambiguities and strong target reflections. The validation of the algorithm is made with acquisitions over the Elbe estuary river in Germany and the Eyjafjallajökull volcano in Iceland.

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.

Synthetic aperture radar (SAR) image intensity is disturbed by additive system noise. During SAR focusing, pattern corrections that are adapted to the characteristics of the wanted signal, but not to the characteristics of the noise, influence the spatial distribution of the noise power. Particularly in the case of ScanSAR, a distinct residual noise pattern in low backscatter areas results. This necessitates a noise-adapted radiometric correction of the focused image for almost all applications except interferometry. In this paper, we thoroughly investigate this topic. Based on signal theoretical and stochastic considerations, we develop a radiometric correction scheme. Simulations and the application of the algorithm to TerraSAR-X datatakes support the theoretical results.

The TerraSAR-X mission was launched in June 2007. After successful completion of the commissioning phase, the mission entered its operational phase in January 2008. Since that time, TerraSAR-X provides the scientific remote sensing community and commercial customers with high-quality spaceborne synthetic aperture radar (SAR) data products. The intention of this paper is to present the SAR data processing concept and the comprehensive portfolio of products reflecting the instrument's diverse imaging capabilities together with options of processing and achieved product quality as well as the essentials of SAR processing. Furthermore, it shall also provide details on how to fully exploit the precision of the TerraSAR-X products.

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.

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.

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.

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.

TerraSAR-X, the first national German radar satellite, was launched in June 2007. It carries an X-band high-resolution synthetic aperture radar instrument featuring Stripmap, ScanSAR and, particularly, Spotlight imaging in a variety of different polarization modes. The mission completed its commissioning phase (CP) in December 2007, the provision of the SAR products for both the scientific and commercial user community was started in January 2008. One central TerraSAR-X element on ground is the pay-load ground segment PGS. From the beginning of the mission, PGS was nominally operated. About ten thousand data takes were already acquired and processed in 2007, not only for SAR verification and calibration purposes, but also for the operational ground segment validation. This paper provides the commissioning and early operational phase results from the SAR payload data processing perspective addressing data reception and SAR processing. Specifically the tuning and adjustment of the TerraSAR-X multi-mode SAR processor TMSP to meet the in-orbit data characteristics and to optimize the SAR focusing results is addressed. Relevant issues are the high-bandwidth chirp replica processing, side lobe suppression, Doppler frequency determination, processor normalization and phase-preservation.

Compared to conventional optical images, the classification of remote sensing SAR images represents a rather difficult task. As a rule, the various SAR imaging and product options, the high dynamic range of SAR images, and the presence of speckle noise prevent us from obtaining robust classification results. In the following, we try to circumvent these difficulties by proper pre-processing and despeckling of high resolution SAR images. Our approach aims at adapting the radiometry and the resolution of the scenes to the optimal target recognition capabilities of a classification algorithm. To this end, we have to apply systematic adaptations that provide optimized multilooking of our input data. Then these adapted data can be fed into a scene classification system.

In February 2007 the German TerraSAR-X satellite will be launched and the TerraSAR-X mission will enter its approximately 5 months commissioning phase. At that time, the challenging developments on both sides, the advanced high- resolution multi-mode SAR instrument on the one hand and the corresponding sophisticated TerraSAR-X ground segment on the other hand will prove correct interaction and functioning. Screening and processing of the SAR data is the task of the DLR developed TerraSAR Multi Mode SAR Processor TMSP. Preceded by data reception, transcription including decryption and followed by archiving, cataloguing and product delivery, processing of the data by the TMSP is the central part of the SAR data workflow implemented in the Payload Ground Segment PGS. Space and ground segment have been subject to intense complete system testing on ground. Here, the compatibility of SAR instrument commanding, SAR instrument operations and subsequent SAR data processing has been successfully proven for the various acquisition modes of the sensor. Compliance of specified and measured product performance has been investigated as far as possible utilizing simulated point target SAR data. However, the real challenge will be the screening and SAR processing of TerraSAR-X data acquired in orbit and linked down to the receiving station. Therefore, the complete reception and processing chain will be properly tuned and adjusted to the properties of the received TerraSAR-X payload data. The TMSP algorithms have to be configured, e.g. thresholds for calibration pulse analysis, estimation window sizes for SAR data analysis, parameterization of estimation algorithms. Also the configuration of product variants with respect to resolution and radiometric quality will be checked and refined. This paper presents the very first experiences in reception, transcription, screening and processing of TerraSAR-X data with respect to performance, throughput and quality. During the TMSP checkout phase the compatibility of instrument commanding and SAR processing have to be verified and the accordance of SAR performance prediction and the obtained product performance and quality have to be investigated. First characteristics of the SAR data with respect to raw data statistics, calibration pulse analysis and Doppler centroid measurements will be shown. As far as available examples of SAR image products featuring the different image modes, Stripmap, ScanSAR and Spotlight at different incidence angles and polarizations will be displayed and a first estimate of product performance parameters will be given.

Both atmospheric and oceanic processes play an important role in the dynamics of tropical cyclones. Due to the relatively small amount of in situ data available for extreme events like hurricanes or typhoons remote sensing techniques play an important role in the measurement of the relevant geophysical parameters. In this paper some recent results on SAR observation of extreme wind conditions are summarized. The study is based on the use of ENVISAT ASAR wide swath images. The objective of this study is to investigate the performance of SAR to improve the existing model for the retrieval of information on wind field under extreme wind and wave conditions using information from a parametric Holland type model.

A new empirical algorithm-CWAVE [1] was developed at the German aerospace center (DLR). A global dataset of two years (September 1998 to December 2000) of ERS SAR data was reprocessed to more than one million SAR imagettes. Met ocean parameters like significant ocean wave height (Hs), wind speed (U10) and mean wave period (Tm-10) are derived from the SAR images using the CWAVE algorithm [1]. The results are compared to collocated ERS altimeter data and in situ measurements from NOAA buoys and observations taken onboard the vessel Polarstern. It is shown that the SAR derived Hsis comparable in quality to altimeter measurements and can thus be used for real time assimilation.

Summary form only given. The advanced high-resolution German SAR satellite TerraSAR-X is scheduled to be launched at the end of May 2007. Due to its daylight and weather independent applicability in combination with a large spatial coverage and a short acquisition time, SAR has become a promising tool for traffic monitoring in recent years. Ground moving target indication (GMTI) techniques shall be applied to TerraSAR-X data in order to demonstrate the capability of a space borne SAR sensor to monitor traffic flows on highways. A series of GMTI experiments were to be carried out during the commissioning phase of the TerraSAR-X satellite. In first trials, cars, which are equipped with special radar reflectors and GPS receivers, were to be used as moving target references that are imaged in TerraSAR-X data takes. In a follow on experiment, arbitrary cars on motorways were to be imaged simultaneously by TerraSAR-X and by an airborne high-resolution camera. Car tracks extracted from the series of the optical images shall serve as a reference for the evaluation of the TerraSAR-X moving target data in this case. The paper presents first results of the data evaluation. An experimental GMTI processing system is used to detect and measure moving targets in both single-channel and dual-channeldata. The dual-channel data, which enable the application of well established GMTI methods like the along-track interferometry (ATI) or displaced phase centre array (DPCA) techniques, are acquired either in the so-called "aperture switching" mode with virtual multiple receiving channels or in the dual-receive antenna (DRA) mode with physically separated receiving channels. The paper reports on the analysis of the first experimental GMTI data by using different detection and measurement strategies. This includes the adapted processing of the SAR raw data with respect to the moving target signals, the incorporation of GIS data in the detection and measurement process and the application of different detectors for across- and along-track velocity components of the moving cars. The quality of the data is thoroughly analyzed and conclusions are drawn for the development and the performance of a fully automatic GMTI processing system for TerraSAR-X. Furthermore, an outlook on the planned experiments is given.

TerraSAR-X is a German satellite to be launched towards end of the year 2006. The SAR instrument built by EADS/Astrium has a multipolarized active phased array antenna and is programmable in wide ranges. This allows to operate imaging modes such as e.g. stripmap, spotlight, ScanSAR and dual-polarization modes. By further dividing the antenna in halves with different position or different polarization, experimental modes for ground moving target detection (GMT) or Quad-Polarization are possible. As a consequence of the various sensor capabilities the tree of SAR products offered by the DLR Ground Segment contains many different variants of polarization, resolution and scene size. Even more product variants are caused by different processing parameters selectable by the user. Some product performance parameters, such as the radiometric accuracy as a function of geometric resolution or the image resolution as a function of incidence angle depend on both, sensor settings (PRF, pulse duration) and on processing parameters (bandwidth, weighting, multi-looking). The processing parameters for the products have been designed by DLR in a way so that valuable products with maximum information content are offered to the user. In order to generate the specified products, the TerraSAR Multi Mode SAR Processor TMSP has been designed by DLR. This processor will be operated at the DLR payload ground segment in Neustrelitz for commercial and scientific customers. The processor will also be operated at additional direct access partner stations of Infoterra GmbH, who will serve private and public customers.

We present one of the first studies on ocean current retrievals from interferometric synthetic aperture radar (InSAR) data acquired during the Shuttle Radar Topography Mission (SRTM) in February 2000. The InSAR system of SRTM was designed for high-resolution topographic mapping of the Earth's land surfaces, using two SAR antennas on a Space Shuttle with a cross-track separation of 60 m. An additional along-track antenna separation of 7 m resulted in an effective time lag of about 0.5 ms between the two images, which could theoretically be exploited for target velocity retrievals. However, the feasibility of ocean current measurements with SRTM has been questionable, since the time lag was much shorter than the theoretical optimum (about 3 ms at X-band) and the signal-to-noise ratio over water was quite low. Nevertheless, some X-band InSAR images of coastal areas exhibit clear signatures of tidal flow patterns. As an example, we discuss an image of the Dutch Wadden Sea. We convert the InSAR data into a line-of-sight current field, which is then compared with results of the numerical circulation model KUSTWAD. For tidal phases close to the conditions at the time of the SRTM overpass; we obtain correlation coefficients of up to 0.6 and rms differences on the order of 0.2 m/s. Furthermore we find that SRTM resolves current variations down to spatial scales on the order of 1 km. This is consistent with predictions of a numerical InSAR imaging model. Remaining differences between SRTM- and KUSTWAD-derived currents can be attributed mainly to residual motion errors in the SRTM data as well as to a limited representation of the conditions at the time of the SRTM overpass in the available KUSTWAD results.

An update of the status of SAR Along Track Interferometry (ATI) is given and examples from X-SAR/SRTM for the waters around the Orkney Islands are provided. A special InSAR "Ocean-Processor" is described. The derived SRTM "velocity maps" demonstrate how valuable this method can be for a world-wide mapping of tidal ocean currents and for the detection of promising sites for hydropower installations

The TerraSAR-L system, currently being designed in a Phase B definition study, provides ESA with its most powerful radar-imaging programme to date. The platform is optimized for and built around the 11 mtimes2.9 m active phased array antenna of the L-band Synthetic Aperture Radar (L-SAR). The L-SAR features, on top of standard Stripmap and ScanSAR operations, full polarimetric capabilities, repeat-pass ScanSAR interferometry and a Wave Mode. Specification of the L-SAR has been guided by a careful analysis of the product requirements resulting in a robust baseline design with considerable margins. Besides, a major contribution to applications in areas of climate change and oceanography, the TerraSAR-L design responds specifically to requirements from interferometric applications. One key element of the TerraSAR-L operations strategy is a long-term systematic and repetitive acquisition scenario to ensure consistent data archives and to maximize the scientific and commercial exploitation of this SAR system. The other important factor is a systematic processing of all acquired data to Single-look Slant-range Complex (SSC) products to facilitate higher level product generation and services based on these products. This paper describes the TerraSAR-L basic product tree and explains the rationale behind. The nominal TerraSAR-L imaging modes are introduced and their performance characteristics are described. The generation concept of multi-look detected products based consistently on complex products - generated either at an intermediate stage during ground segment processing or serving as input product into a stand-alone tool - is introduced. A modular SAR processor design using the same complex product generation algorithms as far as possible independent of the underlying imaging mode is addressed

TerraSAR-X is a national German satellite providing a high-resolution X-Band SAR instrumentation featuring StripMap, ScanSAR and SpotLight imaging modes in various polarizations. The experimental full polarization mode is carried out using a dual receive antenna capability. An active lifetime of five years is anticipated, the launch is expected in 2006. The TerraSAR-X mission serves both commercial and scientific needs. It is implemented in the frame of a public-private partnership between EADS Astrium GmbH (providing the TerraSAR-X Space Segment) and the German Aerospace Center DLR (providing the TerraSAR-X Ground Segment). The commercial product exploitation rights are with Infoterra GmbH, the scientific ones remain at DLR. The Infoterra business concept foresees the inclusion of a Direct Access Service through which TerraSAR-X data may be directly received by stations either for the sole purpose to produce geo-information products for their own and private applications or to generate and distribute commercial products. Essential tasks of the TerraSAR-X Payload Ground Segment developed (and later operated) by DLR are the SAR payload data reception, their archiving and processing, and the distribution of the generated SAR products to users. This paper summarizes the TerraSAR-X Payload Ground Segment design. It describes the SAR data workflow ranging from the payload data reception at the Neustrelitz Ground Station to the long-term data archiving in the multi-mission Product Library at the German Remote Sensing Data Center. Supplementary data as provided by the TerraSAR-X Mission Operation Segment and used during the data-driven generation of the SAR level 0 products are described together with their dissemination concept. The adaptability of the chosen approach for the operation of a Direct Access Station is addressed

In February 2000, the shuttle radar topography mission SRTM was flown as the first single-pass SAR interferometer in space. The goal was to obtain a global digital elevation model. Therefore, a secondary receive-only antenna on the top of a 60 m long deployable mast supplemented the primary transmit and receive antenna mounted in the shuttle cargo bay. Due to mechanical constraints, the interferometric baseline did not only consist of the terrain height sensitive across track component but also contained an along track component of 7 m. The resulting time lag between the data acquisitions by the two antennas causes phase differences, which are proportional to the line-of-sight velocity of moving targets. Since this system ability has been expected prior to the SRTM launch, a traffic monitoring experiment has been carried out during the mission, which showed a surprisingly good agreement between the GPS velocity measurements on ground and the measurement from space. A strategy to exploit the along track interferometric (ATI) phase and the coherence together with the azimuth displacement of moving vehicles in focused SAR images has been developed and applied on sample test sides for traffic monitoring on highways.

Latest concepts for the TerraSAR-X hardware design include the availability of a split antenna mode, in which the fore and aft halves of the SAR antenna array with a total length of 4.8 m can act as separate receiving antennas for along-track interferometry (ATI). The effective ATI time lag is 0.17 ms. We discuss in this paper whether the split antenna mode is suitable for ocean current measurements. Typical (tidal) currents in coastal areas are on the order of 1 to 2 m/s. To measure them, the ATI time lag should ideally be 20 times longer than the time lag of TerraSAR-X. Furthermore, the backscattered power from the ocean at low wind speeds can be below the noise floor of the instrument. However, the high resolution of TerraSAR-X permits averaging over many pixels to reduce noise. Simulated data products suggest that TerraSAR-X is capable of measuring currents with satisfactory accuracy at a spatial resolution of about 1 km, which is sufficient for many applications.

The feasibility of exploiting the along-track separation of 7 m between the two X band antennas of SRTM for ocean current measurements was already demonstrated by us at IGARSS 2002. We presented an SRTM image of the Dutch Waddenzee with clear phase variations in water-covered areas, which could be converted into line-of-sight current variations by about 1.3 m/s. A comparison with a current map from a tidal atlas showed good qualitative agreement, but digital reference data for a quantitative validation of the SRTM results were not available. In this work we present a detailed comparison of the SRTM data with simulated current fields from the numerical circulation model KUSTWAD. We obtain an overall correlation of 0.558, a regression coefficient of 1.011, and an rms difference between SRTM and KUSTWAD currents of 0.24 m/s. Furthermore, we analyze the correlation of spatial variations in the measured and simulated current fields on different length scales. We find that SRTM can resolve the current variations which are relevant to KUSTWAD down to scales on the order of 1 km.

In February 2000, the Shuttle Radar Topography Mission acquired a global digital elevation model (DEM) within only 11 days. During the years 2000 and 2001 extensive testing and calibration activities primarily based on ocean surface data took place at DLR. The calibration phase is now finished and the quality of the products analyzed so far met all specifications. The routine processing of the German X-band data at DLR began in November 2001, starting with Europe. Now, that hundreds of image frames over Europe are being processed and mosaicked to a continent-wide DEM, the largescale system stability becomes visible. Our paper reports the experiences from mosaicking based on self-consistency tests and on comparison with independent reference data. While the overall quality is promising, there are still some outliers to be investigated and compensated. The calibration concept based on the use of ocean data will be presented and discussed. Adaptations and improvements based on self-consistency will be presented, that allow a more robust and automated processing.

We present one of the first studies in which interferometric synthetic aperture radar (InSAR) data from the Shuttle Radar Topography Mission (SRTM) are analyzed with regard to the detectability of ocean surface current variations. The InSAR system of SRTM was designed for high-resolution topographic mapping, using two SAR antennas on a Space Shuttle with a cross-track separation of 60 m. For technical reasons, there was an additional along-track antenna separation of 7 m, which results in a time lag of about 0.5 ms between the acquisitions of images by the two antennas. In theory, this time lag causes additional phase differences, which are proportional to the line-of-sight velocity of moving targets and can thus be exploited, to some extent, for measuring oceanic currents. Indeed, some SRTM images acquired over water exhibit clear signatures of typical flow patterns. We show an example of an X-band phase image of the Dutch Waddenzee and discuss the plausibility of interpreting it in terms of sea surface height or current variations or the effect of waves. We find that only currents can be responsible for phase variations of the observed magnitude on spatial scales of a few 100 meters. We convert the data into a surface current field, which is found to be consistent with the theoretical current field at the time of the SRTM overflight according to a current atlas. Based on this encouraging result and theoretical findings, we discuss the general potential of SAR interferometry from space for oceanic applications.

A global dataset of complex synthetic aperture (SAR) images is processed from wave mode raw data acquired by the ERS-2 satellite. Using these data, different algorithms for wind and wave measurements recently developed in view of future ENVISAT ASAR data are analyzed on a statistical basis. Different aspects of complex SAR wave mode processing with the DLR processor BSAR are discussed and global statistics of processing parameters are presented. Single-look complex (SLC) imagettes give the opportunity to apply multilook techniques in range as well as in azimuth. Such methods are used to reduce speckle noise or to analyze the time evolution of the ocean surface cross section during SAR integration time. A global analysis of different new algorithms for wind and ocean wave measurements, taking advantage of SLC data, is given. Wind speed is estimated with the azimuthal cross-correlation algorithm (CCA). As a modification of the existing CCA, range multilooking is used to deal with the speckle bias. Homogeneity of the imagettes is considered. Wind speed is derived from mean SAR image intensities taking into account wind direction (CMOD algorithm). Comparison with collocated ERS-2 scatterometer data shows reasonable agreement with the CCA and good agreement for the CMOD approach. Using imagettes instead of image power spectra allows the authors to study ocean surface features caused by natural slicks, sea ice, or atmospheric processes. The impact of these phenomena on SCAT measurements is considered. Cross spectral methods are used to derive the ocean wave propagation direction from complex imagettes on a global basis. Comparison with model data provided by the European Center for Medium Range Weather Forecast (ECMWF) shows good agreement.

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.>