Susanne Lehner

German Aerospace Center (DLR), Köln, North Rhine-Westphalia, Germany

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Publications (363)109.42 Total impact

  • Rudolf Ressel, Anja Frost, Susanne Lehner
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    ABSTRACT: In contrast to SAR single-pol data, which allow only classical image analysis, SAR dual-pol imagery can be analyzed by means of complex polarimetry. Our work investigates the potential of different dual-pol configurations (co-pol, compact polarimetry) in different satellite SAR sensors (TerraSAR-X, Sentinel; Radarsat) for automatic sea ice classification. The first step of our analysis comprises the extraction of polarimetric features. To enrich the information content of image segments, second order statistics on these polarimetric features are additionally computed. The discriminative power and relevance of the different features are ranked by utilizing the concept of mutual information. Different selections of the most relevant features are then fed into a neural network classifier. We explore different network configurations for optimal classification results. Performance is compared for different selections of relevant features. In order to evaluate the generalizability of trained classifiers, data for classification is taken from various geographical regions (Svalbard, Kara Sea, Baffin Island Coast, Antarctic). The outcome for the different sensors is then also discussed in terms of reliability and applicability. The implemented dual-pol processing chain exhibits improved performance over classical single-pol texture based ice classification approaches and is well-suited for fully automated ice charting purposes in near real-time situations. The promising results we achieved for our single-pol based classification algorithm during field campaigns (Akademik Shokalskyi, Polarstern, Lance) can therefore also be expected for dual-pol data, complementing our portfolio of navigation assistance products.
    36th Canadian Symposium of Remote Sensing; 06/2015
  • Rudolf Ressel, Anja Frost, Susanne Lehner
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    ABSTRACT: Over the last three decades, the Arctic summer sea ice coverage has decreased significantly. This trend is expected to continue due to persistent climate change. Besides increased research efforts in this field, this phenomenon has also attracted attention from maritime end-users. To keep Arctic shipping routes safe, monitoring of icebergs and drift ice are crucial. Satellite borne remote sensing, in particular Synthetic Aperture Radar (SAR), is ideally suited to this purpose. Wide coverage, high-frequency availability, and Independence from daylight and cloud coverage are among the major advantages of this data source. We propose automated iceberg detection and sea ice classification algorithms based on TerraSAR-X imagery and their application for near real-time purposes. Operational data acquired during several cruises into ice-infested waters are discussed. We show how maritime users benefit from such value-added SAR based products.
    36th International Symposium on Remote Sensing of Environment (ISRSE); 01/2015
  • Suman Singha, Domenico Velotto, Susanne Lehner
    PolinSAR 2015, Frascati, Italy; 01/2015
  • Rudolf Ressel, Anja Frost, Susanne Lehner
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    ABSTRACT: In this work, we examine the performance of an automated sea ice classification algorithm based on dual polarimetric TerraSAR-X data. Polarimetric features are extracted from HHVV dualpol stripmap images. In a second step, the feature vectors are fed into an artificial neural network to classify each pixel into an ice type. The first part of our analysis addresses the predictive value of different subsets of features for our classification process (by means of measuring mutual information). Different neural network configurations are then explored for optimal classification performance. The results on a TerraSAR-X dataset indicate a high reliability of a trained dual polarimetric classifier. Performance speed and accuracy promise applicability for near real time operational use.
    ESA POLinSAR 2015; 01/2015
  • Xiao-Ming Li, Susanne Lehner, Sven Jacobsen
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    ABSTRACT: The fast development of offshore wind farms have drawn increasing attention to monitoring the offshore wind turbine wakes, which are of significant importance on improving layout of large offshore wind farms, prediction of accurate output power, and safety operation of wind turbines. Spaceborne synthetic aperture radar shows its unique advantages of observing the offshore wind turbine wakes due to high spatial resolution and large coverage. Since the launch of RADARSAT-2, TerraSAR-X and Cosmo-SkyMed in 2007, the new generation spaceborne SAR in high spatial resolution up to 1 m offers a unique advantage to investigate the fine structures of oceanic and atmospheric phenomena occurred in the air-sea interface. In the paper, we present some TerraSAR-X images acquired at the offshore wind farms in the North Sea and the East China Sea. The high spatial resolution SAR images show different sea surface wake patterns downstream of the offshore wind turbines. The analysis suggests that there are major two types of wakes among the observed cases. The wind turbine wakes generated by movement of wind around wind turbines are the most often observed cases. In contrast, due to the strong local tidal currents in the near shore wind farm sites, the tidal current wakes induced by tidal current impinging on the wind turbine piles are also observed in the high spatial resolution TS-X images. The discrimination of the two types of wakes observed in the offshore wind farms is also described in the paper.
    36th International Symposium on Remote Sensing of Environment (ISRSE); 01/2015
  • Susanne Lehner
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    ABSTRACT: High resolution remote sensing Synthetic Aperture Radar (SAR) data from TerraSAR-X/Tandem-X satellites are used to determine and monitor the sea surface in near real time and all weather and illumination conditions. The radar backscatter of the sea surface is determined by the sea surface roughness caused by the wind field and the sea state. These meteo parameters are modelled by the newly developed algorithms XMOD and XWAVE relating the wind field and sea state, depending on incidence angle and directionality to the radar backscatter Sigma 0. The TerrasAR-X Modes Stripmap, Scan SAR and Scan SAR Wide are used to detect ships, oil spills and icebergs. The detectability depending on the background conditions is discussed. Several examples from near real time campaigns performed together with users are given.
    36th International Symposium on Remote Sensing of Environment (ISRSE); 01/2015
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    ABSTRACT: Obviously, radar resolution and swath width are two very important factors when it comes to synthetic aperture radar (SAR) maritime targets detections. The dilemma of using single polarization SAR imagery with higher resolution and coverage or quad- (or dualpolarimetric) imagery with its richness of information, is still unsolved when it comes to this application. In the framework of ESA project MARISS and EU project DOLPHIN, in situ campaigns aimed at solving this dilemma have been carried out. Single and multipolarimetric SAR data acquired by TenaSAR-X, RADARSAT-2 and COSMO-SkyMed have been acquired with close time gaps and partial coverage overlap. In this way several moving and non-moving maritime targets have been imaged with different polarization, geometry and working frequency. Available ground truth reports provided by Automatic Identification System (AIS) data, nautical chart and wind farm location are used to validate the different types of maritime targets.
    ESA POLinSAR 2015; 01/2015
  • Source
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    ABSTRACT: The estimation of marine and meteorological parameters is an important task for operational oceanographic services. Remote sensing Synthetic Aperture Radar (SAR) data from TerraSAR-X/Tandem-X satellites have been used for validation and verification of new developed coastal forecast models. In comparison to in-situ buoy measurements at a single location allows remote sensing to cover large areas and to estimate the spatial distribution of investigated characteristics. Methods to derive wind speed and sea state using simple empirical models from high-resolution SAR data have been developed by DLR especially for coastal applications: a new nonlinear wind algorithm XMOD-2 and new empirical model function XWAVE-2 for obtaining significant wave height from X-band data. Both algorithms are capable of taking into account finescale effects in coastal areas. In order to improve forecast accuracy in coastal area a new high-resolution Coastal Wave Model (CWAM) for the German Bight and the western Baltic Sea has been developed by the German Meteorological Service (DWD) and the German Maritime and Hydrographic Agency (BSH) with resolution of ~900m and more accurate mapping of coastlines (1) and coupled to a irculation model (2). Uncertainties are present when dealing in coastal areas: wind input implying well-timed atmospheric front propagation and physical interaction-processes in shallow water caused by interplay between waves, currents and bottom become important. Due to currents and bottom influence the waves refract, dump, become steeper and break. The lost wave energy dissipates and transfers into turbulence and acceleration of flow currents (radiation stress). Sea state estimated from series of TerraSAR-X images cover strips with length of ~200km and width of 30km over German Bight from East-Frisian Islands to Danish coast. The acquired data with more than 50 images were used for model validations. The comparisons show a number of local variations due to variety in bathymetry and wind fronts.
    36th International Symposium on Remote Sensing of Environment (ISRSE); 01/2015
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    ABSTRACT: High resolution remote sensing Synthetic Aperture Radar (SAR) data from TerraSAR-X/Tandem-X satellites are used to determine and monitor the waterline in the German Wadden Sea area. The Wadden Sea is a very unique and dynamic coastal region located in the North Sea in the German bight. Tidal flats extend several kilometres away from the coast during low tide with features like tidal inlets and sand banks. Under the influence of tidal water currents transporting large amounts of eroded material, these sand banks are also moved over time; heavy storms can even cause large variations in sand bank extensions in merely a few hours. The Wadden Sea is also subject to high ship traffic to the ports of Hamburg, Bremerhaven, Wilhemshaven and others. Hence, observation of obstacles like sand banks and decreasing water depth is crucial for maritime security. Conventional monitoring campaigns with ships or airplanes are economically expensive and can only provide limited coverage. From the high resolution TerraSAR-X/Tandem-X data of the Wadden Sea and large river estuaries like Elbe and Weser, the waterline at the time of recording is extracted using an automatic algorithm with Near-Real-Time capability. This allows for a fast and large scale determination of changes in island and coastal outlines. Additionally, the bathymetry of the observed area can be determined from the extracted waterlines by combining data from many different flybys at different tidal levels.
    36th International Symposium on Remote Sensing of Environment (ISRSE); 01/2015
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    ABSTRACT: The increasing demand for renewable energy resources has promoted the construction of offshore wind farms e.g. in the North Sea. While the wind farm layout consists of an array of large turbines, the interrelation of wind turbine wakes with the remaining array is of substantial interest. The downstream spatial evolution of turbulent wind turbine wakes is very complex and depends on manifold parameters such as wind speed, wind direction and ambient atmospheric stability conditions. To complement and validate existing numerical models, corresponding observations are needed. While in-situ measurements with e.g. anemometers provide a time-series at the given location, the merits of ground-based and space- or airborne remote sensing techniques are indisputable in terms of spatial coverage. Active microwave devices, such as Scatterometer and Synthetic Aperture Radar (SAR), have proven their capabilities of providing sea surface wind measurements and particularly SAR images reveal wind variations at a high spatial resolution while retaining the large coverage area. Platform-based Doppler LiDAR can resolve wind fields with a high spatial coverage and repetition rates of seconds to minutes. In order to study the capabilities of both methods for the investigation of small scale wind field structures, we present a direct comparison of observations obtained by high resolution TerraSAR-X (TS-X) X-band SAR data and platform-based LiDAR devices at the North Sea wind farm alpha ventus. We furthermore compare the results with meteorological data from the COSMO-DE model run by the German Weather Service DWD. Our study indicates that the overall agreement between SAR and LiDAR wind fields is good and that under appropriate conditions small scale wind field variations compare significantly well.
    36th International Symposium on Remote Sensing of Environment (ISRSE); 01/2015
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    ABSTRACT: South Africa has a coastline of approximately 3900 km in length and is the 39th longest coastline in the world. The coastal area over which South Africa has jurisdictional rights (Exclusive Economic Zone, EEZ) cover approximately 1.5 million square km. This area represents a key zone for South Africa economical activities (fishing, shipping, etc.), international maritime trading: one may think that about 30% of European- and Americans-bound oil from the Middle East comes through the Cape Sea Route, one of the most important routes between East and West. Furthermore, South Africa is located at a maritime choke point surrounded by three oceans, the Indian, the South Atlantic and the Southern one. The South Africa large number of well-developed ports and related maritime infrastructures accounts for a value excess of 90% of its imports and exports carried by sea. Moreover, the commercial fishing industries emply approximatively 25000 people producing a gross income of more than 300 million euros annually. This information witnesses that a continous and updated surveillance of the maritime environment with its activities is of fundamental importance for South Africa trading and security. Traditionally, ships have been monitored using transponder cooperative systems such as the Automatic Identification System (AIS). Damages and sabotages of transponder-based systems, together with delays in communication, make this monitoring unsatisfactory. Hence, by combining the advantages of Synthetic Apeture Radar (SAR) observations with the information gathered by AIS, such drawbacks can be overcome. In such a context, a controlled experiment has been set up off the South Africa coasts from December 10 to December 16, 2013 to analyze the performance of different multi-polarization/multi-frequency algorithms. SAR acquisitions have been planned together with the Italian Space Agency ASI, the German Aerospace Center DLR and the Indian Space Research Organization ISRO, while available ground reference and ancillary data have been provided by the South African Maritime Safety Authority (SAMSA) and the Council for Scientific and Industrial Research (CSIR). The processed data set consists of multi-polarization and multi-frequency SAR data collected by several satellite missions: the Italian X-band Cosmo-SkyMed (CSK), the German TerraSAR-X, and the Indian C-band RISAT-1. All the scenes have been collected from December 10 to December 16, 2013. Eight CSK Stripmap scenes have been collected (7 by the single-polarization Himage mode and 1 by the incoherent dual-polarimetric PingPong mode). Six coherent dual-polarimetric TerraSAR-X scenes have been collected by the Stripmap dual-polarimetric mode, while four Fine Resolution Stripmap scenes have been collected by RISAT-1 according to the new hybrid-polarity mode. The data set is processed using different algorithms. Single-pol co-polarized SAR data are processed using the approach proposed in [1][2]. Single-pol cross-polarized SAR data are processed using the approach proposed in [2]. Dual-polarimetric coherent SAR data are processed using the approach proposed in [3][4]. Hybrid-polarity SAR data are processed using new tailored methodologies developed starting from wave-polarimetry concepts.
    ESA POLinSAR 2015; 01/2015
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    ABSTRACT: A TerraSAR-X (TS-X) Synthetic Aperture Radar (SAR) image acquired at the East China Sea offshore wind farm presents distinguished wakes at a kilometer scale on the lee of the wind turbines. The presumption was that these wakes were due to wind movement around blades. However, wind analysis using spaceborne radiometer data, numerical weather prediction and in situ measurements suggest that the prevailing wind direction did not align with the wakes. By analyzing measurement at the tidal gauge station and modeling of the tidal current field, these trailing wakes are interpreted to have formed when a strong tidal current impinged on the cylindrical monopiles of the wind turbines. A numerical simulation was further conducted to reproduce the tidal current wake under such conditions. Comparison of the simulated surface velocity in the wake region with the TS-X sea surface backscatter intensity shows a similar trend. Consequently, turbulence intensity (T.I.) of the tidal current wakes over multiple piles is studied using the TS-X observation. It is found that the T.I. has a logarithmic relation with distance. Furthermore, another case study showing wakes due to wind movement around turbine blades is presented to discuss the differences in the tidal current wakes and wind turbine wakes. The conclusion is drawn that small-scale wakes formed by interaction of the tidal current and the turbine piles could be also imaged by SAR when some conditions are satisfied. The study is anticipated to draw more attentions on the impacts of offshore wind foundations on local hydrodynamic field.
    Journal of Geophysical Research: Oceans 08/2014; 119(8). DOI:10.1002/2014JC009822 · 3.44 Impact Factor
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    Suman Singha, Domenico Velotto, Susanne Lehner
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    ABSTRACT: Continuous operational monitoring by means of remote sensing contributes significantly towards less occurrence of oil spills over European waters however, operational activities show regular occurrence of accidental and deliberate oil spills over the North Sea, particularly from offshore platform installations. Since the areas covered by oil spills are usually large and scattered over the North Sea, satellite remote sensing particularly Synthetic Aperture Radar (SAR) represents an effective tool for operational oil spill detection. This paper describes the development of a semi-automated approach for oil spill detection, optimized for near real time offshore platform sourced pollution monitoring context. Eight feature parameters are extracted from each segmented dark spot. The classification algorithm is based on artificial neural network. An initial evaluation of this methodology has been carried out on 156 TerraSAR-X images. Wind and current history information also have been analyzed for particular cases in order to evaluate their influences on spill trajectory.
    Marine Pollution Bulletin 07/2014; DOI:10.1016/j.marpolbul.2014.06.041 · 2.79 Impact Factor
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    Carlos Bentes, Domenico Velotto, Susanne Lehner
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    ABSTRACT: A growing interest in global maritime surveillance has mo-tivated the development of new methods and sensors to overcome the limitations of coastal-based systems cover-age. Among the available state of the art tools, the Synthetic Aperture Radar (SAR) stands out as a powerful sensor for oceanographic observation and maritime surveillance, due to its capability of wide swath coverage and the ability to operate in all weather conditions. This paper examines the minimum ship size detectable by the Constant False Alarm Rate (CFAR) algorithm over different TerraSAR-X product modes, using simulation-based analyses. These analyses allow a comparison of the expected CFAR ship detection per-formance for different TerraSAR-X products, over different sea state conditions.
    IEEE International Geoscience & Remote Sensing Symposium, 2014; 07/2014
  • Suman Singha, Domenico Velotto, Susanne Lehner
    IEEE GOLD Remote Sensing Conference, Berlin, Germany; 06/2014
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    Domenico Velotto, Susanne Lehner
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    ABSTRACT: In the framework of Maritime Security and Safety, Synthetic Aperture Radar (SAR) ship detection is nowadays cru-cial. Nevertheless, automatic SAR ship detection is not an easy task due to the speckle and SAR artifacts that may cause false alarms. Although speckle can be reduced with proper filter algorithms or multi-looking technique, SAR artifacts are still open issues. From the ship detection point of view, those artifacts that are often generating false alarms are the azimuth ambiguity. Regarding this issue, the aim of this paper is twofold. First, the complete processing chain developed for Very High Resolution (VHS) X-band quad-pol TerraSAR-X (TS-X) data is extended to High Resolution (HR) C-band fine quad-pol Radarsat-2 (RS-2) data. Second, the proposed azimuth ambiguity re-moval strategy is compared to alternative methodologies existing in literature. A set of C-and X-band PolSAR data has been processed and the performances of the analyzed methods evaluated.
    European Conference on Synthetic Aperture Radar, 2014; 06/2014
  • Carlos Bentes, Björn Tings, Susanne Lehner
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    ABSTRACT: Maritime surveillance is an important application of Synthetic Aperture Radar (SAR) satellite systems, and ship detection plays an important role in the maritime scenario observation. The new Wide ScanSAR TerraSAR-X mode allows larger swath coverage than previous TerraSAR-X modes, therefore the new mode is a natural choice for oceanographic observations. This abstract analyses the use of Wide ScanSAR images for ship detection applications, using a constant false alarm rate (CFAR) algorithm. Preliminary results are shown and a comparison with the StripMap mode is performed.
    EUSAR 2014; 10th European Conference on Synthetic Aperture Radar, Berlin, Germany; 06/2014
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    ABSTRACT: In this article, the polarization ratio (PR) of TerraSAR-X (TS-X) vertical–vertical (VV) and horizontal–horizontal (HH) polarization data acquired over the ocean is investigated. Similar to the PR of C-band synthetic aperture radar (SAR), the PR of X-band SAR data also shows significant dependence on incidence angle. The normalized radar cross-section (NRCS) in VV polarization data is generally larger than that in HH polarization for incidence angles above 23°. Based on the analysis, two PR models proposed for C-band SAR were retuned using TS-X dual-polarization data. A new PR model, called X-PR hereafter, is proposed as well to convert the NRCS of TS-X in HH polarization to that in VV polarization. By using the developed geophysical model functions of XMOD1 and XMOD2 and the tuned PR models, the sea surface field is retrieved from the TS-X data in HH polarization. The comparisons with in situ buoy measurements show that the combination of XMOD2 and X-PR models yields a good retrieval with a root mean square error (RMSE) of 2.03 m s–1 and scatter index (SI) of 22.4%. A further comparison with a high-resolution analysis wind model in the North Sea is also presented, which shows better agreement with RMSE of 1.76 m s–1 and SI of 20.3%. We also find that the difference between the fitting of the X-PR model and the PR derived from TS-X dual-polarization data is close to a constant. By adding the constant to the X-PR model, the accuracy of HH polarization sea surface wind speed is further improved with the bias reduced by 0.3 m s–1. A case acquired at the offshore wind farm in the East China Sea further demonstrates that the improvement tends to be more effective for incidence angles above 40°.
    International Journal of Remote Sensing 06/2014; DOI:10.1080/01431161.2014.916059 · 1.36 Impact Factor
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    ABSTRACT: A continuous and updated surveillance of coastal areas is of paramount importance for many application. In particular, when dealing with metallic target at sea monitoring, this continuous surveillance is of key importance for fisheries, oil extraction, commercial traffic control, etc. On this purpose, the key role played by Synthetic Aperture Radar (SAR) has been unanimously recognized. It is a microwave active tool that allows all day and almost weather-independent non-cooperative imaging of the observed scene. However, SAR-based target at sea observation is a challenging task, due to both speckle and meteo-marine factors, i.g. sea state, wave patterns, etc. that may generate false targets or missed ones. In this framework, it is of paramount importance to test SAR-based detection algorithms using a controlled experiment. In this context, in 2013 a controlled experiment has be performed, together with the South African Maritime Safety Authority (SAMSA), off the South Africa coast. Several ships were deployed at sea and ancillary wind/sea state information were retrieved from Automatic Identification System (AIS) data together with spatially and timely co-located SAR observations. Preliminary results presented in this study are related to a multi-polarization analysis of target detection techniques undertaken using both X- (TerraSAR-X, Cosmo-SkyMed) and C-band (RISAT-1) actual SAR data. The dataset was collected between December 9th -13th, 2013, with different polarizations and incidence angles, and different wind/sea state conditions apply (average wind speed from 5 kt up to 35 kt). The latter information is obtained from a ground truth dataset composed by WindSAT and OceanSAT-2 scatterometer data and by coastal ground stations data. Furthermore, satellite and coastal AIS data have provided information about ships position and their descriptive characteristics.
    IEEE GOLD Remote Sensing, Berlin, Germany; 06/2014
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    ABSTRACT: Ship detection is an important application of monitoring of environment and security or safety issues in African Waters. In order to overcome the limitations by other monitoring systems, e.g. coastal radar, surveillance with satellite synthetic aperture radar (SAR) is used because of its potential to detect ships at high resolution over wide swaths and in all weather conditions and independent from sun illumination. TerraSAR-X (TS-X) is an X-band polarimetric SAR capable of imaging up to 1m resolution in Spotlight mode. TS-X can be used fo r a wide variety of applications and methods of analysis including visual interpretation, mapping, digital-elevation-model creation, disaster monitoring, and oceanography. Results on the combined use of TS-X ship detection, automatic identification system (AIS), and satellite AIS (Sat AIS) are presented. Using AIS is an effective terrestrial method for tracking vessels in real time typically up to 40 km off the coast. SatAIS is a space-based system with nearly global coverage for monitoring of AIS equipped ships. Since not all ships operate their AIS and smaller ships are not equipped with AIS, space borne SARs provide complimentary means for ship monitoring. As cases , images were acquired over the Somali Coast Area, South African Coast and Gibraltar in Stripmap mode with a resolution of 3m at a coverage of 30km×50km. The rapid tasking performance as well as the short response time of the TS-X data acquisition of the ground segment DLR-BN (Ground Station Neustrelitz, Germany), are very helpful to monitor hotspot areas such as the Gulf of Aden . For ascending orbits the delivery time of ship detection products is less than 20 min. Along with the detected ship positions, estimated wave heights and wind fields derived from large-area TS-X imagery can be used to get a detailed maritime picture of the situation
    Remote Sensing of the African Seas, 01/2014;

Publication Stats

2k Citations
109.42 Total Impact Points

Institutions

  • 2000–2014
    • German Aerospace Center (DLR)
      • Remote Sensing Technology Institute (IMF)
      Köln, North Rhine-Westphalia, Germany
    • Technische Universität München
      München, Bavaria, Germany
  • 2010
    • Parthenope University of Naples
      • Department of Technologies
      Napoli, Campania, Italy
  • 2004
    • Miami University
      Oxford, Ohio, United States
  • 2002
    • Remote Sensing Solutions GmbH
      Baierbrun, Bavaria, Germany
  • 1999–2002
    • Helmholtz-Zentrum Geesthacht
      • Institute for Coastal Research
      Stadt Geesthacht, Schleswig-Holstein, Germany
    • Delft University of Technology
      Delft, South Holland, Netherlands