Susanne Lehner

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

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Publications (347)109.41 Total impact

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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; · 2.53 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
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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. 07/2014;
  • Suman Singha, Domenico Velotto, Susanne Lehner
    IEEE GOLD Remote Sensing Conference, Berlin, Germany; 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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    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
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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; · 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
    01/2014;
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    Acta Adriatica 07/2013; 54(1):13-26. · 0.36 Impact Factor
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    Xiaoming Li, Susanne Lehner
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    ABSTRACT: A geophysical model function (GMF), which is denoted by XMOD2, is developed to retrieve sea surface wind field from X-band TerraSAR-X/TanDEM-X (TS-X/TD-X) data. In contrast to the previously developed XMOD1, XMOD2 consists of a nonlinear GMF, and thus, it depicts the difference between upwind and downwind of the sea surface backscatter in X-band synthetic aperture radar (SAR) imagery. By exploiting 371 collocations with in situ buoy measurements that are used as the tuning data set together with analysis wind model results, the retrieved TS-X/TD-X sea surface wind speed using XMOD2 shows a close agreement with buoy measurements with a bias of −0.32 m/s, a root-mean-square error (RMSE) of 1.44 m/s, and a scatter index (SI) of 16.0%. Further validation using an independent data set of 52 cases shows a bias of−0.17 m/s, an RMSE of 1.48 m/s, and an SI of 17.0% comparing with buoy measurements. To apply XMOD2 to TS-X/TD-X data acquired at HH polarization, we validate three X-band SAR polarization ratio models that were tuned using TS-X dual-polarization data by comparing the retrieved sea surface wind speed with buoy measurements.
    IEEE Transactions on Geoscience and Remote Sensing 07/2013; · 3.47 Impact Factor
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    ABSTRACT: Some early results of the TerraSAR - X observations of the northeastern Adriatic bora wind are presented in this paper. TerraSAR - X is a German X - band radar satellite launched in 2007 that carries phased array X - band synthetic aperture radar (SAR) operating in different polarizations and providing multiple imaging modes. SAR backscatter can be used to derive wind fields at spatial re solution that no other instrument can provide. Terrain - induced jet and wake patterns are particularly conductive to the SAR - instrument examination. Bora, a cold and dry downslope wind blowing from north - easterly directions on the e astern side of the Adriatic Sea, exhibits such a response. Since bora is primarily winter wind and the town of Senj is known for frequent and severe bora episodes we focus on TerraSAR - X scenes collected in the winter s of 2011 and 2012 over an area with Senj roughly in its center. Recently developed XMOD2 geophysical model function is used for wind magnitude derivation, whereas the WRF model was employed to estimate the wind direction. The selected TerraSAR - X scenes have captured representative bora events exhibiting rich details in bora - induced jet and wake patterns on the lee of the Dinaric Alps. T h e details registered in the normalized radar cross section response strongly suggest the need for still higher resolution numerical simulations in order to properly model the orographic impact on and the fine details in the surface wind field. Comparisons with both research and operational modeling results indicate that the currently used geophysical model function may benefit from enlarging the matchup data base with samples of severe winds.
    Acta Adriatica 06/2013; 54(2013-06-1):13-26. · 0.36 Impact Factor
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    ABSTRACT: New radar satellites image the sea surface with resolutions as high as 1 m. A large spectrum of ocean processes can be estimated using such Earth observation data. These data have been applied to investigations of geophysical processes as well as to forecast model validations and near-real-time services. The numerous processes, parameters, and features observed in high-resolution synthetic aperture radar images include winds, waves (with wavelengths as small as 30 m), oil slicks, waterline changes, changes in seabed morphology in shallow waters, wakes and bow waves of ships, underwater topography, wave energy flux along wave tracks from deep water to the coast, and breaking waves. New algorithms have been developed that are capable of taking into account fine-scale effects in coastal areas. - See more at: http://www.tos.org/oceanography/archive/26-2_lehner.html#citation
    Journal of Oceanography 06/2013; 26(2):80–91. · 1.46 Impact Factor
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    ABSTRACT: This paper discusses the potential for automatic ocean surveillance using compact linear polarization (CL-pol) synthetic aperture radar (SAR), with large area coverage. Here, the target is a wind farm in the North Sea. The relative phase, as derived from CL-pol SAR, is employed for detection of the wind turbines, apart from the wind turbines' wakes, based on fine-mode quad-polarization (quad-pol) RADARSAT-2 (RS-2) images. The relative phase of CL-pol measurements improves the contrast between the wind turbines and their wakes, because it has opposite signs for these two entities. Moreover, there is almost no variation in the relative phase with respect to wind speed or incidence angle. The results are verified by high-sea-state cases, up to 8.7-m significant wave height and 24.3-m/s wind speed, and also 641 quad-pol RS-2 SAR images collocated with 52 National Data Buoy Center buoys at different incidence angles and sea states. Thus, the relative phase of CL-pol SAR provides new light into the problem of operational autodetection of man-made targets, under high-sea-state conditions, over large areas.
    IEEE Transactions on Geoscience and Remote Sensing 06/2013; 51(6):3299-3305. · 3.47 Impact Factor
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    Xiaoming Li, Susanne Lehner, Thomas Bruns
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    ABSTRACT: Simultaneous measurements of significant wave height (SWH) obtained from two independent active microwave sensors of Radar Altimeter 2 (RA-2) and Advanced Synthetic Aperture Radar (ASAR) onboard the ENVIronmental SATellite (ENVISAT) are used for a global verification of ocean wave models (WAMs). In the present study, SWH is retrieved from ASAR wave mode data using the empirical algorithm called C-band WAVE algorithm for ENVISAT, which is capable of representing total SWH irrespective of the cutoff of SAR. Comparisons of two radar measurements with the reanalyses ERA-Interim model (with assimilation of RA-2 measurements) and the German operational WAM (Deutscher Wetterdienst Global Sea wave Model, without assimilation before 2008) show that both WAMs agree well with ASAR and RA-2 measurements. However, the discrepancies of agreement indicate to which extent that the assimilation of RA-2 measurements can improve the performance of WAMs. Moreover, differences in the comparisons of ASAR and RA-2 measurements with the same WAM of ERAInterim reveal that, although assimilation of RA-2 significantly improves the accuracy of model on grids near the RA-2 tracks, the improvement decreases along with the increase of distance between model grids and RA-2 tracks.
    IEEE Transactions on Geoscience and Remote Sensing 05/2013; · 3.47 Impact Factor
  • Xiaoming Li, Susanne Lehner
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    ABSTRACT: A new Geophysical Model Function (GMF), denoted XMOD2, is developed to retrieve the sea surface wind field from X-band TerraSAR-X/Tandem-X (TS-X/TD-X) data. In contrary to the previous XMOD1, XMOD2 is based on a nonlinear GMF, and moreover it also depicts the difference between upwind and downwind of the sea surface backscatter. By exploiting 371 collocations, the retrieved TS-X/TD-X sea surface wind speed U10 by XMOD2 agrees well with in situ buoy measurements with a bias of 0.39 m/s, an RMSE of 1.52 m/s and a scatter index (SI) of 16.1%. To apply XMOD2 to TS-X/TD-X data acquired at HH polarization, we verify the X-band SAR Polarization Ratio (PR) models by comparing the retrieved sea surface wind speed to in situ buoy measurements as well. Based on 62 collocated pairs, it is found that by using the Elfouhaily type PR model and XMOD2 yields better U10 retrieval with a bias of -0.27 m/s, an RMSE of 2.06 m/s and a SI of 22.7% than using the X-PR model which yields a bias of -0.98 m, and RMSE of 2.30 m and a SI of 23.4%. Several TerraSAR-X and TanDEM-X ScanSAR images are acquired in October, 2012 to track the Hurricane Sandy. Three of the images are acquired in the open sea, which are presented in this chapter to demonstrate observations of sea surface wind and wave extracted from X-band ScanSAR image with high spatial resolution of 17 m in the hurricane. In the case of the TerraSAR-X image acquired on October 26, 2012, we analyze the peak wave direction and length of swell generated by Hurricane Sandy, as well as interaction of swell with the Abaco Island, Bahamas. In the other two cases, sea surface wind field derived from the TerraSAR-X and TanDEM-X acquired on October 27 and 28 are presented. The sea surface wind speed retrieved by the X-band Geophysical Model Function (GMF) XMOD2 using wind direction derived from SAR images and the NOAA Hurricane Research Division (HRD) wind analyses are both presented for comparisons. We also compare the retrieved sea surface wind speed with Stepped Frequency Microwave Radiometer (SFMR) to quantify effect of rainfall on X-band SAR images.
    04/2013;
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    ABSTRACT: A physical dual-polarimetric model to observe man-made metallic targets at sea in dual-polarimetric coherent X-band synthetic aperture radar (SAR) data is proposed. The model exploits the intrinsic different symmetry properties of man-made targets and sea surface and is tested over actual StripMap TerraSAR-X HH–HV and VV–VH dual-polarimetric SAR data and colocated ground truth measurements. Then, an operational physically based filter to observe targets at sea is proposed. The filter is very attractive in terms of both detection performances and processing time. A typical SAR scene is processed in seconds by a conventional PC processor.
    IEEE Geoscience and Remote Sensing Letters 03/2013; 10(05):1114 - 1118. · 1.82 Impact Factor
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    ABSTRACT: Synthetic aperture radar (SAR) ship detection is an important application in the field of maritime security. Azimuth ambiguities caused by the aliasing of the Doppler phase history of each point are often visible in SAR images particularly in ocean areas of low wind speed condition, e.g., in coastal areas, in harbors, etc. The main sources of azimuth ambiguities are man-made metallic structures over the ocean, e.g., ships, oil platforms etc., and over land near the coast, e.g., big tanks, bridges' pylons etc., that have a high SAR backscatter responses. Although the ambiguities' backscatter is generally low, in many situations, it is above the surrounding ocean clutter and are mistaken by classic detection techniques, like constant false alarm rate, as real targets causing false positives. This paper addresses both the discrimination of real targets from non-trivial false positives, namely those due to azimuth ambiguities and the detection itself using a Generalized-$K$ distribution approach. The methodology is firstly proposed and demonstrated over a significant data set of full polarimetric X-band SAR data, which have been acquired by the German satellite TerraSAR-X during the experimental dual receive antenna campaign in April and May 2010. It is based on the intrinsic configuration of monostatic two-channel PolSAR systems and relies on the different signature of azimuth ambiguities in cross-polarized channels. Automatic Identification System messages collected and collocated with the data set analyzed are used as ground truth to evaluate and validate the proposed methodology.
    IEEE Transactions on Geoscience and Remote Sensing 02/2013; · 3.47 Impact Factor
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    ABSTRACT: to get introduced to applications of SAR oceanography, to get introduced to geophysical background, to learn methods application possibilities, limitations and to learn the results applicability
    Education TOOL; 01/2013

Publication Stats

1k Citations
109.41 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
  • 2012
    • The International Society for Optics and Photonics
      International Falls, Minnesota, United States
  • 2011
    • Deutscher Wetterdienst
      Offenbach, Hesse, Germany
  • 2010
    • Nova Southeastern University
      Florida, New York, United States
  • 2007
    • Remote Sensing Solutions GmbH
      Baierbrun, Bavaria, Germany
  • 2004
    • Miami University
      Oxford, Ohio, United States
  • 1999–2002
    • Helmholtz-Zentrum Geesthacht
      Stadt Geesthacht, Schleswig-Holstein, Germany
    • Delft University of Technology
      Delft, South Holland, Netherlands