Stephan Brusch

Ship Surveillance With TerraSAR-X

Ship detection is an important application of global monitoring of environment and security. In order to overcome the limitations by other systems, surveillance with satellite synthetic aperture radar (SAR) is used because of its possibility to provide ship detection at high resolution over wide swaths and in all weather conditions. A new X-band radar onboard the TerraSAR-X (TS-X) satellite gives access to spatial resolution as fine as 1 m. In this paper, first results on the combined use of TS-X ship detection, automatic identification system (AIS), and satellite AIS (SatAIS) is presented. The AIS system is an effective terrestrial method for tracking vessels in real time typically up to 40 km off the coast. SatAIS, as a space-based system, allows almost global coverage for monitoring of ships since not all ships operate their AIS and smaller ships are not equipped with AIS. The system is considered to be of cooperative nature. In this paper, the quality of TS-X images with respect to ship detection is evaluated, and a first assessment of its performance for ship detection is given. The velocity of a moving ship is estimated using complex TS-X data. As test cases, images were acquired over the North Sea, Baltic Sea, Atlantic Ocean, and Pacific Ocean in Stripmap mode with a resolution of 3 m at a coverage of 30 km 100 km. Simultaneous information on ship positions was available from TS-X and terrestrial as well as SatAIS. First results on the simultaneous superposition of SatAIS and high-resolution radar images are presented.

An algorithm for the retrieval of sea surface wind fields using X-band TerraSAR-X data

TerraSAR-X (TS-X) is a new, fully polarized X-band synthetic aperture radar (SAR) satellite, which is a successor of the Spaceborne Imaging Radar X-band Synthetic Aperture Radar (SIR-X-SAR) and the SRTM. TS-X has provided high-quality image products over land and oceans for scientific and commercial users since its launch in June 2007. In this article, a new geophysical model function (GMF) is presented to retrieve sea surface wind speeds at a height of 10 m (U 10) based on TS-X data obtained with VV polarization in the ScanSAR, StripMap and Spotlight modes. The X-band GMF was validated by comparing the retrieved wind speeds from the TS-X data with in situ observations, the high-resolution limited area model (HIRLAM) and QuikSCAT scatterometer measurements. The bias and root mean square (RMS) values were 0.03 and 2.33 m s−1, respectively, when compared with the co-located wind measurements derived from QuikSCAT. To apply the newly developed GMF to the TS-X data obtained in HH polarization, we analysed the C-band SAR polarization models and extended them to the X-band SAR data. The sea surface wind speeds were retrieved using the X-band GMF from pairs of TS-X images obtained in dual-polarization mode (i.e. VV and HH). The retrieved results were also validated by comparing with QuikSCAT measurements and the results of the German Weather Service (DWD) atmospheric model. The obtained RMS was 2.50 m s−1 when compared with the co-located wind measurements derived from the QuikSCAT, and the absolute error was 2.24 m s−1 when compared with DWD results.

Tropical Cyclone Intensity Estimated From Wide-Swath SAR Images

Due to the relatively small amount of in situ data available for the open oceans, remote sensing techniques take an important role in the retrieval of geophysical information, particularly during extreme events. The work presented here aims at the improvement of prediction of cyclone intensity using synthetic aperture radar (SAR) images. A new method to measure the hurricane intensity using SAR images, in combination with a parametric Holland-type model of wind speed, is presented. The algorithm is based on a least square minimization of the difference between the parametric model results and the SAR measurement. The radius of the maximum wind speed, required as input for the minimization procedure, is estimated from the SAR image using wavelet analysis. Information on wind direction is extracted from the SAR image through analysis of image features caused by boundary layer rolls. The root-mean-square error of the suggested method has been validated to be equal to 3.9 m/s. The study is based on a data set of wide-swath SAR images of about 400 km × 400 km coverage, acquired by the European Envisat satellite, over tropical cyclones. As a case study, hurricane Katrina is investigated in detail. A total of five tropical cyclone images will be used to validate the results of the new algorithm.

Sonar Measurements in Ship Wakes Simultaneous With TerraSAR-X Overpasses

A pilot experiment was conducted in the period from April to June 2008 in the Straits of Florida near Port Everglades, Florida, in order to study the dynamics of far wakes of ships. In this experiment, a small boat with downward-looking sonar made ¿snakelike¿ sections through wakes of ships of opportunity during the TerraSAR-X overpasses. The ship and its parameters (length, speed, course, etc.) were identified utilizing an automated identification system. The sonar responded to the clouds of microbubbles generated in the ship wake by the propulsion system and ship-hull turbulence. The ship wakes were traced in the sonar signal typically from 10 to 30 min after the ships passage. A preliminary analysis of the measurements suggests that the visibility of the centerline ship wake in synthetic aperture radar (SAR) images is correlated with the presence of microbubbles in the wake. This supports the hypothesis that natural surfactants scavenged and brought to the surface by rising bubbles play an important role in the wake visibility in SAR. The influence of the wind-wave field on the ship wake, as well as the effect of screening of the wind-wave field by the ships hull, adds another level of complexity to wake patterns observed in SAR images.

Underwater bottom topography in coastal areas from TerraSAR-X data

In this article, wave refraction and shoaling in coastal areas were investigated and used to derive the bathymetry. With its high spatial resolution, which can achieve up to 1 m in SpotLight mode, and its low cut-off wavelength, the TerraSAR-X satellite provides images that are particularly suitable for the observation of wave behaviour in transient and shallow waters. By computing the two-dimensional (2D) spectra, shoaling waves were tracked from the open sea to the shoreline. The observed wave refraction and shoaling were compared with wave refraction laws and first-order wave theory (Airy theory). The retrieved bathymetry was compared against depth data from other sources such as ETOPO1, the US Coastal Relief Model and sea charts from the British Admiralty. A further aim of this article was the investigation of breaking waves showing up as near-shore image patterns. A theory is presented of how to derive the height of breaking waves by use of this pattern. Synthetic aperture radar (SAR) images with azimuth as well as range travelling waves were investigated. As test sites, we chose the entrance of Port Phillip near Melbourne (Australia) and the Duck Research Pier in North Carolina (USA).

Synergetic Use of Radar and Optical Satellite Images to Support Severe Storm Prediction for Offshore Wind Farming

In this paper, we show how satellite images taken by space-borne radar sensors can be used to determine mesoscale high-resolution wind fields in synergy with cloud parameters from optical data and, thus, help in the task of maintenance and planning offshore wind farms. The aim of this paper is to use synthetic aperture radar (SAR) and medium resolution imaging spectrometer (MERIS) onboard the environmental satellite (ENVISAT) in synergy to analyze severe weather systems, in particular, to describe the spatial evolution of the atmospheric boundary layer processes involved in cold air outbreaks. We investigated the fine-scale structure of a severe weather case on November 1, 2006 over the North Sea using satellite data. The satellite data are compared with numerical model results of the German Weather Service ldquoLokal Modellrdquo (LM) and the high-resolution limited area model (HIRLAM). LM and HIRLAM show differences in mesoscale turbulent behavior and coastal shadowing. Maximum wind speeds of up to 25 m/s are measured by SAR and are confirmed by the models. Significant differences are observed in the location of the maxima. High-resolution ENVISAT ASAR measurements provide very detailed information on small-scale atmospheric features, which seem to not be captured well by the analyzed numerical models, in particular, in coastal areas. Meteosat second generation (MSG) is used to determine the movement of cloud patterns. Cloud patterns seen in the optical data and radar cross-section modulation give a consistent dynamical picture of the atmospheric processes. The relevance for offshore wind farming is discussed.

Wind and wave observations off the south Pacific Coast of Mexico using TerraSAR-X imagery

High-resolution TerraSAR-X images are analysed for the first time to investigate the effects of gap winds on the sea surface off the south Pacific Coast of Mexico. StripMap and ScanSAR scenes were acquired over the Gulf of Tehuantepec on March 2008, coinciding with a strong Tehuano event with a wind speed of about 24 ms−1. Although spatial variability of wind and wave parameters derived from the image analysis was fairly consistent with model data, Synthetic Aperture Radar (SAR)-derived wind speed was generally underestimated by 33%. The wave-retrieval algorithm, contrastingly, performed well enough to show evidence of shoaling and refracting ocean swell close to the shore. Furthermore, spatial evolution of image intensity spectral peaks suggests the presence of wave energy propagating to the SW, closely aligned with the offshore winds. Although further analyses are required, it could be the first time that growing waves in fetch-limited conditions have been detected through SAR remote sensing.

Ship surveillance by joint use of SAR and AIS

The detectability of ships on synthetic aperture radar (SAR) images is validated by coastal (live) AIS and space AIS. The monitoring area chosen for surveillance is mainly the German coast and the Panama Canal. The delectability in respect to environmental parameters like wind field, sea state, currents and changing coastlines due to tidal effects is investigated.

Use of tandem pairs of ERS-2 and ENVISAT SAR data for the analysis of oceanographic and atmospheric processes

Currently the European satellites ERS-2 and ENVISAT are flying on the same orbit with a time separation of 30 minutes. In this presentation pairs of the respective synthetic aperture radar data are analyzed with respect to different atmospheric and oceanic processes. The presented results were obtained in the framework of the ESA AO project COTAR. The tandem configuration exists since the launch of ENVISAT in 2002. In the presentation an overview will be given of the available image pairs acquired over the ocean on a global scale. Combinations of ERS-2 SAR data with both image mode and wide swath mode scenes provided by the ENVISAT ASAR are considered. The two SAR images enable the analysis of the change of radar cross section within half an hour. This temporal separation is very interesting for oceanographic applications because there are many processes like atmospheric fronts, convective cells, ocean tides, etc., which are detectable on this time scale. In the presentation tandem pairs acquired over the research platform FINO west of the island Borkum in the North Sea will be presented. The area is of high practical interest because of the planned Offshore Windpark Borkum West. The platform provides wind measurements at different heights and additional oceanic information to support the planning activities for this windpark. The dynamics of atmospheric structures is analysed. It is well known that the near surface wind field is a dominating factor for the normalized radar cross section of the sea surface. For this reason SAR scenes are well suited to study atmospheric effects with high spatial resolution. It is shown that the use of tandem pairs enables the study of processes like the propagation of an atmospheric fronts or the evolution of convective cells. Both effects are illustrated with different examples using additional information from in situ measurements. Furthermore existing techniques for the estimation of the wind field in 10 m height from SAR data are applied to both images. The evolution of the spatial structure of both wind speed and wind direction is analysed. The observations are related to some theoretical issues like, e.g., the Taylor hypothesis. Particular emphasize is put on the connection between the spatial and the temporal structure of the wind field. This topic is of high practical relevance, e.g., in the context of offshore windenergy exploitation. Additional applications like ship tracking, oil spill detection and the study of ocean wave field dynamics are briefly discussed as well.

Ship Surveillance with High Resolution TerraSAR-X Satellite in African Waters

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 1 m resolution in Spotlight mode. TS-X can be used for 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 (SatAIS) 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 3 m at a coverage of 30 km× 50 km. 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.