Miguel Bruck

High-resolution satellite measurements of coastal wind field and sea state

Methods to derive wind speed and sea state by simple empirical models from synthetic aperture radar (SAR) data are presented and applied for use in high-resolution numerical modelling for coastal applications. The new radar satellite, TerraSAR-X (TS-X), images the surface of the sea with a high resolution up to 1 m. Therefore, not only wind information and integrated sea state parameters but also individual ocean waves with wavelengths down to 30 m are detectable. Two-dimensional information on the ocean surface retrieved using TS-X data is validated for different oceanographic applications: derivation of finely resolved wind fields (XMOD algorithm) and integrated sea state parameters (XWAVE algorithm). Both algorithms are capable of taking into account fine-scale effects in coastal areas. Wind and sea state information retrieved from SAR data are applied as the input for a wave numerical spectral model (wind forcing and boundary condition) running at a fine spatial horizontal resolution of 100 m. Results are compared to collocated buoy measurements. Studies are carried out for varying wind speeds and comparisons against wave height, simulated using original TS-X-derived wind data, showing the sensitivity of waves to local wind variation and thus the importance of local wind effects on wave behaviour in coastal areas. Examples for the German Bight (North Sea) are shown. The TS-X satellite scenes render well-developed ocean wave patterns of developed swell at the sea surface. Refraction of individual long swell waves at a water depth shallower than about 70 m, caused by the influence of underwater topography in coastal areas, is imaged on the radar scenes. A technique is developed for tracking wave rays depending on changes in swell wavelength and direction. We estimate the wave energy flux along wave tracks from deep water to the coastline based on SAR information: wave height and wavelength are derived from TS-X data.

Coastal wave field extraction using TerraSAR-X data

Abstract The main highlights of TerraSAR-X (TS-X) synthetic aperture radar (SAR) imagery are a higher resolution of up to 1 m, when compared to conventional C-band SAR data, and a reduction of nonlinear imaging effects of a moving target by lower platform altitude. Thus, ocean waves with wavelength < 30     m are detectable. This makes TS-X particularly useful to observe coastal areas where complex bathymetry strongly impacts the approaching waves. TS-X images acquired in different coastal areas are presented, including three case studies at the German coast. Wave fields (significant wave height and peak wavelength) are derived from the TS-X imagery using the proposed XWAVE algorithm and compared not only to in situ buoy wave measurements but also to results of a high-resolution numerical wave model. The objective is to study the quality of significant wave height field estimation in the spatial domain in highly variable conditions, which are typically dominant in coastal areas. The results show that the empirical XWAVE algorithm allows estimating wave fields from TS-X data with high resolution, thus showing the spatial information on wave variations. Therefore, it is a new useful tool to characterize sea state in coastal areas by remote sensing.

TerraSAR-X/TanDEM-X sea state measurements using the XWAVE algorithm

The high-resolution TerraSAR-X (TS-X) satellite, which was successfully launched in 2007, and its twin TanDEM-X (TD-X), which was launched in 2010, deliver high-quality radar images over land and ocean, for scientific and commercial applications. We present an empirical algorithm, called XWAVE, to derive significant wave height () from TS-X and TD-X data without the need for a priori information. A set of 200 TS-X/TD-X VV polarized StripMap synthetic aperture radar (SAR) images are collocated with in-situ measurements from buoys in water depths over 100 m. From that data set, 100 data pairs are used for tuning and the remaining 100 are taken for validation of the algorithm. The algorithm is based on Fourier analyses of TS-X/TD-X data and with coefficients which are fitted with collocated in situ buoy wave measurements. The algorithm takes into account the dependence of on incidence angle, sea surface wind speed, and wave propagation direction in relation to satellite heading. The validation results of derived by the XWAVE algorithm against in situ wave measurements show good agreement with a correlation coefficient, r, of 91%, a scatter index, Si, of 0.21, and a bias of 0.01 m. In addition, we also compare the wave peak period and peak direction derived from the SAR image spectra to those measured by buoys. Both comparisons showed good agreement, with verification results for peak period giving an r of 96% and Si of 0.13 and those for peak direction giving an r of 95% and Si of 0.14. Case studies are additionally shown for TS-X/TD-X acquisitions taken over different wave conditions and incidence angles, where the derived one-dimensional spectrum is compared graphically to the one derived from buoy data.

Sea state measurements using TerraSAR-X data

An empirical algorithm (XWAVE) to derive integrated sea state parameters from TerraSAR-X (TS-X) SAR data is developed and validated using NOAA in-situ buoy wave measurements. The comparison for significant wave height and peak wave length was performed as well for deep water locations in open ocean as well as for coastal areas. The significant wave height validation results show a correlation of 0.93% and a scatter index of 0.21 when using in-situ wave buoy data. Verification of the TS-X derived peak wavelength against in-situ buoy data resulted in a correlation of 0.96 and a scatter index of 0.13. The main highlights of TS-X imagery are a higher resolution of up to 1m, when compared to conventional C-band SAR data and a reduction of non-linear imaging effects of a moving target by lower platform altitude. Thus, ocean waves with wavelength less than 30m are detectable. This makes TS-X particularly useful to observe coastal areas where complex bathymetry strongly impacts the approaching waves. In this paper, TS-X images acquired in different coastal areas are presented, including three cases of the German coast and one case near the coast of the Azores Archipelago in the North Atlantic Ocean. Wave fields are derived from the TS-X imagery using the proposed XWAVE algorithm and compared not only to in-situ buoy wave measurements but also to results of a high resolution numerical wave model. The objective was to study the quality of significant wave height field estimation in the spatial domain in highly variable conditions which are typically dominant in coastal areas. The results show that the empirical XWAVE algorithm allows estimating wave fields from TS-X data with high resolution thus showing the spatial information on wave variations. Therefore it is a new useful tool to characterize sea state in coastal areas by remote sensing.

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.

Sea state variability and coastal interaction processes observed by high resolution TerraSAR-X satellite radar images

Methods to derive wind speed and the sea state from Synthetic Aperture Radar (SAR) satellite data with a high resolution are presented and applied for use in numerical modeling for coastal application. The wind and sea state information retrieved from SAR data are applied as input for a wave numerical spectral model (wind forcing and boundary condition of sea state) running at fine spatial horizontal resolution of 100m. Examples for the German Bight, North Sea are shown.