TerraSAR-X oceanography

Kurzfassung

TerraSAR-X is a high resolution right looking radar satellite that launched on June 15, 2007. TSX carries a high frequency X-band SAR sensor that can be operated in different modes (resolutions) and polarization. The satellite design is based on technology and knowledge achieved from the successful Synthetic Aperture Radar missions X-SAR/SIR-C and SRTM. The sensor operates in different modes: • the "Spotlight" mode with 10 x 10 km scenes at a resolution of 1-2 meters, • the "Stripmap"mode with 30 km wide strips at a resolution between 3 and 6 meters, • the "ScanSAR" mode with 100 km wide strips at a resolution of 16 meters. • additionally TerraSAR-X supports the reception of interferometric radar data for the generation of digital elevation models. Radar Images are of particularly interest for oceanographers due to the fact that using radar the sea surface can be observed through clouds and independent of illumination by sunlight. Several ocean images acquired over the coastal areas of the Pacific are shown. These image can be used to measure the wind field and the sea state to a high resolution and are thus well suited to investigate the variable conditions in coastal areas. The changing wind field caused by coastal topography can be observed as well as the change in wave length and direction as the ocean waves arrive at the shore. The streaks on the sea surface of the image are used to determine the wind direction. The dark shadows behind the coast help to resolve the 180 degree ambiguity, it can be determined whether the wind is blowing too or away from the coast. The intensity of the image is used to derive the wind speed. Higher wind speeds cause a rougher sea surface and thus more of the incoming radar beam is scattered back to the satellite antenna. Thus, depending on the incidence angle high wind speeds cause bright intensity on the image. To derive the wind field from the SAR images, first the direction of these visible wind streaks is determined. Then the intensity of the calibrated images is used together with the direction to determine the wind speed. For the use with TerraSAR X images a new X band wind speed algorithm XMOD had to be developed. From the radar images ocean wave length, direction and significant wave height can be determined. The change of ocean waves approaching the shoreline and breaking finally can is measured and used to determine underwater topography.