Signatures Of Extended Meterological Targets Measured With The Space-borne Synthetic Aperature Radar TerraSAR-X And Their Comparison With Simultaneous Weather Radar Measurements

Abstract

TerraSAR-X, the first civil German space-borne synthetic aperture radar (SAR) satellite, has been successfully launched on 15th of June, 2007. The main purpose of Synthetic aperture radar systems is to map the Earth-surface in high resolution and synthetic aperture radars are often considered as day/night and all-weather imaging systems. Whereas the first argument is true, the second does not hold in every case depending on the operating frequencies as will be shown in more detail. Indeed, recent examples of typical rain-induced signature modification have been recorded with the X-band TerraSAR-X system. It is well known that the specific attenuation of the signals may be around 1 dB/km assuming a rain-rate of 40 mm/hr and such an occurrence may frequently take place for tropical areas over rain-forest. Attenuation up to 20 dB and beyond may occur through the precipitation volumes in the cases of heavy precipitation, such as the Brazilian rainforest. However, as will be shown, even the northern latitudes are vulnerable to precipitation induced distortions. In Effect, the disadvantage of rain features in SAR imagery may turn out to be a useful source for assessing precipitation intensity over SAR surveyed areas. For instance, there is a great potential in quantifying precipitation over oceanic surfaces, a problem hitherto only poorly addressed. During the commissioning phase, a total of 12000 SAR-images (scenes) have been investigated for potential “propagation effects” and some scenes have been selected that revealed visible atmospheric effects. In this contribution, we will present recent example of rain cell signatures in SAR-images and we will focus on a particularly interesting example acquired over New York, where the SAR image will be compared with weather-radar data acquired nearly simultaneously (within the same minute). The comparison of the images show a good overall agreement and it can be clearly shown that reflectivities in the weather radar image of 50 dBz may cause visible artefacts in the SAR images. Due to the polarimetric capabilities of TerraSAR-X it was further possible to visualize the polarimetric behaviour of the signal attenuation through extended meteorological targets. The main advantage of assessing weather volumes, using the SAR-method presented here, is the high resolution obtained indirectly from the analysis of propagation effects in SAR-pixels. The methodology permits us to exploit the high ground-resolution of SAR systems (typically 1 m x 1 m) in order to obtain precipitation mapping bearing similar resolutions. The proposed algorithm suggests further studies exploiting data fusions of space-borne SAR measurements with space-borne weather radar measurements in order to improve the intensity mapping of precipitating clouds over Earth-surface.