Precipitation Effects for Ka-band SAR

Abstract

Space-borne Synthetic Aperture Radar (SAR) imaging is often considered to possess both day/night and all weather operational capabilities. Whereas the first argument is true since we are dealing with an active sensor; the second does not hold in cases for which the operating frequencies are above ~ 3 GHz. Indeed, the SAR performance can be significantly affected by atmospheric effects (losses), especially at unfavourable weather conditions. The principal reasons for the restriction on the use of these higher frequencies can be found in clear air losses (water vapour and oxygen), cloud attenuation and attenuation due to precipitation, primarily rain. The scope of this paper covers the activities performed for an ESA-study (Danklmayer, 2009 [1]) under Task 1 (Survey on Precipitation Effects for SAR) and Task 2, the quantitative assessment of the propagation effects and the associated modelling. For the Ka-band rain rates of 10 mm/h are capable to produce visible artefacts. Depending on the climatic region on Earth, the availability of the investigated Ka-band system will vary. Assuming a 5 dB acceptance of the attenuation due to rain, which corresponds to 2 mm/h at 30° incidence angle for the modelled rain cell, the availability will be better than 98% for the European regions and better than 95% for rain-forest in Brazil. Finally, measures and possibilities are suggested, how to mitigate propagation effects in SAR images, together with an outlook on remaining issues to be addressed in future studies.