Cloud and rain effects on ALTIKA/SARAL Ka band radar altimeter. Part I: modeling and mean annual data availability

Type Article
Date 2009-06
Language English
Author(s) Tournadre Jean1, 2, Lambin J3, Steunou N3
Affiliation(s) 1 : IFREMER, Lab Oceanog Spatiale, F-29280 Plouzane, France.
2 : IFREMER, UMR 6523, F-29280 Plouzane, France.
3 : Ctr Natl Etud Spatiales, F-31401 Toulouse, France.
Source Transactions on Geoscience and Remote Sensing (0196-2892) (IEEE), 2009-06 , Vol. 47 , N. 6 , P. 1806-1817
DOI 10.1109/TGRS.2008.2010130
WOS© Times Cited 32
Keyword(s) geophysical parameters estimates, rain and cloud impact, Ka band altimeter
Abstract The AltiKa project developed by the French Centre National d'Etudes Spatiales is based on a wide-band Ka-band altimeter (35.75 GHz). The technical characteristic of the instrument will offer higher performance both in terms of spatial and vertical resolution that will lead to improved observation of ice, coastal areas, inland waters and wave height. An Indian Space Research Organization satellite, Saral will embark the AltiKa altimeter. The launch is scheduled at the end of 2010. The major drawback of Ka band use is the attenuation of the radar signal by atmospheric liquid water. Clouds and rain effects will thus be a strong constraining factor, because the altimeter link budget imposes an attenuation of less than 3 dB. The impact of rain and clouds on Ka-band altimeter data are analyzed and quantify using an analytical model that compute Altika waveforms in presence of rain or clouds. The results are then used to quantify the waveforms attenuation and distortion as well as the error induced on the altimeter geophysical parameters estimates. Because of the non-linearity of attenuation relations, the impact of clouds/rain depends more on the clouds/rain variability within the altimeter footprint than on the mean characteristics, which makes correction using coincident rain or cloud data difficult. Small rain cell and small dense clouds can thus strongly distort the waveforms and lead to erroneous geophysical parameters estimates. The probability of 20 Hz and 1-sec averaged data loss are computed from the models results and from cloud and rain climatologies. On a global scale, about 3.5% of the 20 Hz data will be lost because of rain and clouds and 2.5% of the 1-sec averaged data. However, the probability strongly varies over the global ocean and can exceed 10% in the Tropics.
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