Cloud and Rain Effects on AltiKa/SARAL Ka-Band Radar Altimeter-Part I: Modeling and Mean Annual Data Availability

The AltiKa project, developed by the French Centre National d'Etudes Spatiales, is based on a wideband Ka-band altimeter (35.75 GHz). The technical characteristic of the instrument will offer higher performance both in terms of spatial and vertical resolutions that will lead to the improved observation of ice, coastal areas, inland waters, and wave height. An Indian Space Research Organization satellite, called Satellite with ARgos and AltiKa, 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 is analyzed and quantified using an analytical model that computes AltiKa waveforms in the presence of rain or clouds. The results are then used to quantify the waveform attenuation and distortion, as well as the error induced on the altimeter geophysical parameter estimates. Because of the nonlinearity of attenuation relations, the impact of clouds/rain depends more on the cloud/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 parameter estimates. The probability of 20 Hz and 1-s averaged data loss are computed from the model 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-s averaged data. However, the probability strongly varies over the global ocean and can exceed 10% in the Tropics.