Abstract
We investigated the sensitivity of fully focused SAR (FF-SAR) processing of Cryosat-2 altimeter data to Earth rotation. Earth’s rotation causes scatterers at varying cross-track locations to have a different relative velocity with respect to the satellite. This second-order effect of Earth rotation on the phase is currently not corrected for in FF-SAR processing of altimetry data. The difference is largest near the poles, where the satellite flies parallel to the equator. Not correcting for the second-order effect yields a parabolic shape in the counter-rotated phase, which increases with the cross-track distance. Its effect is, however, limited by the time-in-view of the scatterer, which is shorter at the edge of the altimeter footprint, and therefore destructive interference will not occur when using Cryosat-2 data. For Cryosat-2, the only expected effect is a reduction in power and along-track resolution in the waveform tail and in the grating lobes. If the FF-SAR processor focuses on one point, and there is a bright scatterer at another, then there is a residual parabolic phase, whose sign and shape depend on the cross-track distance and whether the signal is left or right of the chosen focal point. In theory, if the viewed scene only has few bright coherent scatterers, then it might be possible to determine the cross-track position of each. In practice, however, natural targets are rarely coherent over the integration time.
Highlights
With the launch of Cryosat-2 in April 2010, a new era of satellite radar altimetry began
This study addressed the uncorrected second-order effect of Earth rotation on the range for off-nadir scatterers, which affect the fully focused Synthetic Aperture Radar (SAR) (FF-SAR) processing of satellite radar altimetry data
Earth rotation primarily affects the residual range phase (RRP) correction, which is the largest phase counter rotation to be applied in the FF-SAR process
Summary
With the launch of Cryosat-2 in April 2010, a new era of satellite radar altimetry began. The two Sentinel-3 satellites are operated in SAR mode, which allows to apply FF-SAR processing to their data as well. Sentinel-6 is scheduled to be launched in 2021, which will be the first SAR altimeter to apply interleaved pulsing This removes the along-track contamination caused by the grating lobes. A small residual video phase (RVP) correction (step 3) accounts for quadratic effects of the phase, which is applied in Egido and Smith (2017) for all cross-track positions separately. This combines a parabolic term R02(η) for a nadir target with an offset, to determine the range history for a scatterer cross-track. Barber (1985) shows that in a side-looking SAR system, the effect of Earth rotation on slow time range history depends on cross-track position. We will discuss the consequences and potential benefits for FF-SAR processing
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