The second-order effect of Earth’s rotation on Cryosat-2 fully focused SAR processing

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.

Validity assessment of SAMOSA retracking for fully-focused SAR altimeter waveforms

“…The furthermost range gates are sampled over a shorter integration time than, e.g., the leading edge (see e.g. Guccione et al, 2018;Kleinherenbrink et al, 2020b;Buchhaupt et al, 2021). This goes hand in hand with a lowered along-track resolution of FF-SAR in these bins.…”

“…15). This can be regarded as a pure technicality, because asymmetric rotation-related range history differences dominate away from the equator in any case (Kleinherenbrink et al, 2020b).…”

“…Also Guccione et al (2018) reported a remaining phase misalignment at this step, without providing more details. Also, at off-nadir positions and with increasing latitude, there will always be a remaining parabola in the phase history due to Earth's rotation (Kleinherenbrink et al, 2020b), which cannot be…”

Introducing the Azimuth Cutoff as an Independent Measure for Characterizing Sea-State Dynamics in SAR Altimetry

“…Moreover, an essential consideration when applying the azimuth cutoff autocorrelation method to other SAR altimeters is that CryoSat-2 and Sentinel-3A/B operate in closed-burst mode, unlike Sentinel-6A which operates in open-burst mode [57]. The former's radar signal transmission-reception configuration was found to introduce ambiguities in the azimuth direction, visible as spatial aliases approximately every 90 m [58]. While these aliases are anticipated to impact the shape of the along-track autocorrelation function, the extent of this effect requires in-depth investigation, a topic beyond the scope of this study.…”

Benefits of Fully Focused SAR Altimetry to Coastal Wave Height Estimates: A Case Study in the North Sea

“…We process the received pulse echoes from the L1a products to acquire the return power waveforms at the L1b data level. This is established using a multi-mission FF-SAR processor implementation originally developed for CS2 by Kleinherenbrink et al (2020), which applies a back-projection algorithm as presented in Egido and Smith (2017). The extension to the S3 and S6-MF missions is described in detail in Ehlers et al (2022).…”