Can anyone explain the science behind the need to frequency unwrap Sentinel-1 TOPS IW azimuth data?

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John Wegrzyn

I do not see a reference to warping in the linked document. The level of processing detail is not as high in this reference as it is in the following: "https://sentinels.copernicus.eu/web/sentinel/user-guides/sentinel-1-sar/document-library/-/asset_publisher/1dO7RF5fJMbd/content/sentinel-1-level-1-detailed-algorithm-definition" - which is the basis of my image formation algorithm. Section 6.2.4 deals with the Azimuth Frequency UFR (UnFolding and Resampling). I am using "wrapping" synonymously with "folding". Figure 6.6 shows the collected data to be wrapped into PRF extent and the subsequent sections discuss how to remove this. This is truly wrapped information and can only properly be processed when unwrapped. There likely is some sort of sampling/hardware interaction that is specifically causing this, but I have not seen or experienced it in any SAR system or collection modality before. I am looking for a better description than I have found in the other posted/published ESA documentation.

Also an interesting reference, but does not directly address the question. Is it really as simple as the PRF generates harmonics that wrap through the digitizer during the deramp? If so, why don't other SAR systems (spotlight in particular) have the same artifact in the collected data? How is it it only shows up in the azimuth processing direction? I still don't understand the physical interpretation of this wrapping in azimuth processing. Anyone else game to concretely explain the phenomenon?