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As I understand, for elevation mapping using InSAR, one typically requires an out-of-plane baseline to create the required phase difference between images to detect objects at height. This usually requires either multiple satellites flying in formation or waiting for a repeat pass of a single satellite.

This paper, A New Single-Pass SAR Interferometry Technique with a Single-Antenna for Terrain Height Measurements, suggests that one could achieve this in a single pass (along-track interferometry), if we are able to image at a high squint angle. The idea is that a grazing-angle difference is still present in the along-track case when squint angle is high. This is not the case for broadside imaging.

However apart from this paper, I was not able to find any other sources to cross-reference the viability of this approach, nor does it seem that anyone else has reproduced the results. The mathematical principle behind seem sound to me. If it works, why is this concept not being used more often, and if it doesn't actually work, where is the error in the logic?

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So I was in NASA's SAR group for 20 years, and I don't think this question will be answered here. Anyway, phase difference is about baseline dot wave vector ($\vec B \cdot \vec k$)...where if it's pulsed it the mixed down phase, not the carrier phase, so somehow by high squint and along track phase center separation you get a decent dot product. Once you start looking with $\vec k \cdot \vec v_{\rm platform} \ne 0$ you get Doppler ambiguity problems, range correction problems...it just gets harder and harder to process. (For instance, in discussing decorrelation mechanisms, the paper explicitly mentions $\gamma_{\rm rotation}$, which is due to the weird geometry).

So to answer the main question: why hasn't this been done before: because it's sub-optimal vs a normal InSAR geometry (eq: https://en.wikipedia.org/wiki/Shuttle_Radar_Topography_Mission). Just spend the money and build the system that works best, instead of mixing side looking SAR and along track SAR, which have different purposes, because you only have one antenna.

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