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I was exploring this satellite's payload characteristics on ESA's website and I saw that it's azimuth resolution varies a lot. For example azimuth resolution for IW mode is 22m (https://sentinel.esa.int/web/sentinel/user-guides/sentinel-1-sar/resolutions/level-1-single-look-complex). But this is very weird, and I can't wrap my head around why that is, since Sentinel 1a is a SAR, therefore it's azimuth resolution should be L/2 (L being the length of the antenna) or 6.15m (since it's antenna is 12.3 meters i believe).

Why is its resolution different?

I assume this will have something to do with the doppler shifts but I can't see it in my head.

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The formula that gives the azimuth resolution as being equal to half the antenna length is only true in Stripmap mode, where the satellite scans the ground as fast as it moves along its orbit.

The Sentinel-1 IW acquisition mode is done using the TOPSAR technique, which is more complex than Stripmap. Quoting the Sentinel-1 reference page:

With the TOPSAR technique, in addition to steering the beam in range as in ScanSAR, the beam is also electronically steered from backward to forward in the azimuth direction for each burst

Azimuth resolution is reduced compared to SM due to the shorter target illumination time of the burst.

This explains why the azimuth resolution in IW mode is worse.

Source: https://sentinel.esa.int/web/sentinel/technical-guides/sentinel-1-sar/products-algorithms/level-1-algorithms/topsar-processing

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  • Ah I see. But does that mean that the resolution is equal to N * L/2, where N is the number of sub-swaths (3 in the case of Sentinel-1a)? Or in other words is the azimuth resolution N * L/2 = 3*12.3/2=18.45m? – Slav Dec 1 '19 at 13:45
  • I guess the ground is scanned faster than three times the satellite's speed, because there is some "lost time" when the beam is being steered from one swath to another, which could explain that the real resolution is higher than 18.45m. This is just a guess, I am not a SAR expert. – guampi Dec 1 '19 at 15:11

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