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On this link (Sentinel1 webpage for a SLC product), the resolution is written as a range from the highest corresponding to low incidence angle, to the lowest resolution corresponding to high incidence angle.

My question is how is the resolution related in general to the incidence angle? And if they are really related, why is the resolution affected by the change in the incidence angle?

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2 Answers 2

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Think of the geometry. The incidence angle refers to the angle from nadir, or directly beneath the satellite, which would be 0°. As the sensor looks out to the sides from this nadir, the angle of incidence increases as does the fov (field of view). This is why the resolution decreases with increase in incidence angle. This illustration from the Sentinel website might help to visualize this concept.

enter image description here

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  • According to the illustration above, as FOV (or incidence angle) increases, the swath increases too. But, does an increase in the swath mean a decrease in the resolution, and why? Thanks.
    – Hakim
    Commented Jul 29, 2015 at 19:50
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    It helps to think of the sensor like an imaging sensor of a digital camera in the sense that there is only a finite amount of information it can receive: If you zoom in (decrease in swath width) you'll cover a smaller area with the same amount of pixels and if you zoom out (increase in swath width) you'll cover a larger portion of the earth with the same amount of pixels.
    – Kersten
    Commented Jul 30, 2015 at 8:15
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    There are two parts to the idea of resolution and angle. The first is the sensor, as @Kersten mentioned. The second part is that this is radar, and you are trying to capture reflections of an emitted signal. The closer you are to directly above something, the more waves fall on a given area and are reflected straight back at you, increasing the odds that they will be picked up by the sensor. With a larger angle, the waves encounter a larger surface area on the ground, and the potential for scatter, or waves reflected in different directions, increases. Both factors reduce resolution.
    – Get Spatial
    Commented Jul 30, 2015 at 15:20
  • @GetSpatial I do not see the connection between resolution and amount of reflected energy. The resolution will be what it is, based on the geometry as described here and the characteristics of the sensing system (including the sensor). The amount of reflected energy will determine the quality of the image--its brightness and contrast in particular--but will not change the inherent resolution of the system.
    – whuber
    Commented Aug 5, 2015 at 13:33
  • @whuber I think that what was meant is not the reflected energy per pixel (or any other unit) but the quantity of information returned to the sensor instead (depending on the resolution and the swath).
    – Hakim
    Commented Aug 5, 2015 at 21:15
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One short comments regarding your other questions: Why is azimuth resolution worse than range resolution in Sentinel1 (SAR)

You are absolutely right that sentinel TOPS have a worse azimuth resolution comparing with other data. This exactly the unique characteristic of TOPS. The working mode of TOPS (also including ScanSAR) is designed to sacrifice azimuth resolution to increase SAR image coverage on range direction.

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