Page "Synthetic aperture radar" Paragraph 15

It is important to realize that only those sub-wavelength differences of successive ranges from the transmitting antenna to each target point and back, which govern signal phase, are used to refine the resolution in any geometric dimension.

The central direction and the angular width of the illuminating beam do not contribute directly to creating that fine resolution.

Instead, they serve only to select the solid-angle region from which usable range data are received.

While some distinguishing of the ranges of different scene items can be made from the forms of their sub-wavelength range variations at short ranges, the very large depth of focus that occurs at long ranges usually requires that over-all range differences ( larger than a wavelength ) be used to define range resolutions comparable to the achievable cross-range resolution.