Under the Open Skies Treaty, observation aircraft may be equipped with sideways-looking synthetic aperture radar. SAR is a high resolution ground mapping technique which takes advantage of the forward motion of a vehicle that is carrying a pulsed radar to synthesise the effect of a large antenna aperture. In other words, the larger the radar antenna (aperture), the higher the radar picture's resolution. SAR uses a radar with a very small antenna (such as can be carried in an aircraft) that synthesises a series of recurring radar pulse returns to simulate the effect of a much larger antenna aperture; thus, the name "synthetic" aperture.

SAR is effective in detecting large objects day or night, even through cloud cover.