Synthetic aperture radars differ from other radars by transmitting radar energy sideways along the flight path and reassembling the echoes using doppler interferometry to give the effect of a longer antenna than it's practical for a plane or spacecraft to carry. The distance flown creating the synthetic antenna is known as the synthetic aperture, and can be several hundred meters long, which yields finer resolution than possible from a smaller antenna. Sometimes called range-doppler interferometry.

High-res pictures through inclement weather are not all SAR is good for. The data can be used to produce 3-D views of natural and man-made terrain. It's also possible to tune the radars to detect subsurface features, which can be particularly useful in oceanography and petroleum exploration.

It takes advanced signal processing to continuously adjust the radar's range component for the continuously changing position of the aircraft. The azimuthal component is not really parallel to the flight path either but is actually radial to the instantaneous position of the antenna. This makes SARs an expensive tool. Gear this sophisticated only recently became available outside the military and space community.

There is a related technique called inverse synthetic aperture radar which depends on target motion to provide the required doppler shift. Look for it on an anti-aircraft installation near you soon.

Can anyone comment on possible maximum resolutions of SAR? Seems to me spacecraft ought to be able to make apertures that are *miles* long…

Update 19August01:

Richard Branson is testing an airship-deployed ultra-wide-band SAR for mine clearing. Developed by the UK Defence Evaluation and Research Agency, the UWB SAR penetrates vegetation and surface soil and can detect plastic mines as small as 10 cm, scanning the terrain at 100 m2 per second.