A radar on a moving aircraft or satellite sweeps past the area and combines all the echoes as if it were one enormous antenna. The result is a sharp image — at night and through cloud as well.
In practiceIntelligence analysts use such radar images to count aircraft on an air base, see vehicle tracks in snow or mud and follow the build-up of a position, even after weeks of cloud. A unit therefore cannot hide behind bad weather; only camouflage, moving and decoys help.
A radar can also image: the terrain and what stands on it, day and night and through cloud. The problem is angular resolution (Angular resolution): it gets worse with range. The first solution was side-looking airborne radar (SLAR): a long antenna along the fuselage of an aircraft, looking sideways and building up an image strip by strip through the aircraft's motion. But even an antenna of a few metres gives a resolution of hundreds of metres at tens of kilometres.
Synthetic aperture radar (SAR) breaks through that limit. The aircraft (or satellite) continuously transmits pulses during its flight and stores the amplitude and phase of every echo. A point on the ground is illuminated as long as it is in the beam: over a whole stretch of flight. By combining all those echoes coherently, the radar behaves like one enormous antenna with the length of that stretch — the synthetic aperture.
X-band (λ = 3 cm), a 3 m antenna, range 50 km. With the real aperture the resolution is 50,000 × 0.03 / 3 = 500 m. As SAR it becomes 3 / 2 = 1.5 m — independent of range. The synthetic aperture itself is 500 m long here.
That SAR resolution gets better as the real antenna gets smaller sounds contradictory, but follows logically: a small antenna has a wide beam, illuminates each point for longer, and therefore yields a longer synthetic aperture. In the range direction the resolution comes, as always, from the bandwidth (Range resolution).
Variants:
- Spotlight SAR: the beam stays pointed at one area while the platform flies past — longer illumination, even finer resolution, smaller image.
- ISAR (inverse SAR): now the target moves (a ship pitching and rolling, a turning aircraft) and the radar stays still. The result is an image of the target itself — useful for classification.
- GMTI (ground moving target indication): a related mode that detects moving vehicles among the ground clutter, as on the now-retired E-8 JSTARS.
SAR satellites (for example the European Sentinel-1 and commercial constellations such as ICEYE) make it hard to hide: they see through cloud and in the dark. Countermeasures are noise jamming of the satellite (which blurs the image but gives itself away), deceptive jamming with a DRFM (EW receivers & DRFM under the bonnet) that places false objects in the image, and passive means: corner reflectors as decoys, radar-absorbing camouflage nets and breaking up recognisable shapes. Because a SAR satellite has a predictable orbit, simple timing is also a measure: do not move, or cover up, during the pass.
Radartutorial.eu: synthetic aperture radar.