Synthetic aperture radar (SAR) refers to a technique for producing fine-resolution images from a resolution-limited radar system. It requires that the radar be moving in a straight line, either on an airplane or,rocket. The basic principle of any imaging radar is to emit an electromagnetic signal (which travels at the speed of light) toward a surface and record the amount of signal that bounces/echoes back, or “backscatters,” and it's time delay.

The SAR works similar to a phased array, but contrary to a large number of the parallel antenna elements of a phased array, SAR uses one antenna in time-multiplex. The different geometric positions of the antenna elements are the result of the moving platform now.

The wavelengths that remote sensing radars use to observe Earth’s surface are microwaves, typically in the range of a few to tens of centimetres. Because the radar signal loses energy as it travels – at a rate equivalent to the beam width (wavelength / antenna size) – by the time it hits the surface, the beam has spread dramatically. For example, with a signal wavelength of 10 centimeters and an antenna of 10 meters in diameter, the beam width is 1/100 radians (0.6 degrees).

From an altitude of 1,000 kilometers, the resulting beam width on the ground becomes a very large 10 km, producing an image resolution which is insufficient for most applications. SAR is the solution to this dilemma as it can vastly improve the resolution.

SAR techniques take advantage of the fact that the radar is moving in orbit to synthesize a virtual 10-km-long antenna from the physical 10-m antenna in the direction of flight. As the radar moves along its path, it sweeps the antenna’s footprint across the ground while continuously transmitting pulses – short signal bursts separated by time – and receiving the echoes of the returned pulses.

As a target (like a ship) first enters the radar beam, the backscattered echoes from each transmitted pulse begin to be recorded. As the platform continues to move forward, all echoes from the target for each pulse are recorded during the entire time that the target is within the beam. The point at which the target leaves the view of the radar beam some time later, determines the length of the simulated or synthesized antenna. The synthesized expanding beamwidth, combined with the increased time a target is within the beam as ground range increases, balances each other, such that the resolution remains constant across the entire swath.

The SAR-processor stores all the radar returned signals, as amplitudes and phases, for the time period T from position A to D. Now it is possible to reconstruct the signal which would have been obtained by an antenna of length v · T, where v is the platform speed. As the line of sight direction changes along the radar platform trajectory, a synthetic aperture is produced by signal processing that has the effect of lengthening the antenna. Making T large makes the „synthetic aperture” large and hence a higher resolution can be achieved