SAR stands for synthetic aperture radar. Radar sends out a pulse of radio waves which bounces off the object to be depicted. Then the scattered pulses return to the radar, where they are captured by the receiving antenna. The antenna is the radar's aperture, or its opening on the world. SAR antennas are a type of radar antennna designed to take advantage of their satellite's movement, thus creating a "synthetic" aperture or opening.
SAR is an active remote sensing system. Active systems transmit their own signal and measure the energy that is reflected or scattered back from the target material.
Passive systems measure natural radiation emitted by the target material and energy from other sources reflected from it. For instance, the reflection of solar radiation in a camera is an example of a passive system. A difficulty with passive systems in studying the polar regions is that good quality images in the visible portion of the spectrum cannot be obtained in the presence of clouds, rain, or darkness.
SAR images, which resemble photographs, are actually maps in which the brightness shown is a measure of the radar energy reflected back to the antenna. Water droplets in fog and clouds are transparent to radio waves of the proper frequency just as window glass is to light waves of the visible frequency. Hence, a SAR instrument can gather data in conditions where optical sensors would be useless, i.e. it can provide excellent images of what the radar detected even in fog, clouds or darkness. SAR data is used to study agriculture, ecology, geology, oceanography, and hydrology as well as shipping in ice-covered seas, oil exploration, ocean pollution monitoring, and ocean research.
This figure shows a SAR antenna on a satellite that is orbiting the earth at an altitude of 705 km, with a trace of sub-satellite track, a radar beam (yellow) that is pointing 23 degrees off the sub-satellite track to continuously illuminate the surface with a 100 km wide swath (red).