About SAR



In addition to its core advantages of cloud penetration and day or night collection, radar imaging has other interesting capabilities. Radar sensors illuminate the ground using pulses of microwave radiation. The sensors emit thousands of pluses each second and record the resulting echoes. They measure the time it takes each pulse to travel from the sensor to the ground and back, as well as other characteristics of the return wave. From these raw measurements, the SAR technique is used to form images. SAR was a breakthrough in radar imaging because it synthesizes a large virtual antenna from a small physical antenna, allowing imaging from a small platform that can be put into orbit.

In addition, radar’s microwave pulses are coherent. They are controlled and consistent in structure from pulse to pulse. This contrasts to the random illumination of sunlight. The coherent nature of SAR enables high resolution from great distances. In fact, the resolution of a SAR sensor has the remarkable characteristic that it is not dependent on the distance between the sensor and the ground. Coherence also enables special image processing techniques like the precise measurement of surface structure changes over time. Commercial SAR systems have been used, for example, to measure ground subsidence in oil fields at a level of a few millimeters per year.

SAR imaging is not constrained by a fixed focal length like optical telescopes. Therefore imaging can be done in various modes with differing resolution and coverage. Another benefit of SAR is that the images have a natural geometric fidelity and accuracy. Multiple radar images taken over the same location can be managed so that they are all alike with consistent illumination and geometric characteristics. This makes change detection very reliable and one of SAR’s principal applications.

XpressSAR leverages all these technologies to provide the world's best commercial SAR data.