Synthetic Aperture Radar
The Synthetic Aperture Radar (SAR) is a coherent, active, microwave imaging method that improves natural radar resolution by focusing the image through a process known as synthetic aperture processing. This typically requires a complex integrated array of onboard navigational and control systems, with location accuracy provided by both Doppler and inertial navigation equipment.
Imaging radar is an active illumination system which, in contrast to passive optical systems, does not require the Sun's illumination. An antenna, mounted on an aircraft or spacecraft, transmits a radar signal in a side-looking direction towards the earth's surface. The reflected signal, known as the echo, is backscattered from the surface and received a fraction of a second later at the same antenna. The brightness, or amplitude, of this received echo is measured and recorded and the data are then used to construct an image. For coherent Radar systems such as SAR, the phase of the received echo is also measured and used to construct the image. Radar uses a single frequency for illumination; therefore there is no color associated with raw Radar imagery. However, Radar provides at significant benefits such as the ability to image through clouds, and the ability to image at night.
SAR takes advantage of the Doppler history of the radar echoes generated by the forward motion of the spacecraft to synthesise a large antenna, enabling high azimuthal resolution in the resulting image despite a physically small antenna. As the radar moves, a pulse is transmitted at each position. The return echoes pass through the receiver and are recorded in an echo store. 
Combining the series of observations requires significant computational resources. It is normally done at a ground station after the observation is complete, using Fourier transform techniques. The result is a map of radar reflectivity (including both amplitude and phase). The phase information is, in the simplest applications, discarded. The amplitude information, however, contains information about ground cover, in much the same way that a black-and-white picture does. 
The software used by PECS-GRID project is aimed exlusively for processing of SAR data acquired by ESA's ERS-1,2 and Envisat missions.
ERS-1 and ERS-2 Earth observation satellites were launched in 1991 and 1995 respectively. They are considered as an ESA success thanks to the originality, quality, and reliability of the on-board instruments. Currently, ERS-2 is the nominal mission. ERS-1 was retired after nine years of service in March 2000. 
In March 2002, ESA launched Envisat, an advanced Earth observation satellite. The Envisat ensures the continuity of the data measurements of the ERS-1,2 satellites. Envisat is the nominal mission. 
|||The ASAR User Guide - 1.1.2 Scientific Background|
|||Wikipedia - Synthetic aperture radar|