Synthetic Aperture Radar (SAR) refers to a technique for producing fine resolution images from an intrinsically resolution-limited radar system. The wavelengths, λ, that are used for radar remote sensing of the earth’s surface are typically in the range of a few to tens of centimeters. At these wavelengths, the energy radiated from a radar antenna of dimension D disperses quickly at a rate that is equivalent to the beam width λ/D of the antenna. For a typical spaceborne SAR configuration with wavelengths of ~10 cm and an antenna of 10 m size, this beam width is 1/100 radians, or about 0.6 degrees. If we are in space observing the Earth 1000 km below, the beam size on the ground is then 1000 λ/D = 10 km. This intrinsic resolution of the radar system is insufficient for many applications and practical solutions for improving the resolution needed to be found.
Through the outlined principles, SAR defeats the intrinsic resolution limits of radar antennas in the along-track direction. In the cross-track or range direction, orthogonal to the satellite path, the resolution is not defined by the antenna beam width, but rather the width of the transmitted pulse. This is because the transmitted pulse intersects the imaged surface as it propagates in the beam. After a two-way trip of a transmitted pulse from sensor to the ground and back, two objects can be distinguished if they are spatially separated by more than half the pulse width. Hence, range resolution is controlled by the transmitted waveform that is generated by the radar and not the size of the antenna footprint on the ground. Wider bandwidth signals generate finer resolution images in range.
For most purposes, the transmitted signal can be thought of as a single frequency sinusoid with a well-defined amplitude and phase. Thus the image constructed from the SAR processing is a complex image – each resolution element, or pixel, has an amplitude and phase associated with it. Once calibrated, the amplitude is proportional to the reflectance of the surface. The phase is proportional to the distance the wave traveled between the radar and the ground, any propagation phase delays due to the atmosphere or ionosphere, and any phase contribution imparted by the reflectance from the surface.