Aperture Radar
Synthetic-aperture radar (SAR) is a form of radar that is used to create two- or three-dimensional images of objects, such as landscapes. SAR uses the motion of the radar antenna over a target region to provide finer spatial resolution than conventional beam-scanning radars. SAR is typically mounted on a moving platform, such as an aircraft or spacecraft, and has its origins in an advanced form of side looking airborne radar (SLAR). The distance the SAR device travels over a target in the time taken for the radar pulses to return to the antenna creates the large synthetic antenna aperture (the size of the antenna). Typically, the larger the aperture, the higher the image resolution will be, regardless of whether the aperture is physical or synthetic this allows SAR to create high-resolution images with comparatively small physical antennas.
To create a SAR image, successive pulses of radio waves are transmitted to “illuminate” a target scene, and the echo of each pulse is received and recorded. The pulses are transmitted and the echoes received using a single beam-forming antenna, with wavelengths of a meter down to several millimeters. As the SAR device on board the aircraft or spacecraft moves, the antenna location relative to the target changes with time. Signal processing of the successive recorded radar echoes allows the combining of the recordings from these multiple antenna positions. This process forms the synthetic antenna aperture and allows the creation of higher-resolution images than would otherwise be possible with a given physical antenna.
As of 2010, airborne systems provide resolutions of about 10 cm, ultra-wideband systems provide resolutions of a few millimeters, and experimental terahertz SAR has provided sub-millimeter resolution in the laboratory.
Basic principle
A synthetic-aperture radar is an imaging radar mounted on a moving platform. Electromagnetic waves are sequentially transmitted, and reflected echoes are collected, digitized and stored by the radar antenna for later processing. As transmission and reception occur at different time, they map to different positions. The well ordered combination of the received signals builds a virtual aperture that is much longer than the physical antenna length. This is why it is named “synthetic aperture”, giving it the property of being an imaging radar.[4] The range direction is parallel to flight track and perpendicular to azimuth direction, which is also known as along-track direction because it is in line with the position of the object within the antenna’s field of view.
Basic principle
The 3D processing is done in two steps: the azimuth and range direction are focused for the generation of 2D high-resolution images, after which a digital elevation model is used to measure the phase differences between complex images, which is determined from different look angles to recover the height information. This height information, along with the azimuth-range coordinates provided by 2-D SAR focusing, gives the third dimension, which is the elevation direction. The first step requires only standard processing algorithms, for the second step, an additional pre-processing stage such as image co-registration and phase calibration is used.
The Technology 