SYNTHETIC APERTURE RADAR (SAR) IMAGING BASICS
The word “radar” is an acronym for “radio detection and ranging.” A radar measures the distance, or range, to an object by transmitting an electromagnetic signal to and receiving an echo reflected from the object. Since electromagnetic waves propagate at the speed of light, one only has to measure the time it takes the radar signal to propagate to the object and back to calculate the range to the object. The total distance traveled by the signal is twice the distance between the radar and the object, since the signal travels from the radar to the object and then back from the object to the radar after reflection. Therefore, once we measured the propagation time (t), we can easily calculate the range (R) as
where c is the speed of light in a vacuum. The factor ½ accounts for the fact that the radar signal actually traveled twice the distance measured: first from the radar to the object and then from the object to the radar. If the electric property of the propagation medium is different from that of the vacuum, the actual propagation velocity has to be estimated for advanced radar techniques, such as synthetic aperture radar (SAR) interferometry.
Radars provide their own signals to detect the presence of objects. Therefore, radars are known as active, remote-sensing instruments. Because radars provide their own signal, ...