Vision is perhaps the most critical component of the human sensory system. The lens in the human eye collects the optical waves that are being reflected from the objects in its surrounding medium, which are then interpreted as visual information in the brain.
Human-made vision systems imitate the function of the eye–the sensor—and that of the brain—the processor. Human-made vision systems are primarily built to improve on our ability to resolve targets, for example, binoculars, radars, and sonars, or to capture the image of a scene, for example, cameras. In either case the ability to view or capture a scene improves with a larger lens aperture (in a binocular or camera), a larger radar antenna aperture, or larger acoustic transducer aperture; the key to better vision is a larger aperture. Unfortunately, it is extremely difficult to either build or maintain a physically large aperture vision system. (Recall the Hubbel telescope.)
In the 1950s an invention in radar by Wiley revolutionized the way human-made vision systems are constructed. This invention was called synthetic aperture radar, or SAR. The principal idea behind SAR is to synthesize the effect of a large-aperture physical radar, whose construction is infeasible. What is the significance of a larger aperture? This becomes evident in the following example.
The lateral or cross-range resolution of a D = 1-meter diameter radar antenna with wavelength λ = 1 meter at the range R = 1000 ...