Synthetic aperture radar (SAR) is a detection and imaging technique that enables scientists to perform such tasks as monitoring the environment, mapping out natural resources, and guiding advanced weapons systems for the military. Astronomers can create detailed maps of the surfaces of distant planets and moons using SAR. It combines radar technology with the advanced information processing capabilities of today's digital electronics to produce much more detailed images than can be obtained by more conventional methods.
In many ways, SAR is like a standard radar system. A radio signal is sent out, and the system analyzes the returning echoes to determine the target's distance from the signal source. This is known as "range," and it forms one dimension of a 2D SAR image. The degree of fine detail that can be seen in the resulting image is called the resolution. Resolution depends on the width of the radar pulses that are emitted: narrow pulses yield finer range resolution.
The second dimension in a SAR image is called the azimuth. It is an indicator of direction. To get this information, scientists use a very large antenna to focus the sent and received radar signals into a sharp beam. The sharper the beam, the finer the azimuth resolution. Since SAR systems use very low frequencies, a very large antenna as long as several hundred feet would be needed -- too large for an airborne system. So most SAR systems collect data while flying a comparable distance. That distance the is known as the synthetic aperture.
What is Doppler shift?
A Doppler shift in sound occurs when a moving object passes you. For instance, if an ambulance is moving away from you, its siren will sound lower in pitch than if the ambulance were moving toward you. Extreme shifts in sound wave frequency can cause sonic booms. Sounds waves can't travel any faster than the speed of sound, so if the sound source is moving just as fast, the sound waves "stack up" and hit all at once, causing a very loud boom.