SAR Phase

A synthetic aperture radar (SAR) works by illuminating the Earth with a beam of coherent microwave radiation such as a laser. This radiation can be thought of as a wave, such as a water wave or a sound wave.

As you may remember from trigonometry or physics class, a wave can be described by just three properties: its wavelength, amplitude, and phase.
The wavelength is the distance between peaks on the wave. For SAR, the wavelength is fixed.
The amplitude is the displacement of the wave at the peak. This corresponds to brightness for light and loudness for sound. In SAR, the amplitude shows mountains, rivers, lakes, cities, and other useful information.
The phase describes the shift of the wave from some other wave. Phase is usually measured in angular units, such as degrees or radians.

If two waves are not shifted with respect to one another, they oscilate together and are "in phase", or have a phase shift of 0 degrees. However, if two waves are oriented oppositely, so one wave is at a crest while the other is at a trough, they are "out of phase", or have a phase shift of 180 degrees.

In SAR, the phase of the echoing signal is compared to a reference wave, so the phase of a SAR image is actually the phase difference between the echo and this reference. Since the wavelength of most SARs is under one meter, and the distance between pixels is several meters, the phase of a single SAR image does not carry any information. Subtracting it from another SAR image's phase that covers approximately the same area results in an interferogram.