From AMS Glossary
Techniques for measuring properties of a scattering medium by comparing the signals received at separate antennas or at separate points on the same antenna, or by comparing signals received using two slightly different carrier frequencies.
For example, in spatial interferometry the angle of arrival of signals is found by comparing the phases of signals received at two or more separated or spaced antennas. This information, combined with Doppler frequency measurements, can be used to estimate the velocity of the target. In frequency domain interferometry, the phases of signals at two slightly different carrier frequencies are compared to improve the range resolution.