Direction finding
Radio surveillance spectrum - Goniometry and Localization

A direction finder measures the direction of arrival of an electromagnetic wave relative to a reference direction. The process is purely passive and undetectable by the transmitter.

Whatever their principles, conventional direction finders all use the assumption of planar wavefronts associated with each transmitter: the isophase (i.e., constant-phase) loci of the wave emanating from a transmitter are assumed to be parallel planes at sufficient distance from the transmitter (i.e., beyond the Fresnel distance $D_{\text{Fresnel}}=D_{\text{Tx}}^2\text{/}\lambda$...