Abstract: A method for estimating the location of the source of a (possibly unfriendly) electromagnetic probe signal (threat radar) received by an aircraft as the aircraft moves along a chosen path. A signal direction finder on the aircraft determines geometric parameters describing a probe signal plane that contains a probe signal propagation direction, as received at the aircraft, at two or more selected times. A location determination system on the aircraft determines the location of the aircraft at each of the selected times. Information on the probe signal plane and the aircraft location is used to estimate initial location coordinates and, optionally, velocity coordinates and acceleration coordinates, for the probe signal source, in two dimensions or in three dimensions. Probe signal planes at additional selected times can be used to provide a check on, or provide a more reliable estimate for, the probe signal source location.