A cross-ambiguity function (CAF) is a generalization of cross-correlation. Calculating a cross-ambiguity function between two versions of the same signal received on separated moving platforms (like aircraft) is a means for estimating simultaneously the differences in both the time of arrival of the two versions at the two platforms and the frequency difference between them caused by different Doppler shifts that result from the platform motions. In the simplest scenarios, both these parameters remain essentially constant over an observation interval and when the signal emitter is stationary, each of these parameters defines a locus on the earth's surface whose intersection identifies the geolocation of that emitter. In more complex signal scenarios, modified versions of a CAF calculation provide equivalent estimates. Importantly, the estimates derived via a CAF or modified CAF are generally independent of the nature of a signal's (usually unknown) modulation.