Abstract: Passive location of an emitter is achieved by sensing a signal propagated from the emitter at multiple sensing locations and determining its phase at each sensing location. A three-dimensional region is searched to find an emitter location for which phase estimates of the signal at the emitter location are in good agreement among the sensing locations. An iterative search from a set of starting points in the region may be performed. The region may be subdivided and each region searched in parallel using multiple processors in parallel. Phase at the sensing locations may be determined locally, using synchronized clocks at the sensing locations, or at a common receiver. In the latter case, signal propagation time from the sensing location to the receiver location is taken into account. The emitter may be a wireless endoscopy capsule, for example.