Abstract: A sensor system (10) for locating transmitters incorporates three vehicle-mounted patch antennas (30 to 34), a signal processing unit (40), a navigation unit (50) and a computer (60). The navigation unit (50) measures sensor system position using the Global Positioning System. The antennas (30 to 34) receive radiation from a transmitter (210) to be located and respond by generating output signals which are frequency downconverted and processing coherently by a digital signal processing unit (230). This produces elevation and azimuth phase data for the computer (60), which determines measured phase differences. The computer (60) also calculates expected phase differences from antenna positions and trial transmitter positions. It determines transmitter location from correlation between measured and expected phase differences. The position of a transmitter (210) is determined for a number of sensor system positions.