Abstract: A method for testing each one of a plurality of receiving sites operative in a time difference of arrival geo-location system. The method comprises determining a theoretical time of arrival of a test message at each receiving site assuming the test message was transmitted from a test message transmitting antenna at a known location; determining a measured time of arrival of the test message at each receiving site, the test message transmitted from the test message transmitting antenna; determining a true time of arrival of the test message at each receiving site, the true time of arrival comprising the measured time of arrival less a start time of the test message; determining a difference between the theoretical and the true times of arrival for each receiving site, wherein the difference for each receiving site is indicative of a health of the receiving site.