Abstract: Three fixed reference stations having known positions comprise Omega receivers. Phase measurements made at all three stations are assembled at a master station, utilizing landlines or radio links and, using the known positions of the stations, a central processing unit calculates not merely the offset of the master station as measured from the true position but also the errors in the azimuths of the propagation directions of the received signals. The position measurement of a vehicle with an Omega receiver is corrected on the basis of the offset and also the errors in the said azimuths. Sufficient accuracy is achieved for finding the position of land based vehicles whose position may be calculated subject to the corrections in the master station on the basis of raw data transmitted to the master station over a link. Alternatively the azimuth corrections may be transmitted to the vehicle over the link. Two fixed stations suffice if they can measure absolute phase.