Abstract: An aircraft position determining system including an on-board radio altimeter which cooperates with a ground based transponder/beacon located at a position A. The radio altimeter is adapted as a distance meter and comprises means (9, 14, 15, 16) for shifting the frequency of the local oscillation signal. The system has a range sufficient to enable it to operate in a second predetermined volume defined with respect to the position A which includes a first volume defined by the lateral distance accuracy (d) and vertical distance accuracy (h) of the position determined by an independent on-board guidance device. The exact position of the aircraft when close to A is determined by at least two successive measurements of distance from to the position A.

Abstract: A passive system and related methods for detecting range and/or elevation of a target by using an X beam antenna system and by measuring the time delay between receipt of signals in the two planes of the X beam which are received directly from the target and by measuring the time delay between receipt of signals in the two planes of the X beam which signals are emitted by the target but reflected from a remote surface. By knowing these two time delays, the height of the antenna system, the fixed angle between the two planes of the X beam and the rate of rotation of the X beam, a direct calculation is established to determine the range and/or elevation of the target.

Abstract: A passive system and related methods for detecting the location of a plurality of N targets by using two X-beam antenna systems and by (i) measuring the time delay between receipt of signals in the two planes of each X-beam which are received directly from the target and by (ii) measuring the time delay between the receipt of signals in the two planes of each X-beam which signals are emitted by the target but reflected from a remote surface. By known these two time delays for each antenna system, the heights of the antenna systems, the fixed angle between the two planes of the X-beam for each system, and the rate of rotation of the X-beam for each system, a direct calculation is established to determine the altitude of each target as viewed from each of the two antenna systems. Bearing or azimuth angles are determined for each target at each antenna system, and the actual N target locations of the N.sup.