Abstract : The report describes an algorithm that is used to determine the maximum likelihood position of an aircraft from the measurement of one radar's range, azimuth, and elevation and from an altitude measurement of the aircraft. The variances of these measurements must also be known. The report also shows how much the altitude-aided algorithm improves the accuracy as compared to a one-radar determination. It was concluded that: The position error in the low radar elevation angle ACMTS(Air Combat Maneuvering Test System) geometry at the Pacific Missile Range, obtained by using a telemetered altitude measurement and the R, A, and E from one radar in the algorithm described in the report, could be reduced from 1/2 to 1/5 that of the position error obtained when only the R, A,E measurement from one radar is used. The altitude measurement greatly reduces the error in determination of the position point when the aircraft being tracked is a large distance from the radar.