A method of passively determining the range to a target from a test platform is disclosed. It comprises traveling at a constant velocity a course (28) in the test plat­form which is straight; detecting at the platform the presence of an energy signal radiating form the target (20), and sampling said energy signal at three discrete times; generating for each of said energy signal samples a first detection signal responsive to its angle (ϑ) of arrival at the platform, and a second detection signal responsive to its frequency on arrival at the platform; storing said detection signals; determining the range to the target utilizing said stored detection signals and said test platform velocity and position; and generat­ing a target indication signal representative of the range of the target. The target may be an aircraft. The test platform may be stationary or a test aircraft (24). The energy signal may be a continuously generated or pulsed signal detectable frequency.