A method for locating an emitter of electromagnetic waves utilising a plurality of receivers, the method comprising the steps of: a. detecting the times of arrival of said electromagnetic waves at said receivers; b. computing the relative time difference of arrival between said receivers using said detected times of arrival; c. estimating the position of the emitter from said computed relative time differences of arrival; and d. correcting said detected times of arrival for path length discrepancies caused by the earth's atmosphere; characterised in that step (d) includes the additional steps of: e. measuring time differences of arrival between pairs of said receivers; f. obtaining an estimate of emitter position assuming a straight-line path between each receiver and said emitter; g. for each receiver using said estimate and obtaining the ground range from emitter to receiving platform; h. using said ground range known receiving platform height and an assumed refractive profile in selected ray-tracing integral equation to predict actual path length; i. obtaining the difference between said predicted path length and said straight-line paths to form corrections to said time differences of arrival; j. repeating steps (e) to (i) until said corrections converge; and k. applying said converged corrections to correct said detected times of arrival of said electromagnetic waves at said receivers. a. detecting the times of arrival of said electromagnetic waves at said receivers; b. computing the relative time difference of arrival between said receivers using said detected times of arrival; c. estimating the position of the emitter from said computed relative time differences of arrival; and d. correcting said detected times of arrival for path length discrepancies caused by the earth's atmosphere; characterised in that step (d) includes the additional steps of: e. measuring time differences of arrival between pairs of said receivers; f. obtaining an estimate of emitter position assuming a straight-line path between each receiver and said emitter; g. for each receiver using said estimate and obtaining the ground range from emitter to receiving platform; h. using said ground range known receiving platform height and an assumed refractive profile in selected ray-tracing integral equation to predict actual path length; i. obtaining the difference between said predicted path length and said straight-line paths to form corrections to said time differences of arrival; j. repeating steps (e) to (i) until said corrections converge; and k. applying said converged corrections to correct said detected times of arrival of said electromagnetic waves at said receivers. e. measuring time differences of arrival between pairs of said receivers; f. obtaining an estimate of emitter position assuming a straight-line path between each receiver and said emitter; g. for each receiver using said estimate and obtaining the ground range from emitter to receiving platform; h. using said ground range known receiving platform height and an assumed refractive profile in selected ray-tracing integral equation to predict actual path length; i. obtaining the difference between said predicted path length and said straight-line paths to form corrections to said time differences of arrival; j. repeating steps (e) to (i) until said corrections converge; and k. applying said converged corrections to correct said detected times of arrival of said electromagnetic waves at said receivers.