Abstract: This invention relates to a method for determining the reply efficiency of a DME navigation beacon and to an apparatus for performing the method. The invention involves locating an RF receiver nearby a DME beacon to be tested. The RF receiver analyses all signals received on the interrogation frequency of that beacon to determine pulse pairs which correspond to a valid interrogation of that beacon. Other pulse events of interest may also be detected. The RF receiver also records all signals on the reply frequency of the beacon and detects all replies sent by the beacon. Particular interrogations can then be correlated with replies and the reply efficiency of the beacon determined. Several RF receivers may be located round the beacon to better provide correlation between particular interrogations and responses.