A TACAN-compatible accurate DME for short ranges

DME accuracy is improved greatly by using pulses having a short rise-time (0.2 μs) instead of the 2.5 μs TACAN pulses. The greatest improvement is in the reduction of errors caused by pulse-distorting echoes and by the large variation (80 dB) in the amplitude of interrogations from near and far aircraft. All existing ground transponders are equipped with Ferris discriminators which will not answer interrogations from adjacent channels and therefore will not supply erroneous distance data. Such discriminators can be improved to pass sharply rising pulses without integration while maintaining their near-absolute selectivity. This paper describes characteristics of ground and airborne DME, using sharply rising pulses, which provide very high accuracy at short ranges, especially in collaboration with ILS. The equipment operates within the TACAN-VORTAC system in that “short rise-time” airborne DME is used to interrogate existing en route VORTAG beacons, and existing airborne DME can obtain distance information from the “short rise-time” beacons although with less accuracy than “short rise-time” interrogators. Existing airborne and ground DME were modified to emit and use short rise-time pulses and were flight-tested. Repeated passes over a measured course provided an accuracy of ±85 feet at three miles.