In the field of air navigation, the controversy between DME (Distance-Measuring Equipment) and Tacan, a new system sponsored by International Telephone & Telegraph Co., has tended to obscure their many basic similarities, especially in the case of the present civil VOR (Very High Frequency Omnirange)-DME. Both operate on the same basic principle. A set in an aircraft transmits an interrogation pulse. This is received by a ground station, causing a pulsed reply to be transmitted back. In interrogation and reply, pairs of pulses are sent to avoid confusion with spurious noises or with other DME pulses, in the case of civil DME.
The differences which are claimed to give Tacan its superiority and growth potential include:
1. Clear-channel DME, in which individual channels are provided solely by frequency separation between stations in place of the pulse-coding (multiplexing) required by civil DME to provide sufficient channels. This Tacan characteristic makes multiplexing available for additional functions, such as bearing, instrument landing, data link, and, possibly at some future date, voice communication via pulse-code modulation.
2. UHF-band (1,000 megacycles) operation of both bearing and distance functions, so that Tacan’s omnirange bearing service is much less affected by terrain and obstructions near the antenna than is VOR, which operates in the range of 112-118 megacycles. For the same reason, Tacan can use a much smaller ground-station antenna, an important consideration on shipboard and in various USAF mobile installations. A combined ‘coarse’- and ‘fine’-bearing indicating system utilized in Tacan enables it to provide greater accuracy than VOR.
3. In civil DME, pulses 2.5 microseconds wide are given one of ten different possible spacings which are 14 to 77 microseconds apart, to provide ten discrete channels at one operating frequency. In Tacan, the DME pulses are 3.2 microseconds wide and always spaced 12 microseconds apart, since each channel operates at a different frequency. Both the ground and airborne DME receivers contain twin-pulse decoders set to pass only pulse pairs having the prescribed spacing and to reject all others.
During the ‘search’ phase of the operation, when either type is looking for replies from ground stations, approximately 150 pulse-pairs are transmitted per sec. When contact has been established with the ground station, this rate is lowered to approximately 30 pulse-pairs per sec. So that an airborne receiver will not mistake ground replies intended for other aircraft as the answer to its own interrogation, the repetition rate of each airborne transmitter is ‘wobbled’ or varied slightly to give it an instantaneous repetition rate which differs from that of other aircraft in the area.
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