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Just because comm is VHF doesn't mean it can't hurt. What about DME?

An internationally standardized navigation system which allows an aircraft to measure its distance from a selected ground-based beacon. Such beacons are used throughout the world by all airliners, most of the military aircraft of the West, and a large number of general-aviation aircraft. The range of service is line-of-sight up to 300 mi (480 km) and system accuracy is usually 0.1 mi (0.16 km) but precision equipment, intended for use during landing, has accuracy of 100 ft (30 m).

The airborne equipment, called an interrrogator, transmits pulses of 1 kW peak power on 1 of 126 frequencies. These are in the 1025–1150-MHz band and are spaced 1 MHz apart. Each pulse is of 3.5 microseconds duration and is paired with another, spaced 12 or 36 microseconds later. The combination of frequencies and pulse spacings therefore provides 252 operating channels.

The beacon on the ground, called a transponder, receives these pulses, delays them by 50 μs, and then retransmits them, usually with a power of 1 kW, on 252 frequencies lying between 962 and 1213 MHz. The pulse-pair spacing is 12 μs on those frequencies not used by the interrogator, and 30 μs on those frequencies shared with the interrogator. The transponder transmission is called the reply. The frequency difference between interrogation and reply is always 63 MHz. This arrangement allows each transmitter frequency to act as the local oscillator for its associated superheterodyne receiver, the intermediate frequency of which is 63 MHz. For landing purposes, some transponders have powers as low as 100 W.

In the aircraft the replies to its own interrogations are recognized by their phase coherence with their own transmissions, and by the elapsed time measured between transmissions and reception (minus the 50-μs transponder delay), usually by means of a crystal clock. This elapsed time is about 12 μs for each nautical mile (7 μs for each kilometer), and is displayed in the cockpit on a digital meter, which is usually calibrated in miles and tenths of miles. See also Electronic navigation systems; Instrument landing system (ILS).

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