15

(DME) Equipment Measuring Distance 15

Distance Measuring Equipment (DME)

Frequencies

Channels

DME (emission code P0N) is a secondary radar system operating between 960 and 1215 MHz in the UHF band at 1 MHz spacing; this provides 252 spot frequencies or channels.

There is always a difference of +/- 63 MHz between the interrogation and transponding frequencies. The channels are numbered 1 to 126X and 1 to 126Y. A channel number is selected by the pilot of a TACAN (TACtical Air Navigation) equipped military aircraft; this equipment provides the pilot with range and bearing. Civil aircraft have the cheaper VOR/DME equipment and select the appropriate paired VHF frequency to obtain range from either a DME or military TACAN facility.

Example Channel Numbers and Paired Frequencies

DME PairedWith ILS LocalizerTransmitter

DME is also frequency paired with the ILS localizer frequencies. These DME supplement or replace the range information provided by the Marker Beacons. The range information is zero referenced to the ILS runway threshold. DME is obtained by selecting the ILS frequency.

Uses of DME

A DME:

•provides very accurate slant range, a circular position line and in conjunction with another DME, or a co-sited VOR, two position line fixes.

•integrates the change of slant range into groundspeed and elapsed times when the aircraft is fitted with an appropriate computer.

•permits more accurate flying of holding patterns and DME arcs.

•provides range and height checks when flying non-precision approach procedures, e.g. locator only and VOR let-downs.

•indicates accurate ranges to the runway threshold, and heights for range, when flying an ILS/DME procedure.

•facilitates radar identification when the pilot reports his VOR/DME position.

•facilitates the separation and control of aircraft in non-radar airspace, based upon a VOR/ DME fix reported by individual aircraft.

•is the basis for a simple Area Navigation (RNAV) system when the appropriate computerization is fitted.

•provides accurate range inputs into the more complex and accurate RNAV systems; twin, self-selecting DME/DME are used.

Principle of Operation

Figure 15.6 VORTAC

PulseTechnique

DME is a secondary radar system providing slant range by pulse technique.

The aircraft’s interrogator transmits a stream of omni-directional pulses on the carrier frequency of the ground transponder. Simultaneously the interrogator’s receiver starts a Range Search. At the transponder on the ground the received interrogation pulses are re-transmitted, after a delay of 50 µs, at a frequency that is +/- 63 MHz removed from the interrogation frequency.

The airborne system identifies its own unique stream of pulses and measures the time interval, electronically, between the start of the interrogation and the reception of the response from the transponder. The time measurement, and hence range, is very accurate and is based upon the speed of radio waves i.e. 3 × 108 m/s. A modern DME is inherently accurate to +/- 0.2 NM

In theory up to 100 aircraft can interrogate a DME transponder. Thus each aircraft is receiving its own returning paired pulses plus those which result from other aircrafts’ interrogations, as the pulses have the same carrier frequency.

The width of the interrogation pulses is 3.5 µs (1050 m) and they are transmitted in pairs; (the interval between the individual pulses of a pair is 12 µs for X channel and 36 µs for Y channel).

Distance Measuring Equipment (DME) 15

(DME) Equipment Measuring Distance 15

Figure 15.7 The Principle of Range Measurement

For an individual aircraft to achieve an unambiguous slant range measurement and overcome the problem of identification:

•Each aircraft’s interrogator is programmed to transmit its paired pulses at random intervals i.e. the transmission sequence of pulses is irregular or jittered. This differentiates its pulses from all the others.

•At the instant of transmission, the receiver of an interrogator sets up gates to match the random PRF of the transmitted twin pulses.

•The responses on the transponder’s carrier frequency include an individual aircraft’s paired pulses plus those from other aircraft.

•The receiving equipment of an aircraft is designed so that only the responses which match its randomized PRF are allowed through the gates. The pulses have now achieved lock-on i.e. the DME enters the tracking mode.

•As the aircraft’s range from the station increases or decreases (unless the aircraft is circling) the gates move to accommodate the corresponding increase or decrease in the time between transmission and reception of the twin pulses. This lock-and-follow technique ensures that the returning twin pulses are continuously tracked.

•The off-set in time between transmission and reception is the measure of the aircraft’s slant range from the DME transponder.

Distance Measuring Equipment (DME) 15