DISTANCE MEASURING EQUIPMENT

DME

PRINCIPLESUnlike VOR and ADF which are passive systems, DME is an active system.The aircraft equipment transmits UHF pulses. (interrogating signal)The ground station replies with its own UHF pulses.The aircraft equipment measures the time difference between signal sent and signal received. Radio waves travel at a constant speed, allowing a distance to be computed.

In order for this process to occur both ground station and aircraft are equipped with a transmitter and receiver. (transceiver)A DME ground station must be able to differentiate between multiple interrogation signals. (numerous aircraft in the area)This is accomplished by the aircraft transmitting two pulses spaced in a pattern unique to the unit. (jitter)

The ground station responds with a pulse pair that matches.The aircraft unit scans all replies until it locates the one matching its own pulse signature.Once located it ignores all other signals and concentrates on the match.This Lock On process creates a delay from the time the DME frequency is tuned until information is displayed to the pilot.(1-3sec)

OPERATIONDME is possibly the easiest of all navigation equipment to use.Basic units must be tuned separately, while other units are paired to the VOR receiver.Most DME ground stations are collocated with a VOR facility.Most DMEs automatically tune to the DME frequency associated with the VOR.Simply set the VOR receiver to the required frequency and the DME will tune itself.Note: the DME ident must still be identified seperately in order to test ground and airborne DME equipment.

If more than one VOR receiver is installed NAV 1 or NAV 2 must be selected on the DME.A HOLD function allows DME information to be retained when the VOR receiver is reset to a facility without DME.Caution: it is easy to lose track of what distance information is being displayed; NAV 1, NAV 2, or a previously held frequency. Descending or turning based on the wrong distance information could have catastrophic results.

SLANT-RANGE DISTANCEThe distance displayed by a DME is subject to slant-range error.DME measures the straight line distance (slant-range) between the aircraft and the ground facility; not ground distance.The altitude of the aircraft has an effect on the distance information.An aircraft directly over the DME ground facility would display a DME equal to aircraft altitude in nm.This error is negligible when the slant range is more than the altitude in 1000’s of feet.

Slant-range can be converted to ground distance by this formula: ground distance=√(S²-A²)S=slant range nm, A=altitude nmAs a rule of thumb slant-range error can be disregarded when the aircraft is more than a mile away for every thousand feet AGL. Ex: more than 5 miles away at 5000 feet AGL.DME system accuracy is +/-0.5NM or 3% of the distance whichever is greater.

DERIVATIVE INFORMATIONAlthough DME only provides distance directly, it can also derive groundspeed and time to station.The DME computes groundspeed by timing the interval between distance changes.Time to station is calculated from groundspeed and distance.This derivative information is limited by the way the information is obtained. It is accurate only when the aircraft is traveling directly towards or away from the station.

If an aircraft were to fly a constant DME arc around a ground facility the DME would determine the aircraft is standing still, because the distance from the station remains constant.This is an important concept to understand when utilizing DME derived groundspeed or time to station.

AUTO-STANDBY MODEDME is subject to line-of-sight reception.An aircraft below the horizon (out of line-of-sight range) will not receive any signals from the ground facility.Whenever the aircraft DME is not receiving ground signals it enters auto-standby mode and stops transmitting. (saves electricity and wear and tear on unit)As soon as the aircraft is in range of a ground facilities signal it returns to normal operation and begins to transmit once again.

COLLOCATED FACILITIESDME facilities can exist alone but they are usually collocated with a VOR.Although VOR and DME operate on different frequencies, the two are integrated through paired frequencies.Only one frequency must be selected (VOR) and the other (DME) is automatically selected.Care must be taken to ensure the two facilities are actually collocated, separately located facilities will result in distance discrepancies.

VOR/DME, VORTAC, TACANVOR/DME: collocated facilities only useable by civilian aircraft.TACAN: a military installation giving azimuth information on a UHF frequency as well as DME information.The DME may be used by civilian aircraft, however any apparent track information must be ignored.VORTAC: a civilian VOR collocated with a TACAN, civilian aircraft receive track information from the VOR and DME from the TACAN.