DME's

Distance Measuring Equipment (DME) what is it and how does it work? Most large aircraft and some smaller aircraft have DME equipment. It seems to be so easy to use, and when it is there, we just tune to a frequency, and it tells us how far away you are from that station.

The simplest explanation is that the DME box is a transceiver that has a computer in it. The transmitter part sends a signal out to a ground station, usually co-located with a VOR. The ground station then will send a signal back to the DME receiver on the aircraft. The time between the signal being sent, and the return signal being received will be computed by the computer, and converted into miles.

The DME signal isn't actually on the frequency that you tune into. The VOR frequencies and the DME frequencies are paired. The pairing is outlined in the AIM, and other documents. A VOR on 111.0 will be paired with a DME on channel 47 (Aircraft Transmits on 1072MHz, Ground transmits on 1009MHz). The channel concept helps us think about the transmit/receive frequencies.

Each DME transceiver sends a unique set of pulses. The ground station sends the same set of pulses back. If their are several aircraft near the same DME ground station, the aircraft only will do timings on the signal with the matching pulses. The aircraft transmitter will listen for quiet time before transmitting to prevent signals colliding. Most ground stations are capable of servicing about 100 unique aircraft at a time.

The distance measured will be straight line. That means, that if an aircraft is flying at about 10000ft straight over the top of the DME ground station, the DME indicator will read 2miles, not 0. The DME system will try to maintain about a quarter mile, per ICAO requirements. The system will include the ground station and the aircraft equipment.

Some of the RNAV and RNP requirements can be met by using two DME systems along with the inertial reference unit (IRU). Approach plates will sometimes be labeled DME/DME/IRU as needed to meet the requirements of the approach.

GPS is making some of the DME capabilities obsolete. Will the FAA begin decommissioning them any time soon? Probably not. Aircraft upgrades are expensive, and the DME systems are quite reliable, and low maintenance. Potentially when all aircraft are equipped with GPS will the FAA consider removing DMEs from the NAS.

Is this helpful?