What is DME? How does a DME work? Briefly explained.

Distance Measuring Equipment (DME) makes a pilot’s life a lot easier by figuring out how many miles the airplane has to fly to arrive at the next station.

Distance measurement equipment (DME) was invented by James “Gerry” Gerrand under the supervision of Edward George “Taffy” Bowen while employed as Chief of the Division of Radiophysics of the Commonwealth Scientific and Industrial Research Organisation (CSIRO).

Amalgamated Wireless Australasia Limited deployed another engineered version of the system in the early 1950s operating in the 200 MHz VHF band.

The Federal Department of Civil Aviation referred to this Australian domestic version as DME (D) (or DME Domestic), and the later international version adopted by ICAO as DME.

What is DME in aviation?

The DME determines the slant range between an airplane and a selected VOR ground station in NM (line of sight) by timing a signal being sent from the A/C to the GND station, then the GND station replay (interrogation, then replay). 

Commonly DME station is located at a VOR station or near an ILS station at an airport to calculate the slant distance between the aircraft and the station.

When you select a VOR or LOC frequency, the frequency of the associated DME station (if there is one) gets simultaneously chosen as they pair automatically.

To know the distance between the aircraft and a VOR station, the plane needs a DME interrogator and a DME ground station on the ground.

How does a DME work?

The interrogator transmits its uniquely randomly spaced pulse pairs to the ground station. After a short delay (50 μs), the DME ground station retransmits these pulse pairs at a frequency either above or below the frequency of the transmitted signal by 63 MHz

The randomly spaced pulses make it possible for each DME system to discriminate between its retransmitted signal and those of A/C s. Likewise, a random spacing identification makes it possible for the A/C receiver to determine the elapsed time between transmission from A/C & receiving of that signal retransmitted from the ground station. 

The system consists of the following:

1. Transmitter: To transmits the uniquely randomly spaced pulse pairs to the ground station.

The transmitter is connected to a suppressor bus which is also linked to ATC transponders.

When one transmitter connected to the suppressor bus is transmitting, other transmitters attached to the bus are suppressed to prevent interference.

2. Receiver: It receives the retransmitted pulse pairs from the ground station.

3. Duplexer:- This multiplies the wave with the carrier wave.

4. Antenna: Transmits DME interrogation and receives the reply from the selected ground station.

5. Controlled variable delay: Delay the transmitted signal internally. It gives out a signal to the indicator to display the equivalent distance of the delay set up in the controlled variable delay block

6. Matching circuit:- It gives error information by comparing the received signal with the delayed transmitted signal from the controlled variable delay block.

When the system is on, it is in Search mode.

The controlled variable delay servo motor runs from zero to max. Delay causes the indicator readout to change from zero to maximum. And back to zero until it matches the delayed transmitted signal to the received signal.

While searching, the warning flag in the DME indicator is in view showing that the DME isn’t operative. 

The matching circuit causes the servo motor to Lock on a particular delay (equivalent to a specific distance lock on) when the matching circuit sees the same delay for the transmitted and received signals.

7. Audio identification: Sends the information (identification tone in Morse code) on an audio frequency of 1350 HZ to the flight interphone system in the cockpit.

DME and ATC operate in the same frequency range (UHF band), so they have identical and interchangeable antennae.

There are 4 DME system modes:

Search mode: Constantly search for a DME station 

Lock-on mode: The servo motor stops its quick search and runs slowly after Lock-on.

The distance indication is correct if the delayed transmitter signal matches the received signal. 

When the aircraft is near or over the beacon, the DME range is approx. = A/C altitude than the horizontal distance.

This causes the GND speed indicated to be slower than the actual GND speed. 

To overcome this problem, the indicator goes to the Memory mode when the aircraft is 20 NM from the beacon, indicates the last calculated GND speed, and corrects it for the slant range at 20 NM until the plane passes another 20 NM on the other side of the beacon.

Memory mode: DME keeps the location for the selected station frequency in the memory.

DME aids pilots in learning the distance between the aircraft & the desired station.

Image Source: ig_aviaton via Instagram.