Distance Measuring Equipment (DME)
DME is a secondary radar providing the pilot with an accurate slant range from a ground transmitter. Normally paired with VOR the combination provides the standard for ICAO short-range navigation systems (know as rho-theta). More recent uses see the DME paired with ILS and MLS to give range from touchdown during a precision approach.
Principle of Operation
The system works on the principle of secondary radar:
- The interrogator on board the aircraft transmitting an interrogation signal.
- The ground based transponder (transponder meaning a transmitter that is responding to an interrogation)
The interrogation signal from the aircraft and the response are on different frequencies.
- Frequency: UHF – 960 to 1215 MHz
- Emission Characteristics: P0N
- Aircraft Equipment: The airborne unit (interrogator) consists of:
- An omni-directional blade aerial.
- A transmitter.
- A receiver.
- A time measuring device.
- A tracking unit.
The interrogator transmits pulse pairs on the selected frequency. These pulse pairs are spaced by either 12μ seconds (X channel) or 36μ seconds (Y channel).
For the X channel the transponder will reply with a pulse pair spacing of 12μ seconds. The Y channel reply is at 30μ seconds.
When the equipment is switched on, or when new DME channel is selected, the pulse pairs are transmitted at 150 pulse pairs per second (pps). This is the search mode. The equipment will stay in search mode till the equipment:
- Locks on (normally 4 to 5 seconds), or
- 15 000 pulse pairs have been transmitted.
If lock on occurs then the transmitter reduces the PRF of pulse pairs to 24 to 30 pps, this is known as “tracking mode”. If the system transmits 15 000 pulse pairs the PRF will drop to 60 pps until the system locks on.
The ground transponder consists of a receiver and a transmitter. When an interrogating signal is detected, the response is transmitted after a 50 μsecond delay. Response is at a different frequency to that received with the transponder being capable of generating up to 2700 PPS. When a signal is replied to, the ground transponder will reply at a rate of 24 – 30 pps.
Interrogator and transponder operating frequencies are grouped into pairs, the two frequencies being 63 MHz apart. The airborne interrogator uses frequencies from 1025 MHz to 1150 MHz for transmissions, while the ground based transponder answers on frequencies in two groups, 962 MHz to 1024 MHz (low) and from 1051 MHz to 1213 MHz high).
For each airborne interrogation frequency two reply frequencies are allocated, one at + 63 MHz and the other at – 63 MHz. These are the X and Y channels. An interrogation frequency of 1030 MHz will, therefore, have responding frequencies at 1093 MHz and 976 MHz. The responding frequencies at + 63 MHz are referred to as “X” channels, while those at – 63MHz are known as “Y” channels. This allows for 252 channels as opposed to the 126 previously available.
ICAO recommends the pairing of DME channels with VOR/ILS or MLS. Thus a VOR on 112.30 MHz is always paired with the DME on Channel 70X (1094 MHz interrogation – 1157 MHz reply). A VOR on 112.35 MHz would pair with the DME on Channel 70Y (1094 MHz interrogation – 1031 MHz reply). Each DME channel is identified by a number and a letter (X or Y). The following table is an illustration of some of the available channels with their paired frequencies.
DME Channel = VOR/ILS/MLS Paired frequency
- 20X = 108.3
- 20Y = 108.35
- 21X = 108.4
- 21Y = 108.45
- 70X = 112.3
- 70Y = 112.35
- 126X = 117.9
- 126Y = 117.95
The channel numbers and paired frequencies can be found in the relevant communications documents.
As a pilot you will never select a DME frequency because of the pairing. Even though the system works in the UHF band you will select the paired VHF frequency. The major reason for this procedure is to reduce the workload.
If two aircraft transmit to a DME at the same time. The replies are on the same frequency. If both signals received by the aircraft are the same how can any differentiation of the correct reply be made. Which aircraft is being replied to?
The equipment in the aircraft “jitters” the PRF before transmission. This random PRF is unique to the aircraft. When the ground station replies it manufactures exactly the same PRF reply for the aircraft. Any reply taken by the airborne equipment, which does not match the PRF of the initial transmission, is rejected.
The responder will now respond to the new rate and since the interrogator PRF is randomly varied, only the responses to that interrogation will have the same random variation of PRF. Within the airborne receiver the ‘tracking unit’ looks for responses around the anticipated time interval that is compatible with the current range from the ground responder.
Effectively a gate is created and only responses that arrive within that gate are considered. The receiver then determines a match between the PRF of the response and those that were transmitted. Once this match is achieved, the time difference is measured and, allowing for responder fixed delay, a range is derived. This is tracking mode.
The advantage of using secondary radar is that reflected transmissions from the ground or cloud will not be processed by the aircraft equipment, as the frequency of reply is incorrect.
If the responding signals are interrupted whilst the system is in tracking mode a memory circuit is activated. The system holds:
- The last measured range value
- The receiver gate at the last measured time interval
Memory mode can be held for up to 8 to 10 seconds after which the system will return to the search mode.
Since the ground based responder beacon is limited to a maximum PRF of 2700 pps and interrogations occur at 24 - 30 pps (27 pps average), it follows that up to 100 aircraft may be handled by one DME beacon.
The ground transponder has a set gain level that a signal must break to be replied to. This ensures that receiver noise or other returns that are weak are ignored. In the diagram:
- Signal A: Is too weak to break the normal gain level so is not replied to.
- Signal B & D: Both signals have broken the normal gain level and so are replied to by the receiver as long as the beacon is not saturated.
- Signal C: If the beacon is saturated the normal gain level is raised to a saturation gain level and only the strongest 100 signals are replied to.
Co-location of Beacons
As stated in the introduction the DME is usually paired with a VOR to provide the primary short range fixing required by ICAO. Where a VOR/DME transmit the same callsign in a synchronized manner the stations are called “associated”. This means:
- The VOR and DME transmitter are co-located.
- In a terminal area where the VOR/DME is used for approach purposes the aerials are a maximum of 100 feet apart.
- Where the VOR and DME are not used for approach purposes the aerials are at a maximum of 2000 feet apart.
Where co-location occurs the identification is synchronized and transmitted 7½ seconds apart. In a 30 second period:
- The VOR will ident 3 times.
- The DME will ident once.
Where a VOR and DME serve the same area (within 7 nm) they may be frequency paired but the DME will generally use Z as the last letter of the ident.
- VOR ident MAC
- DME ident MAZ
Where a VOR and TACAN are co-located the system is called VORTAC. The VOR uses the DME portion of the TACAN.
Where DME is paired with an ILS or MLS the 50μ second time delay is gradually reduced to a minimum to allow the DME to read zero when the aircraft passes the runway threshold.
All aircraft displays will show the value of the measured slant range while some contain an arithmetic unit, which calculates the instant ground speed and time to the station. On most modern installations, you can select “GS” or “TIME” for this purpose. Some indicators show distance, ground speed and time simultaneously.
It is important to note that the indications of ground speed and time will only be correct when flying directly towards the ground station. If you fly in any other direction, both the DME indicated ground speed and time to the station would be too low. In this case, only slant distance is correct.
All navigational aids provide the pilot with a position line, depending on the type of radio aid. The position line resulting from the DME, is a circle. When your DME indicator shows 55 nm, you know that you are at a slant range of 55 nm from the station, but you don’t know if you are south, east, north or west of the station.
As a result of this, the position line from one DME station alone is only of little help. A radial from a VOR will provide a second position line that could intersect the DME circle at two places. This results in an ambiguity situation as can be seen from the diagram.
If the VOR and DME are associated you will have one clearly defined fix position.
As an approach aid, the DME will provide, together with the tracking facility, positions like initial approach fix (IAF), final approach fix (FAF) and missed approach point (MAPt) etc.
A DME procedure is one that is published for use with a particular ground facility. The most common type of DME procedure is known as “flying the arc”. This procedure requires the pilot to maintain a specific range from a DME, generally between two stated VOR radials.
The DME indicates a slant range that is the straight line from the aircraft to the ground station, not the distance along the ground. The true range can be calculated by using Pythagoras’s theorem.
A2 + B2 = C2
If an aircraft is flying at 45,000 ft with an indicated DME of 175 NM. The true range is?
- 45 000 ft = 7.4 nm
- Range = 1752 - 7.42
- Distance = 174.84 nm
This results in a slant range error of 0.16 nm or 0.09%, which is negligible.
Problems start when the aircraft is closer than 20 nm to the beacon.
The aircraft is at 30 000 ft, indicated DME 20 nm. The true range is?
- 30 000 ft = 4.9 nm
- Ground distance = 19.39 nm
- The error is 0.61 nm or 3%
The slant error is almost negligible at long distances, but increases both with altitude and with decreasing DME distance. If you are using a DME at less than 20 nm range you must apply a slant range correction.
Flight Overhead the DME
When an aircraft passes directly overhead a DME station, the DME will indicate the altitude of the aircraft in nautical miles. For instance, if the aircraft passes at an altitude of 40 000 ft, the indication will be about 6.6 nm.
There is a cone of silence directly above the ground station. However, the arithmetic unit in the aircraft will remember the last computed data and continue to indicate the altitude for some time.
If no signals of an acceptable strength are received the system will unlock and will only regain the tracking mode if the correct signal of an acceptable strength is received. The unlock condition is indicated by:
- An off flag on rotary indicator
- A red bar across the face of the digital indicator
Failure indications are also shown when the equipment is switched on and:
- No signal is being received
- The received signal is below the minimum strength required
- The aircraft is out of range of the transponder
An off indication is also given when the equipment is switched off.
The DME is extremely accurate. ICAO prescribes a maximum system error of ± 0.25 nm (370 metres) or ± 1.25% of the slant range on 95% of occasions whichever is the greater.