5 - 1

DME

NAR and TFHS

5 DISTANCE MEASURING EQUIPMENT

5.1 Principle of operationThe distance measuring equipment - DME - provides the pilot with distance infor-mation from the aircraft to the tuned ground station. The system works on theprinciple of a secondary radar and there are, therefore, two active units. Theinterrogator on board the aircraft and the ground based transponder. The systemoperates in the UHF frequency-band on carrier frequencies between 960 and 1215MHz with a channel spacing of 1 MHz. This results in 252 spot frequencies.

Interrogator and transponder operating frequencies are grouped into pairs, thetwo frequencies being 63 MHz apart. The airborne interrogator uses frequenciesfrom 1025 MHz to 1150 MHz for transmissions, while the ground based trans-ponder answers on frequencies in two groups, 962 MHz to 1024 MHz and from1151 MHz to 1213 MHz. These frequencies are grouped to provide two sets ofchannels.

For each airborne interrogation frequency two reply frequencies are allocated, oneat + 63 MHz and the other at 63 MHz. An interrogation frequency of 1030 MHzwill, therefore, have responding frequencies at 1093 MHz and 976 MHz. Theresponding frequencies at + 63 MHz are referred to as X channels, while thoseat 63MHz are known as Y channels. This allows for 252 channels as opposedto the 126 previously available.

Each DME channel is identified by a number and a letter (X or Y). The followingtable is an illustration of some of the available channels with their paired VOR/LZZ frequencies.

DME VOR/LLZchannel Paired

frequency

20X 108.320Y 108.3521X 108.421Y 108.45- -80X 113.380Y 113.35- -126X 117.9126Y 117.95

In order to simplify tuning procedures in the aircraft, DME channels are pairedwith specific VOR or ILS frequencies. You, the pilot, will never tune a DME byselecting its channel number but only its paired VHF frequency. One of the majorreasons behind the use of paired frequencies is to reduce flight deck work load at

Radio Navigation

5 - 2 NAR and TFHS

times when there is a great amount of activity, such as during take off, initialclimb and approach.

The ground transponder consists of a receiver and a transmitter. On detecting aninterrogating signal, a fixed 50-microsecond delay is triggered after which the re-sponder transmits pulses at exactly the same rate as those of the interrogatingsignal. This is where the necessity of using different frequencies for interrogationand response becomes evident. If both frequencies were the same then the re-sponses would be detected by the responder receiver and self-triggering wouldoccur. The responder is capable of generating up to 2700 PPS.

Responders are normally sited in such a way as to provide a ranging facility tocompliment the bearing information from a VOR. This combination of range andbearing is referred to as a rho/theta (/) navigation system.

5.2 Airborne EquipmentThe airborne unit (interrogator) consists of an aerial, a transmitter, a receiver, atime measuring device and a tracking unit. The interrogator transmits pulsepairs on the selected frequency. On initial switch-on, or when a new DME channelis selected, the pulse pairs are transmitted at 150 PPS. The receiver/timer circuitsearches for responses at that PRF. This is known as the SEARCH MODE.During this mode of operation, which normally lasts for only 4 or 5 seconds, therange indication will be hidden from view. When a response is detected the receiverwill LOCK ON and the transmitter PRF will drop to an average of 27 pulse pairsper second but will be randomly varied over the range 24 to 30 PPS. The re-sponder will now respond to the new rate and since the interrogator PRF is ran-domly varied, only the responses to that interrogation will have the same randomvariation of PRF. Within the airborne receiver the tracking unit looks for responsesaround the anticipated time interval which is compatible with the current rangefrom the ground responder. Effectively, it creates a gate and only responses thatarrive within that gate time are considered. The receiver then determines a matchbetween the PRF of the response and those that were transmitted. Once thismatch is achieved, the time difference is measured and, allowing for responderfixed delay, a range is derived. This is known as the TRACKING MODE.

You should make special note at this time that the measured range is along thedirect path between interrogator and transponder and it is, therefore, a slantrange. (Figure RA 5.1)

5 - 3

DME

NAR and TFHS

Figure: RA 5.1

While operating in tracking mode, if the response signals are interrupted, aMEMORY circuit is activated. This holds the indications at the last measuredrange value and, more importantly, holds the receiver gate at the last measuredtime interval. This is maintained for a period of 8 to 10 seconds, after which theunit returns to search mode.

Beacon SaturationSince the ground based transponder beacon is limited to a maximum PRF of 2700pulses per second and interrogations occur at 27 PPS (average), it follows that upto 100 aircraft may be handled by one DME beacon. If more than 100 interroga-tors are within nominal range of the beacon, responses will be transmitted to the100 strongest signals. In this circumstance, as your range from the DME in-creases, your interrogation signal will become weaker (at the responder) untilfinally it will get no response. After a short period in Memory Mode your DMEinterrogator will revert to Search Mode.

Frequency SelectionUnlike other radio navigation equipment the DME does not need a frequency selec-tor. As mentioned before, the VOR and localiser frequencies are normally pairedwith DME channels. This means that when a certain VOR station is tuned in theNAV 1 and the selector on the DME is set to NAV 1, the indicator will show thedistance to the DME station that has a channel paired with that particular VORor localiser frequency.

You will never have to tune in the UHF operating frequency of a DME, only thepaired VHF frequencies are published on the appropriate charts. The indicatorcan have different types of displays, but a digital display is most common.

Additional FunctionsAll aircraft displays will show the value of the measured slant range while somecontain an arithmetic unit, which calculates the instant ground speed and time tothe station. On most modern installations, you can select GS or TIME for thispurpose. Some indicators show distance, ground speed and time simultaneously.It is important to note that the indications of ground speed and time will only becorrect when flying directly towards or away from the ground station. If you fly inany other direction, both the DME indicated ground speed and time to the stationwould be too low. In this case, only slant distance is correct. Even more advancedairborne DME equipment may have facilities for entering or storing the elevation

Radio Navigation

5 - 4 NAR and TFHS

of the DME ground station. Also having the aircraft altitude available, the DMEmay calculate the horizontal distance between the aircraft and the ground DMEstation. The use of DME in RNAV calls for extensive automatic operation of on-board DME sets.

5.3 Ground Installation SitingThe ideal situation regarding the position of a ground installation, would be wherethe VOR station and DME stations being paired are situated at exactly the sameposition geographically. This would provide the pilot with both bearing and dis-tance information to the same geographical point. Unfortunately, it is not alwaysso. The ground station providing distance information is sometimes a part of themilitary navigation aid TACAN (Tactical Air Navigation). In such cases, the DMEwill not necessarily be co-located with a VOR station. Where a VOR and a TACANare associated and providing a common service, the ground station is called aVORTAC.

When the facilities are so-called associated, they (VOR-DME or VOR-TACAN) areeither co-located or are situated within a certain prescribed maximum distance.Associated stations are frequency paired, they have the same Morse identificationcodes which are synchronously transmitted. The maximum allowed distance be-tween the transmitter aerials depends on the intended use of the installation. Forassociated stations intended for navigation purposes, up to 600 metres is accept-able. However, for such beacons used to provide guidance in the terminal andapproach phases a smaller separation would be necessary.

Where a DME unit is associated with an ILS installation, it will provide approachrange information continuously and is therefore considerably more effective thanthe marker beacon. In this use it is important that the distance between theaerials should be minimised.

It is also worth noting that, in order to be able to provide ranges at very low values,the 50ms responder delay is considerably reduced.

5.4 DME navigation

5.4.1 Position FixingAll navigational aids provide the pilot with a position line, depending on the typeof radio aid. The position line resulting from the DME, is a circle. When your DMEindicator shows 55 NM, you know that you are at a slant range of 55 NM from thestation, but you dont 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 littlehelp. A radial from a VOR not co-located or associated with the DME will providea second position line which could intersect the DME LOP at two places. Thisresults in an ambiguity situation as can be seen from figure RA 5.2.

5 - 5

DME

NAR and TFHS

Figure: RA 5.2

If the VOR and DME are associated you will have one clearly defined fix position.Fortunately, most of todays VOR/DME installations are associated and provide avery useful navigational aid, both for en-route navigation and approach. For en-route navigation, the DME provides a good means of checking the wind situationagainst forecast. It also makes it easy for you to find the correct change-over pointswhen tracking between two VORs.

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 ap-proach point (MAP)etc.

5.4.2 DME ProceduresA 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 is aprocedure that requires the pilot to maintain a specific range from a DME, gener-ally between two stated VOR radials. Figure 5.3 illustrates such a procedure

55 NM

DME groundstation

VOR

Radial 345

Radio Navigation

5 - 6 NAR and TFHS

Figure: RA 5.3

5.5 Limitations and AccuracyDME transmissions are, like the VOR, line of sight transmissions and the sig-nals follow a straight line. This means that the operating range will depend onaltitude and the presence of obstacles between transmitter and receiver. The maxi-mum range will be approximately 200 NM and the formula for calculating therange is the same as for other VHF/UHF aids;Range (NM) = 1.25HTx + 1.25HTrAs explained earlier, the DME will indicate slant range, which is the straight linefrom the aircraft to the ground station and not the distance along the ground.When we know the altitude of the aircraft above DME elevation and the indicatedDME distance, we can calculate the actual ground distance by applying Pythago-ras rule.

Example 1:An aircraft is flying at 45, 000 ft with an indicated DME of 175 NM. What is thecorrect ground distance?First, we convert the 45000 ft to 7.4 NM and by using Pythagoras we can find thecorrect ground distance:

Dist2 = 1752 - 7.42

Dist = 174.84NM

This results in a slant range error of 0.16 NM or 0.09 %, which is negligible.

Example 2:The aircraft is at 30, 000 ft, indicated DME 25 NM. What is the ground distance?

30 000 ft = 4.9 NM, ground distance = 24.52 NM. We get a slant range error of0.48 NM or 1.92%.

5 - 7

DME

NAR and TFHS

As you see, the slant range error does exist. It is almost negligible at long dis-tances, but increases both with altitude and with decreasing DME distance.

When an aircraft passes directly overhead a DME station, the DME will indicatethe altitude of the aircraft in nautical miles above the DME station. For instance,if the aircraft passes at an altitude of 40000 ft, the indication will be about 6.6NM.

Actually, there is a cone of silence (see chapter VOR) directly above the groundstation. However, the arithmetic unit in the aircraft will remember the last com-puted data and present this value on the indicators for some seconds, dependingon the type of equipment.

AccuracyThe DME is extremely accurate. ICAO prescribes a maximum system error of 0.5 NM or 3% of the slant range, whichever is the greater. This means that at 200NM DME distance, the error will be no more than 6 NM.

With todays modern equipment, the system is capable of providing a service wellwithin these requirements and the normal operating accuracy is considered to be 0.2 NM or 0.25 % of the slant range, whichever is greater. The result of this isthat we can expect all indications to be within 0.2 NM which is extremely accu-rate.

The above accuracies apply to DMEs meant for general navigation use (DME/N).Improved types of DME, DME/P are meant for more critical types of navigationsuch as instrument approaches associated with ILS or MLS. The Accuracies ob-tainable with DME/P is considerable higher, up to + less than 10 meters for someapplications.

Radio Navigation

5 - 8 NAR and TFHS

Can you answer these?

1 In the DME system

a) The aircraft equipment is called a transponder.b) The receive and transmit frequency is always split by 63 MHz.c) The operation is similar to a primary radar system.d) The channels are referred to as X channels paired with VORs and

Y channels paired with ILS localizers.

2 The airborne DME equipment will transmit pulse pairs at acomparatively high PRF

a) At all times, except when the panel control LO is operated.b) When the distance presented is above 50 NM.c) Whenever a stable signal is being received from the selected ground

station.d) When first switched on and after a channel selection.

3 System, or beacon, saturation of the DME system

a) Occurs when the aircraft DME set has been in operation for anextended period of time, without being put into the STAND/BYmode.

b) Occurs when many aircraft, being at a long distance from the DME,are demanding a reply.

c) May occur when more than 100 aircraft are demanding replies froma single ground station.

d) All 3 answers are correct.

4 If a VOR station and a DME station, having different locations, areselected to provide a fix:

a) Two sets, with separate frequency control, are required in theaircraft.

b) Two positions, being ambiguous, will be presented.c) Two different IDs will have to be checked.d) All 3 answers above are correct.

SA-RN 1.5

5 - 9

DME

NAR and TFHS

5 Using modern DME equipment meant for general navigation use, theaccuracy expected is

a) 0,2 NM or 0,25% of the slant range, whichever is greater.b) 0,5 NM or 0,25% of the slant range, whichever is greater.c) 0,5 NM or 3,0% of the slant range, whichever is greater.d) 0,2 NM or 3,0% of the slant range, whichever is greater.

6 How many aircraft will saturate a DME station?

a) 200 aircraft.b) 100 aircraft.c) 50 aircraft.d) 2700 aircraft.

7 What frequency does an SSR interrogator transmits on?

a) 1030 MHz 0,2 MHz.b) 1030 KHz 0,2 KHz.c) 1000 MHz.d) 118,4 MHz.

8 An aircraft is passing overhead a DME station at FL 240. What is theDME indication?

a) 0 DMEb) 1 DMEc) 4 DMEd) 6 DME