ATTENTION:
The following material is to be used for reference only.Do not use this material as your only source of information.Antonio Faria - SAIT student December 2011

DME – DISTANCE MEASURING EQUIPMENT


● BASICS
Distance measuring equipment (DME) is one of the most valuable pieces of avionics in the aircraft,especially for the IFR pilot.The main purpose of the DME is to display an aircraft distance to a ground station.DME reduces pilot workload by continuously showing the distance to the station, time-to-station and groundspeed.
Combined with VOR, the DME permits the pilot to determine an exact position from a single ground station.The VOR tells what radial the aircraft is on and DME tells how far out on that radial is the aircraft [1].
DME was developed in Australia, invented by Edward George “Taffy” Bowen.The system is a post-World War II development of the IFF (identification friend or foe) system.An engineered version of the system was deployed by Amalgamated Wireless Australasia Limited in the early 1950s.
This Australian domestic version was referred to by the Federal Department of Civil Aviation as DME(D) (or DME Domestic).The later international version was adopted by ICAO as DME(I).DME has been standardized by the International Civil Aviation Organization (ICAO) as a radio aid for short and medium-distance navigation.

● SYSTEM COMPONENTS
The DME system is composed of a UHF transmitter/receiver (interrogator) in the aircraft and a UHF receiver/transmitter on the ground.
A transmitter/receiver (interrogator) in the aircraft is showed at figure 1.
The ground station can have its own installations, as showed at figure 2.However it is more usually installed co-located with another navigation aid, such as VOR. This kind of station is called VOR/DME, showed at figure 3.

Fig1.jpg

Figure 1: DME receiver/transmitter (interrogator) [9]

Fig2.gif
Figure 2: DME ground station [2]

Fig3.jpg
Figure 3: VOR/DME ground station [2]

● FREQUENCY RANGE
978 MHZ to 1213 MHZ

This frequency range is situated in UHF Band and is characterized by line of sight (LOS), direct wave propagation.
The DME frequencies are paired to VOR frequencies.The DME interrogator on the aircraft is designed to automatically tune to the corresponding DME frequency when the associated VOR frequency is selected on the NAV receiver.
● HOW IT WORKS
DME function by means of two-way transmissions of signals between the aircraft (interrogator) and the DME ground station.

Fig4.jpgFFigure 4: DME signals [2]
Paired pulses at a specific spacing are sent out from the aircraft and are received by the ground station.The ground station then transmits the paired pulses back to the aircraft at the same pulse spacing but on a different frequency.The time required for the round trip of this signal exchange is measured in the aircraft DME unit and is translated into distance (NM) from the aircraft to the ground station.
Sequence of events:
  1. The aircraft interrogates the ground station by transmitting a series of pulse-pairs (interrogations) on the RX frequency of the ground station.
The pulse-pairs have a constant time interval (T1 = 12µsec or 36µsec) between pulses.
The interrogator randomly varies the time interval between pulse-pairs (Jitter).

guassian_pulses.jpg

Figure 5: DME generated pulse pairs [3]
Jitter
Is the randomly varying space between the pulse-pairs sent by the interrogator, represented by T2 at figure 5.Jitter works as an encoding of the pulse sequence sent by the aircraft, allowing the DME to recognize only the replies corresponding to its own interrogations.
2. The ground station receives the series of pulses-pairs and, after a precise time delay (50 µsec), the ground station replies with an identical sequence of reply pulse-pairs.
The reply from the ground station to the aircraft interrogator is made in a frequency 63 MHZ above or bellow the interrogator frequency.
3. The DME receiver in the aircraft receives the reply and measures the elapsed time from when it sent the interrogation until it received the reply.
It subtracts the 50 microsecond delay that the ground station introduced to come up with the round-trip time.
From this, the airborne receiver can calculate its exact distance from the ground station, given the fact that Distance = Velocity x Time.
The DME equipment then displays the computed distance.
Example:
Total time between interrogation and reply: 62.36 microseconds
Reply signal round-trip time: (62.36µsec - 50µsec)/2 = 6.18µsec
Velocity: 3x10^8 m/s
Distance = (3x10^8 m/s) x (6.18µsec) = 1,854 meters or 1.00108 NM


Fig6.jpg


Figure 6: DME signal travel time [2]
4. Considering that the ground station is replying interrogation from several aircraft at the same time, the DME interrogator need to sort its own pulse-pair out from the
ground station replies. To achieve that the DME airborne receiver examines the ground station replies looking for a sequence with the same randomly jittered signature.
When it finds that, it knows they're replies to its own interrogations.

The DME uses rate of change of the Distance to calculate the Ground Speed and Time to Station.Ground Speed is calculated by determining how far the aircraft travels in a short time [3].If an aircraft is known to be at certain distance and travels 1.38NM in 30 seconds, it will have a ground speed of approximately 190 miles/hour or 165 knots.
Once the ground speed is known, the Time to Station can be calculated. If you are flying away from the station, groundspeed will be accurate but "time-to-station"will actually show "time-from-station" and will increase as you get farther and farther from the station.
DME displays distance in nautical miles, groundspeed in knots, and time-to-station in minutes.
Beware, however, that DME groundspeed and time-to-station are only accurate when you are flying directly to (inbound) or from the ground station (outbound).
If you are flying in any other direction, you will see groundspeed that is erroneously low and time-to-station that is erroneously high.

● EFFECTIVE RANGE
DME is limited to line-of-sight.
Regardless of how much power the unit has, if it's not within direct line-of-sight of the ground station, it's not going to work.
The altitude of the aircraft will have a direct relationship with the range that the system can be used. The approximate range can be estimated by the formula [4]:


formula_1.jpg


Where d is distance in nautical miles and h is altitude in feet above the ground level.
For an altitude of 20,000 feet the approximate range will be 155 NM.

Another thing that can limit DME range is high air traffic density [1].
The DME ground station can only respond to a certain number of interrogations in a given period of time.
If too many aircraft are interrogating the ground station, it will automatically desensitize its receiver so that it can only hear and reply to the strongest interrogations.
This can result in shorter-than-normal DME range, particularly for small aircraft with their low-powered DME units.

● DME ACCURACY
DME in fact measures the straight-line distance from the aircraft to the ground station.
This is called "Slant Range" and is slightly more than the actual horizontal distance because of the difference in elevation between the aircraft and the station.




Fig7.gif



Figure 7: Slant Range [2]
The Slant Range (SR) is the actual distance from the aircraft to the VORTAC/DME.
For example, if an aircraft is at an altitude of 24,000ft (approx. 4NM) at a horizontal distance of 3NM, then the Slant Range from the aircraft to the station will be 5NM.


formula_2.jpg


There is than an accuracy problem due to the difference between the actual horizontal distance from aircraft to station and the Slant Range distance, which is thedistance calculated by the DME. At short distances the slant range error increase, however the error decreases at longer distances and is considered negligible atranges greater than 25NM or at altitudes less than 5,000ft [3].

System accuracy is typically 0.5NM or 3% of the calculated distance, whichever is greater.

The most extreme case of "slant range error " occurs when the aircraft passes directly over the station; instead of reading zero, the DME shows the altitude of the
airplane above the station (in nautical miles).

The distance displayed by the DME will be more accurate when flying at low altitude and far from the station.

Slant range error also affects groundspeed and time-to-station displays when you're close to the station.
Displayed DME groundspeed drops below actual groundspeed as you approach the station and then climbs back to normal after you pass it.
Displayed DME time-to-station may not count all the way down to zero as you fly over the station.


Fig8.png

Figure 8: Slant Range Error [2]
● INTERROGATION AND REPLY TIMING
The DME exchanges messages with the ground stations using one of two different timing modes:

X Mode
When the DME is channelled to a frequency ending in .X0 such as 122.00 or 116.70 MHZ.
The interrogation pulse pairs spacing from aircraft to ground station is 12µsec.
The reply pulse pair spacing from ground station to aircraft is 12µsec.
The ground station delay time is 50µsec.

Y Mode
When the DME is channelled with the .X5 frequencies, such as 114.55 or 117.65MHZ.
The interrogation pulse pair spacing from aircraft to ground station is 36µsec.
The reply pulse pair spacing from ground station to aircraft is 30µsec.
The ground station delay time is 56µsec.

The purpose of the ground station delay (50µsec in "X" mode) is to eliminate the possibility of uncoordinated operation when the aircraft is very close to the ground station.
If the ground station returned the pulse pair without a delay, then the interrogator could be still transmitting the second pulse of the pair when the reply from the first pulse was received.With the delay in place, a reply at zero nautical miles would occur 50µsec after the interrogation [3].
It then allows the operation of the DME in close proximity to the ground station.
● DME OPERATION MODES
The DME airborne equipment operates in several modes, described as follows:

Standby Mode
When the system is first powered up, it enters the standby mode. Transmissions are inhibited, the receiver and audio are operative.The DME displays shows four dashes to indicate no computed data. The receiver monitors pulse-pairs received from any local ground station.If sufficient pulse-pairs are counted, the interrogator enters in Search mode [4].
Search Mode
The transmitter now will broadcast the maximum number of pulse pairs (up to 150 pulse-pairs/sec) until it receives a specific number of reply pulses.
Track Mode
As the DME receives a valid reply for its own pulse-pairs, it enters in Track mode and reduces the pulse repetition frequency to 25 pulse-pairs/sec,in order to free up band space for other airborne DMEs. Distance to the ground station will now be showed on the DME indicator.
Memory Mode
If pulse-pairs from any station are not received after a short period of time (approx .2 seconds), the interrogator goes into memory mode, wherebydistance is calculated from the most recently received pulse-pairs. Memory mode expires after a short period of time (approx. 10 seconds) [4].The DME re-enter in search mode.
An interaction involves the rate of pulse pairs per seconds (pps) sent from the various airborne DME to the ground station.The ground station transmits at a constant pulse rate of 2700 pulse pairs per second, no matter how many aircraft are interrogating it.If there are not enough aircraft in the vicinity to make up the 2700 pps then the ground station will transmit randomly spaced pairs as filler pulses for the transmitter duty cycle.This is known as SQUITTER.
A typical DME ground station can provide distance information to 100 aircraft at a time. Above this limit the ground station avoids overload by limiting the gain of the receiver.Replies to weaker more distant interrogations are ignored to lower the transponder load.The technical term for overload of a DME station caused by large numbers of aircraft interrogations is Station Saturation [6].
● DME IDENT
DME also has identification. When a VOR and DME are co-located (as in a VORTAC or VOR-DME station), the DME transmits the same Morse coded ident as the VOR,but sends it during the pause between successive VOR idents.The DME ident is also higher-pitched: 1350 Hz compared with 1020 Hz for a VOR ident [1].
● ANTENNA
As DME uses direct wave propagation, antennas are usually located on the belly of the aircraft.Even with the antenna on this position, banking the aircraft will sometimes shield the antenna from the ground station with the wing.To prevent the display from indicating a malfunction during short periods of signal loss, the Memory mode described above is used.
The antenna is a little shark fin 1/4 wave antenna.

Fig9.jpg


Figure 9: DME aircraft antenna [8]
It should not be installed close to any structure, such as the landing gear, which may shield the antenna from the ground station.Avoid mounting the DME antenna (960 – 1215 MHZ) within six feet from the TRANSPODER antenna (1030 MHZ and 1090 MHZ), as both system works in close frequency ranges [7].Avoid mounting the DME antenna within three feet from any ADF sense or any COMM antenna [7].
● DME BLOCK DIAGRAM
block_diagram.jpg



Figure 10: DME block diagram [3]
● REFERENCES:
[1] AVweb - DME Basics - by Tom Rogers, Ph.D.
September 6, 1998
http://www.avweb.com/news/avionics/183230-1.html

[2] Google Images
http://www.google.ca/searchq=distance+measuring+equipment&hl=en&biw=1366&bih=587&prmd=imvns&tbm=isch&tbo=u&source=univ&sa=X&ei=ZrC9TpvINKKQiQLj2rGnAw&sqi=2&ved=0CHwQsAQ

[3] SAIT ELCM 355 course handouts – by Darrell Bueckert
December 2010

[4] Aircraft Communications and Navigation Systems
Mike Tooley and David Wyatt
Butterworth Heinemann – first edition 2007

[5] Distance Measuring Equipment (DME)
http://www.thalesatminc.com/Technology/DME415.htm

[6] Distance Measuring Equipment
Wikipedia
June 2009
http://en.wikipedia.org/wiki/Distance_measuring_equipment

[7] Bendix/King KN62/62A/64 Manual Number 006-00144-0007
Revision 7
November, 2004
http://www.to-avionics.com/pubs/bendixking/a/006-00144-0007_7.pdf

[8] Peninsula Avionics
Online Catalog
http://www.peninsulaavionics.com/item322.htm

[9] Avionics Sales
Online Catalog
http://www.avionicsales.com/product/GA/dme.htm