Distance Measuring Equipment (DME)

Distance Measuring Equipment

Distance Measuring Equipment provides Accurate Slant Range using secondary radar principle
Combination of VOR and DME is an ICAO approved equipment for Airways
TACAN is military equipment equivalent to DME that is installed in military airfields providing same indications in flight deck

Radar Mile is the time taken for a Pulse to travel 1 NM and return back to radar calculated using speed of radio waves
Since, 1 Radar Mile = 12.36 Micro Seconds One way Distance to the target in NM = Time Taken For Pulse To Return / 12.36

DME equipment in the aircraft sends a pair of interrogation pulses and receives the response from ground transmitter that responds after a delay of 50 micro seconds
The time difference between the pulses gives slant range or the straight-line distance of the aircraft from the DME calculated using the speed of radio waves
Slant Range can be converted to Plan Range using Pythagoras Theorem that states Plan Range² = Slant Range² – Height²
Slant range is equated to plan range when distance exceeds thrice the height but when aircraft is close to the beacon slant range must be converted to plan range

DME operates on the Frequency Range of 962 to 1213 MHz in the UHF Band with 1 MHz Spacing between two DME, therefore accommodate 252 channels
DME uses two different carrier wave frequencies for transmission and reception spaced 63 MHz apart
For example, if the aircraft transmits at 962 MHz the Ground station would respond on 1025 MHz
Transmission Frequency by aircraft and ground station is changed by 63 MHz to avoid self triggering of the transponder due to reflections from clouds
DME is equipped with an Echo Protection Unit which provides a 50 micro second delay to avoid Ground Reflected Waves

DME uses Jittering the PRF or Random Pulse Repetition Frequency Technique
Jittering PRF technique enables the aircraft to pulse pairs at random intervals and opens its receiver gates in to accept its response only
Electronic gates in the aircraft transponder open the receiver to accept only its own response pulse that matches its transmitted PRF
This technique avoids confusion between the response meant for your aircraft from response meant for other aircrafts
Twin pulses are transmitted to avoid accepting matching randomised single pulses from ignition systems or from other radars

DME switches over between Search, Track and Memory modes automatically for efficient operation
Search mode is active on initial contact when the aircraft transmits 150 pulse pairs for 100 seconds, thereafter reduces transmission to 60 pulses until lock on
Tracking mode is active on establishing contact with ground station where aircraft transmits 25 pulse pairs per second
Lock and follow technique by electronic gates that opens receiver based on the changing distance from station is used in Tracking Mode
Memory mode activates on temporary non-availability of response where the DME displays calculated distances based on recent ground speed for 10 seconds

Beacon saturation occurs when more than 100 aircrafts interrogate the beacon and the ground transponder is unable to respond to all the aircrafts
It is assumed 95% of aircrafts would be tracking and 5% in search modes
Average requirement is taken as 27 pulse pairs per second per aircraft, hence the output of ground station is 2700 pulse pairs for 100 aircrafts
Beacon saturation happens when the ground station exceeds 2700 pulse pairs the ground equipment reduces its gain excluding excluding far away aircrafts

Associated VOR and DME are selected using that same selection in aircraft
Associated Terminal VOR-DME are located within 100 Ft (30 M) while associated Route VOR-DME are located within 2000 Ft (600 M)
Non-Associated VOR-DME in the same place within 1 NM apart would have their ident third letter as Z like VOR BBB may have DME BBZ
Non-Associated VOR-DME more than 6 NM apart have their different Identification codes

Accuracy of DME is calculated on a 95% probability meaning that the DME accuracy would be maintained 95% of times
Narrow spectrum DME (DME-N) is 1.25 % of range and 0.25 NM and Precision DME (DME-P) is 0.20 NM
Ground speed accuracy will reduce when overhead beacon or while flying a curved path therefore ground speed is valid only when flying directly to or from the station
ILS-DME is co-located with ILS equipment and measures distances from the threshold of runway and therefore would be correct only on approach

Maximum Range is based on line-of-sight equation, however the practical range is about 250 NM
Designated Operational Coverage (DOC) of DME protects DME from range errors due co-channel interference
When the DOC of DME is mentioned as 20/240 it means that the DOC is 20 NM at 24,000 Ft