Principle of DME for part 2 of RTR(A) exam, also helpful for Navigation paper of DGCA

Distance Measuring Equipment (DME)

General Principle and Operation: It uses the principle of range by echo principle using secondary radar system.  The aircraft interrogator interrogates the ground transponder on a given carrier frequency by sending a pair of pulses at a random PRF. Various aircraft interrogate the same transponder on the same carrier frequency but with different PRF. The transponder replies to the interrogator by sending similar PRF of pulses on a carrier frequency 63 MHz plus or minus of the interrogation frequency.

The aircraft receiver starts timing as soon as the interrogation pulse is transmitted and commences search for the reply.  The pointer or counters start moving and indicates distance when lock on is achieved on a reply.

Frequency Band: UHF band is used in two sets:


  1. a) Low band: Aircraft interrogator 1025 to 1087 MHz}

} – 63 MHz

Ground transponder 962 to 1024 MHz     }


  1. b) High band: Aircraft interrogator 1088 MHz to 1150 MHz}

} + 63 MHz

Ground transponder 1151 MHz to 1213 MHz     }

Range: It is short range navigation aid which indicates slant range of about 200 nm for aircraft at 30,000 ft. Closer the station, greater the error due to slant range. But practically beyond 5 nm, the error almost becomes negligible.

Range = 1.25 (√ HT + √ HR)

Accuracy: ± 0.5 nm or ±3% of distance measured

Beacon Saturation: The transponder beacon replies to 100 strongest signals which means it is not necessary that nearest aircraft are responded to. It saturates with 100 interrogations.

VOR-DME: In some stations VOR and DME are paired.  By selecting VOR frequency at the aircraft both VOR and DME are selected and bearing and distance fix may be obtained.

ILS-DME: DME frequency may be paired with ILS Localizer frequency. Airborne equipment having facility for ILS and DME may get both the information by selecting only ILS Localizer frequency.

Ground Speed Check: When approaching or leaving a station DME may provide G/S check. Flying at an angle to the station shall not indicate G/S accurately.

Working.  DME is normally frequency paired with either VOR or ILS.  In Search/Scan Mode the aircraft interrogator transmits at the rate of 150 pulses per second (PPS).  Ground Transmitter replies to each pair of pulse with a frequency 63 MHz removed from received signal frequency.  In Lock ON mode aircraft interrogates at a changing PRF of 24 – 30 PPS with an average of 27 PPS.  If no response is received aircraft interrogator transmits at 60 PPS.  Ground equipment can respond to a max of 2700 PPS.  Interrogation code is linked to the leading and trailing pulse.

Q)  Number of aircraft required to saturate a DME station is (a) 126 (b) 100 (c) 80 (d) 27.

  1. (b)

 Q)  Number of aircraft required to saturate a DME station in Search/Scan mode is (a) 126 (b) 100 (c) 80 (d) 27.

  1. (a) A DME does not lock on to its own transmission because…………
  1. Transmission and reception takes place at different frequencies.

Slant Range Error.  DME equipment gives Slant Range.  The difference between slant range and ground range is called Slant Range Error.  The error is negligible when ac is than 1000 ft/nm.  A DME never reads zero except ILS DME which indicates zero at ILS threshold.

 Q)  An aircraft is flying at 20000 ft over a DME station, the DME will indicate (a) Zero (b) 3.28 km (c) 20000 ft (d) 3.28 Nm. A. (d)

ILS/DME shows distance of aircraft from the ILS threshold.  When co-located with Glide Slope equipment it gives correct indications only in approach funnel i.e. within localizer coverage area only and is calibrated for this purpose.

 Frequency Pairing with VOR.   A DME can be paired with VOR if it is co-located.  It still can be frequency paired if both VOR and DME are within 7 Nm and are serving the same station.  In such cases identification code will have “Z” at the end for example VVZ.

  1. The letter Z at the end of VOR Identification Code is indicative of

(a)  Both VOR & DME are co-located and serving same Station.

(b) VOR & DME are not co-located and are located > 7nm and are serving the same station.

(c)  VOR & DME are not co-located and are within 7 nm from each other but not serving same station.

(d)  VOR & DME are not co-located but are within 7 nm of each other and serving the same station.  A. (d)

TACAN.  It is a military navigation aid which gives both angular/ azimuth and range information to a suitably equipped military aircraft. TACAN transmits both range & azimuth information.  A civil aircraft can obtain range information only from TACAN.

Swamping.  Pushing out of some older, closer airborne equipment which is transmitting at low power by some modern airborne equipment which transmits at high power is called Swamping (100 most powerful airborne equipment sustain Lock ON)

Accuracy.  ± 0.5 Nm or 3% of range whichever is higher.

 Q)  An ac is flying at a DME indication of 50 nm. Accuracy of indication will be  (a) ±5 nm (b) ±0.5 nm (c) 1.5 nm  (d) None of the above.  A. (c)

  1. Give the approximate theoretical maximum range that an aircraft at 26000 feet may expect if the DME transponder is 81 feet amsl.
  2. Range = 1.25 (26000) + 1.25 (81) nm = (1.25 x 161) + 1.25 x 9) nm = 200 + 11.25 nm

=          211.25 nm

  1. An aircraft at 40 000 feet reads a DME distance of 80 nm to the station. What is its ground distance form the station?
  2. A. As the distance required is in nautical miles, we must convert 40,000 ft into nautical miles before applying it to the Pythagoras formula.

(Ground distance)2 = (slant range)2 – ht2 = 802 400002


= 802 –             6.582 (approx.) = 6400 – 43 (approx.) = 6357, ground distance = (6357)ft = 79.7 nm


  1. An aircraft at 24320 feet is 30 nm, ground range from the station. What is its DME range ?
  2. 42 + 302 = (slant range)2 or 916 = (slant range)2  , slant range = 916m = 30.26 nm

     3.  An aircraft at 20000 feet over a DME station, the DMe will read (a) Zero (b) 20, 000 ft  (c) 3.28 km  (d)  3.28 Nm.



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  • Arunaksha Nandy Post author

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