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3.5.3.4.2 DME/N. If it is desired to decrease the time of search, the PRF may be increased during search but shall not
exceed 150 pairs of pulses per second.
Annex 10 — Aeronautical Communications Volume I
23/11/06 3-42
3.5.3.4.3 DME/N. Recommendation.— After 15 000 pairs of pulses have been transmitted without acquiring
indication of distance, the PRF should not exceed 60 pairs of pulses per second thereafter, until a change in operating
channel is made or successful search is completed.
‡3.5.3.4.4 DME/N. When, after a time period of 30 seconds, tracking has not been established, the pulse pair repetition
frequency shall not exceed 30 pulse pairs per second thereafter.
3.5.3.4.5 DME/P. The interrogator pulse repetition frequency shall not exceed the following number of pulse pairs per
second:
a) search 40
b) aircraft on the ground 5
c) initial approach mode track 16
d) final approach mode track 40
Note 1.— A pulse repetition frequency (PRF) of 5 pulse pairs per second for aircraft on the ground may be exceeded if
the aircraft requires accurate range information.
Note 2.— It is intended that all PRF changes be achieved by automatic means.
3.5.3.5 Aircraft handling capacity of the system
3.5.3.5.1 The aircraft handling capacity of transponders in an area shall be adequate for the peak traffic of the area or
100 aircraft, whichever is the lesser.
3.5.3.5.2 Recommendation.— Where the peak traffic in an area exceeds 100 aircraft, the transponder should be
capable of handling that peak traffic.
Note.— Guidance material on aircraft handling capacity will be found in Attachment C, 7.1.5.
3.5.3.6 Transponder identification
3.5.3.6.1 All transponders shall transmit an identification signal in one of the following forms as required by 3.5.3.6.5:
a) an “independent” identification consisting of coded (International Morse Code) identity pulses which can be used
with all transponders;
b) an “associated” signal which can be used for transponders specifically associated with a VHF navigation or an MLS
angle guidance facility which itself transmits an identification signal.
Note.— An MLS angle guidance facility provides its identification as a digital word transmitted on the data channel into
the approach and back azimuth coverage regions as specified in 3.11.4.6.2.1.
3.5.3.6.2 Both systems of identification shall use signals, which shall consist of the transmission for an appropriate
period of a series of paired pulses transmitted at a repetition rate of 1 350 pulse pairs per second, and shall temporarily
replace all reply pulses that would normally occur at that time except as in 3.5.3.6.2.2. These pulses shall have similar
characteristics to the other pulses of the reply signals.
Chapter 3 Annex 10 — Aeronautical Communications
3-43 23/11/06
‡3.5.3.6.2.1 DME/N. Reply pulses shall be transmitted between key down times.
3.5.3.6.2.2 DME/N. Recommendation.— If it is desired to preserve a constant duty cycle, an equalizing pair of pulses,
having the same characteristics as the identification pulse pairs, should be transmitted 100 microseconds plus or minus
10 microseconds after each identity pair.
3.5.3.6.2.3 DME/P. Reply pulses shall be transmitted between key down times.
3.5.3.6.2.4 For the DME/P transponder, reply pulse pairs to valid FA mode interrogations shall also be transmitted
during key down times and have priority over identification pulse pairs.
3.5.3.6.2.5 The DME/P transponder shall not employ the equalizing pair of pulses of 3.5.3.6.2.2.
3.5.3.6.3 The characteristics of the “independent” identification signal shall be as follows:
a) the identity signal shall consist of the transmission of the beacon code in the form of dots and dashes (International
Morse Code) of identity pulses at least once every 40 seconds, at a rate of at least 6 words per minute; and
b) the identification code characteristic and letter rate for the DME transponder shall conform to the following to
ensure that the maximum total key down time does not exceed 5 seconds per identification code group. The dots
shall be a time duration of 0.1 second to 0.160 second. The dashes shall be typically 3 times the duration of the dots.
The duration between dots and/or dashes shall be equal to that of one dot plus or minus 10 per cent. The time
duration between letters or numerals shall not be less than three dots. The total period for transmission of an
identification code group shall not exceed 10 seconds.
Note.— The tone identification signal is transmitted at a repetition rate of 1 350 pps. This frequency may be used
directly in the airborne equipment as an aural output for the pilot, or other frequencies may be generated at the option of the
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