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3.5.5.1.2 Frequency stability. The radio frequency of operation shall not vary more than plus or minus 100 kHz from
the assigned value.
3.5.5.1.3 Pulse shape and spectrum. The following shall apply to all radiated pulses:
a) Pulse rise time.
1) DME/N. Pulse rise time shall not exceed 3 microseconds.
2) DME/P. Pulse rise time shall not exceed 1.6 microseconds. For the FA mode, the pulse shall have a partial rise
time of 0.25 plus or minus 0.05 microsecond. With respect to the FA mode and accuracy standard 1, the slope
of the pulse in the partial rise time shall not vary by more than plus or minus 20 per cent. For accuracy standard
2 the slope shall not vary by more than plus or minus 10 per cent.
3) DME/P. Recommendation.— Pulse rise time should not exceed 1.2 microseconds.
b) Pulse duration shall be 3.5 microseconds plus or minus 0.5 microsecond.
c) Pulse decay time shall nominally be 2.5 microseconds, but shall not exceed 3.5 microseconds.
d) The instantaneous amplitude of the pulse shall not, at any instant between the point of the leading edge which is
95 per cent of maximum amplitude and the point of the trailing edge which is 95 per cent of the maximum
amplitude, fall below a value which is 95 per cent of the maximum voltage amplitude of the pulse.
e) The spectrum of the pulse modulated signal shall be such that at least 90 per cent of the energy in each pulse shall be
within 0.5 MHz in a band centred on the nominal channel frequency.
f) To ensure proper operation of the thresholding techniques, the instantaneous magnitude of any pulse turn-on
transients which occur in time prior to the virtual origin shall be less than one per cent of the pulse peak amplitude.
Initiation of the turn-on process shall not commence sooner than 1 microsecond prior to the virtual origin.
Note 1.— The lower limit of pulse rise time (see 3.5.5.1.3 a)) and decay time (see 3.5.5.1.3 c)) are governed by the
spectrum requirements in 3.5.5.1.3 e).
Annex 10 — Aeronautical Communications Volume I
23/11/06 3-54
Note 2.— While 3.5.5.1.3 e) calls for a practically attainable spectrum, it is desirable to strive for the following spectrum
control characteristics: the spectrum of the pulse modulated signal is such that the power contained in a 0.5 MHz band
centred on frequencies 0.8 MHz above and 0.8 MHz below the nominal channel frequency is, in each case, at least 23 dB
below the power contained in a 0.5 MHz band centred on the nominal channel frequency. The power contained in a 0.5 MHz
band centred on frequencies 2 MHz above and 2 MHz below the nominal channel frequency is, in each case, at least 38 dB
below the power contained in a 0.5 MHz band centred on the nominal channel frequency. Any additional lobe of the
spectrum is of less amplitude than the adjacent lobe nearer the nominal channel frequency.
3.5.5.1.4 Pulse spacing
3.5.5.1.4.1 The spacing of the constituent pulses of transmitted pulse pairs shall be as given in the table in 3.5.4.4.1.
3.5.5.1.4.2 DME/N. The tolerance on the pulse spacing shall be plus or minus 0.5 microsecond.
3.5.5.1.4.3 DME/N. Recommendation.— The tolerance on the pulse spacing should be plus or minus 0.25 microsecond.
3.5.5.1.4.4 DME/P. The tolerance on the pulse spacing shall be plus or minus 0.25 microsecond.
3.5.5.1.4.5 The pulse spacing shall be measured between the half voltage points on the leading edges of the pulses.
3.5.5.1.5 Pulse repetition frequency
3.5.5.1.5.1 The pulse repetition frequency shall be as specified in 3.5.3.4.
3.5.5.1.5.2 The variation in time between successive pairs of interrogation pulses shall be sufficient to prevent false
lock-on.
3.5.5.1.5.3 DME/P. In order to achieve the system accuracy specified in 3.5.3.1.3.4, the variation in time between
successive pairs of interrogation pulses shall be sufficiently random to decorrelate high frequency multipath errors.
Note.— Guidance on DME/P multipath effects is given in Attachment C, 7.3.7.
3.5.5.1.6 Spurious radiation. During intervals between transmission of individual pulses, the spurious pulse power
received and measured in a receiver having the same characteristics of a DME transponder receiver, but tuned to any DME
interrogation or reply frequency, shall be more than 50 dB below the peak pulse power received and measured in the same
receiver tuned to the interrogation frequency in use during the transmission of the required pulses. This provision shall apply
to all spurious pulse transmissions. The spurious CW power radiated from the interrogator on any DME interrogation or reply
frequency shall not exceed 20 microwatts (minus 47 dBW).
Note.— Although spurious CW radiation between pulses is limited to levels not exceeding minus 47 dBW, States are
cautioned that where DME interrogators and secondary surveillance radar transponders are employed in the same aircraft,
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