Home' RTCA Documents for Review : DO-204B, MOPS Aircraft ELT 406MHZ Contents © EUROCAE, 2018
The type of emission shall be A3A, as defined in the ITU Radio Regulations, and shall
have one of the following characteristics:
carrier keyed in such a manner that full power is applied to the antenna for one
period, followed by reduced or zero power for a further period.
The ON/OFF cycle shall be optimized to allow the precise location of the beacon in all
weather conditions and to ensure compatibility with all VHF homing equipment
currently in service.
If the carrier is keyed to provide an ON/OFF cycle then the minimum duty cycle shall
be 33% and shall meet the following criteria.
For a 33% Duty Cycle the transmitter shall be on for a period of not less than
0.75 seconds and then off for not more than 1.5 seconds repeating.
For duty cycles between 33% and 100% the on time shall be increased beyond
0.75 seconds and the off time reduced accordingly. The carrier signal may be
further interrupted to permit the transmission of 406 MHz satellite transmissions
and/or 406 MHz homing transmissions as defined by Cospas-Sarsat or Morse
code transmissions as defined in § 18.104.22.168.
The homing signal shall not be interrupted or suppressed in the presence of signals
from other 406 MHz beacons nor 121.5 MHz beacons.
22.214.171.124 Sideband Components
The ELT shall have clearly defined sideband components which are symmetric about
the output signal spectrum and distinct from the carrier component at both the 121.5
MHz and 243.0 MHz frequencies.
The ELT spectrum at 121.5 MHz shall have at least 30% of its energy distribution
within a bandwidth of plus and minus 30 Hz about a fixed reference frequency
corresponding to the carrier component, carrier frequency (fc), over the audio/sweep
At 243 MHz, 30% of the energy distribution shall fall within a bandwidth of fc +/-60 Hz.
126.96.36.199 Continuous Wave
To aid SAR system detection and homing capabilities, the transmission may also
provide a period of unmodulated Continuous Wave (CW) power for a duration of 2.0
(+/-0.25) seconds, repeated every 8.0 (+/-0.8) seconds.
188.8.131.52 Voice Modulation
Voice modulation, (ITU-A3E), is permitted provided that its use shall not prejudice the
primary function of the beacon and power shall not be consumed at a greater rate
than in the beacon mode.
Such an ELT unit shall also comply with the following specifications:
the unit shall be clearly marked to caution operators to use the speech facility
only when the rescue craft is seen or heard.
where a speech facility is provided on 121.5/243 MHz the Transmitter/Receiver
key shall revert to beacon operation when not actually being keyed for a period
of more than 20 seconds.
where a second auxiliary speech channel is provided, it must operate on 123.1
MHz, (scene of the search frequency) and shall provide the same reversion to
Beacon mode on 121.5/243 MHz as in b. above.
d. the operating instructions must warn the user that when the speech facility is
used, the homing capability is severely restricted or non-existent.
184.108.40.206 Identification Signal
An identification signal is permitted using the International Morse Code. If this option is
the identification signal shall consist of 5 or 6 letters, (the aircraft registration), at
a repetition rate of once per minute;
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