Goto Section: 87.139 | 87.143 | Table of Contents
Revised as of September 1, 2021
Goto Year:2020 |
§ 87.141 Modulation requirements.
(a) When A3E emission is used, the modulation percentage must not
exceed 100 percent. This requirement does not apply to emergency
locator transmitters or survival craft transmitters.
(b) A double sideband full carrier amplitude modulated radiotelephone
transmitter with rated carrier power output exceeding 10 watts must be
capable of automatically preventing modulation in excess of 100
(c) If any licensed radiotelephone transmitter causes harmful
interference to any authorized radio service because of excessive
modulation, the Commission will require the use of the transmitter to
be discontinued until it is rendered capable of automatically
preventing modulation in excess of 100 percent.
(d) Single sideband transmitters must be able to operate in the
Carrier mode Level N(dB) of the carrier with respect to peak envelope
Full carrier (H3E) O>N>−6.
Suppressed carrier (J3E) Aircraft stations N<−26;
Aeronautical stations N<−40.
(e) Each frequency modulated transmitter operating in the band
72.0-76.0 MHz must have a modulation limiter.
(f) Each frequency modulated transmitter equipped with a modulation
limiter must have a low pass filter between the modulation limiter and
the modulated stage. At audio frequencies between 3 kHz and 15 kHz, the
filter must have an attenuation greater than the attenuation at 1 kHz
by at least 40 log10 (f/3) db where “f” is the frequency in kilohertz.
Above 15 kHz, the attenuation must be at least 28 db greater than the
attenuation at 1 kHz.
(g) Except that symmetric side bands are not required, the modulation
characteristics for ELTs must be in accordance with specifications
contained in the Federal Aviation Administration (FAA) Technical
Standard Order (TSO) Document TSO-C91a titled “Emergency Locator
Transmitter (ELT) Equipment” dated April 29, 1985. TSO-C91a is
incorporated by reference in accordance with 5 U.S.C. 552(a). TSO-C91a
may be obtained from the Department of Transportation, Federal Aviation
Administration, Office of Airworthiness, 800 Independence Avenue SW.,
Washington DC 20591.
(h) ELTs must use A3X emission and may use A3E or NON emissions on an
optional basis while transmitting. Each transmission of a synthesized
or recorded voice message from an ELT must be preceded by the words
“this is a recording”; transmission of A3E or NON emission must not
exceed 90 seconds; and any transmission of A3E or NON emissions must be
followed by at least three minutes of A3X emission.
(i) ELTs manufactured on or after October 1, 1988, must have a clearly
defined carrier frequency distinct from the modulation sidebands for
the mandatory emission, A3X, and, if used, the A3E or NON emissions. On
121.500 MHz at least thirty per cent of the total power emitted during
any transmission cycle with or without modulation must be contained
within plus or minus 30 Hz of the carrier frequency. On 243.000 MHz at
least thirty percent of the total power emitted during any transmission
cycle with or without modulation must be contained within plus or minus
60 Hz of the carrier frequency. Additionally, if the type of emission
is changed during transmission, the carrier frequency must not shift
more than plus or minus 30 Hz on 121.500 MHz and not more than plus or
minus 60Hz on 243.000 MHz. The long term stability of the carrier
frequency must comply with the requirements in § 87.133 of this part.
(j) Transmitters used at Aircraft earth stations must employ BPSK for
transmission rates up to and including 2400 bits per second, and QPSK
for higher rates.
(k) Universal Access Transceiver transmitters must use F1D modulation
without phase discontinuities.
[ 53 FR 28940 , Aug. 1, 1988, as amended at 54 FR 11721 , Mar. 22, 1989;
56 FR 11518 , Mar. 19, 1991; 57 FR 45749 , Oct. 5, 1992; 71 FR 70676 ,
Dec. 6, 2006]
Goto Section: 87.139 | 87.143
Goto Year: 2020 |
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