Sec. 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 percent.
    (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 
following modes:

------------------------------------------------------------------------
                                        Level N(dB) of the carrier with 
             Carrier mode                respect to peak envelope power 
------------------------------------------------------------------------
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

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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 
Sec. 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.
[ 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]