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 log[10]
   (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  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]