§ 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:
   Carrier mode Level N(dB) of the carrier with 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 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]

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