Sec. 22.865  Automatic channel selection procedures.

    Operation of stations using the channels listed in Sec. 22.857 must 
be in accordance with the procedures in this section.
    (a) A communications channel is not available for use by a ground 
station if it is already in use by another ground station at the same 
location. Ground station equipment must automatically determine whether 
channels are in use by other ground stations at the same location, and 
may employ radio frequency signal monitoring to do so. For example, a 
communications channel may be determined to be in use if the received 
signal power on that channel at the ground station exceeds -115 dBm, 
which, assuming a 0 dB gain 895 MHz receive antenna, corresponds to a 
field strength of approximately 19 dBV/m. Ground stations may 
employ an alternative method of determining whether a communications 
channel is in use provided that such procedure is at least as reliable 
as radio frequency signal monitoring.
    (b) Data indicating which communications channels are available for 
use are transmitted by ground stations on the assigned control channels.
    (c) A call is originated when an airborne mobile station selects a 
communications channel based on the received data from ground stations 
and other factors, and transmits an identification code (which 
identifies the specific ground station from which service is requested) 
on the selected communications channel. The ground station from which 
service has been requested may then obtain any necessary billing 
information and complete the call.
    (d) A ground station may not transmit on a communications channel 
unless it has received the proper identification code. After a ground 
station has begun to transmit on a communications channel, that channel 
is not available to ground stations other than the one from which 
service has been requested until the call is terminated.
    (e) A call is terminated by the ground station when either a hang-up 
signal is transmitted by the airborne mobile station, or the signal from 
the airborne mobile station on the communications channel is lost for a 
period of 15 continuous seconds. The hang-up signal is the on-off keying 
(50% duty cycle) of an unmodulated carrier over a period of one second 
with pulse duration of 5 milliseconds. However, if all carriers 
authorized to operate air-ground systems using the channels listed in 
Sec. 22.857 agree that an alternative hang-up signal and/or procedure 
would be more efficient or beneficial, such alternative hang-up signal 
and/or procedure may be used. The carriers must jointly give prior 
notification to the FCC if an alternative hang-up signal and/or 
procedure is used.