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address at:
National Aeronautical Charting Group
Telephone 800-626-3677
http://naco.faa.gov/
9-1
The Air Traffic
Control System
Introduction
This chapter covers the communication equipment,
communication procedures, and air traffic control (ATC)
facilities and services available for a flight under instrument
flight rules (IFR) in the National Airspace System (NAS).
Chapter 9
9-2
Figure 9-2. Audio Panel.
Figure 9-1. Typical NAV/COM Installation.
Communication Equipment
Navigation/Communication (NAV/COM)
Equipment
Civilian pilots communicate with ATC on frequencies in
the very high frequency (VHF) range between 118.000 and
136.975 MHz. To derive full benefit from the ATC system,
radios capable of 25 kHz spacing are required (e.g., 134.500,
134.575, 134.600). If ATC assigns a frequency that cannot
be selected, ask for an alternative frequency.
Figure 9-1 illustrates a typical radio panel installation,
consisting of a communications transceiver on the left and a
navigational receiver on the right. Many radios allow the pilot
to have one or more frequencies stored in memory and one
frequency active for transmitting and receiving (called simplex
operation). It is possible to communicate with some automated
flight service stations (AFSS) by transmitting on 122.1 MHz
(selected on the communication radio) and receiving on a
VHF omnidirectional range (VOR) frequency (selected on
the navigation radio). This is called duplex operation.
An audio panel allows a pilot to adjust the volume of the
selected receiver(s) and to select the desired transmitter.
[Figure 9-2] The audio panel has two positions for receiver
selection, cabin speaker, and headphone (some units might
have a center “off” position). Use of a hand-held microphone
and the cabin speaker introduces the distraction of reaching
for and hanging up the microphone. A headset with a boom
microphone is recommended for clear communications. The
microphone should be positioned close to the lips to reduce
9-3
Figure 9-3. Boom Microphone, Headset, and Push-To-Talk
Switch.
Figure 9-4. Combination GPS-Com Unit.
the possibility of ambient flight deck noise interfering with
transmissions to the controller. Headphones deliver the
received signal directly to the ears; therefore, ambient noise
does not interfere with the pilot’s ability to understand the
transmission. [Figure 9-3]
Switching the transmitter selector between COM1 and
COM2 changes both transmitter and receiver frequencies.
It is necessary only when a pilot wants to monitor one
frequency while transmitting on another. One example is
listening to automatic terminal information service (ATIS)
on one receiver while communicating with ATC on the
other. Monitoring a navigation receiver to check for proper
identification is another reason to use the switch panel.
Most audio switch panels also include a marker beacon
receiver. All marker beacons transmit on 75 MHz, so there
is no frequency selector.
Figure 9-4 illustrates an increasingly popular form of
NAV/COM radio; it contains a global positioning system
(GPS) receiver and a communications transceiver. Using its
navigational capability, this unit can determine when a flight
crosses an airspace boundary or fix and can automatically
select the appropriate communications frequency for that
location in the communications radio.
Radar and Transponders
ATC radars have a limited ability to display primary returns,
which is energy reflected from an aircraft’s metallic structure.
Their ability to display secondary returns (transponder replies
to ground interrogation signals) makes possible the many
advantages of automation.
A transponder is a radar beacon transmitter/receiver installed
in the instrument panel. ATC beacon transmitters send out
interrogation signals continuously as the radar antenna
rotates. When an interrogation is received by a transponder, a
coded reply is sent to the ground station where it is displayed
on the controller’s scope. A reply light on the transponder
panel flickers every time it receives and replies to a radar
interrogation. Transponder codes are assigned by ATC.
When a controller asks a pilot to “ident” and the ident button
is pushed, the return on the controller’s scope is intensified for
precise identification of a flight. When requested, briefly push
the ident button to activate this feature. It is good practice
for pilots to verbally confirm that they have changed codes
or pushed the ident button.
　
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