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feedback reduces the common problem of hearing what
is expected to be heard, which is particularly problematic in ATC clearances and read backs. Not only does
reducing voice communications reduce frequency
congestion, it also eliminates certain opportunities for
misunderstanding.
Controller pilot data link communication (CPDLC) augments voice communications by providing a second
communication channel for use by the pilot and controller, using data messages that are displayed in the
cockpit. This reduces delays resulting from congestion
on voice channels. The initial version of CPDLC will
display a limited number of air traffic messages, but
future versions will have expanded message capabilities
and permit pilot-initiated requests.
Point A
Point B
Figure 6-8. RNP allows parallel tracks along the same route, multiplying capacity along that route.
6-9
AIRCRAFT COMMUNICATIONS
ADDRESSING AND REPORTING SYSTEM
Of course, pilot-controller communication is compromised when the crew is listening to other frequencies or
engaged in other communications, such as talking to
their company. If these communications could be
accomplished silently and digitally, voice communications with ATC would improve. The Aircraft
Communications Addressing and Reporting System
(ACARS) is a commercial system that enables the crew
to communicate with company personnel on the ground.
It is often used to exchange routine flight status messages, weather information, and can serve as a non-voice
communication channel in the event of an emergency.
Many of the messages are sent and received automatically, such as the time the flight leaves the gate
(triggered by the release of the parking brake), takeoff and touchdown times (triggered by landing gear
switches), and arrival time (triggered when a cabin
door is opened). Other information may include
flight plans, significant meteorological information
(SIGMETs), crew lists, cargo manifests, automatic
terminal information service (ATIS) reports, en
route and destination weather, clearances, and fuel
reports. Some ACARS units can interface with
onboard engine and performance-monitoring systems
to inform company ground personnel of maintenance
or operations related issues. [Figure 6-9]
Significant valuable meteorological data can be obtained
by collecting data from aircraft fitted with appropriate
software packages. To date, the predominant sources of
automated aviation data have been from aircraft
equipped with aircraft to satellite data relay (ASDAR)
and ACARS, which routes data back via general purpose
information processing and transmitting systems now
fitted to many commercial aircraft. These systems offer
the potential for a vast increase in the provision of aircraft observations of wind and temperature. Making an
increasingly important contribution to the observational
database, it is envisioned that ACARS data will
inevitably supersede manual pilot reports (PIREPS).
Another use of ACARS is in conjunction with Digital
ATIS (D-ATIS), which provides an automated process
for the assembly and transmission of ATIS messages.
ACARS enables audio messages to be displayed in text
form in the flight decks of aircraft equipped with
ACARS. A printout is also provided if the aircraft is
equipped with an on-board printer. D-ATIS is operational at over 57 airports that now have pre-departure
clearance (PDC) capability.
AUTOMATIC DEPENDENT
SURVEILLANCE-BROADCAST
Unlike TCAS and terrain awareness and warning systems
(TAWS), which have been used in airline and military air-
craft for at least a decade, ADS-B is a relatively new air
traffic technology. It is an onboard system that uses Mode
S transponder technology to periodically broadcast an aircraft’s position, along with some supporting information
like aircraft identification and short-term intent. By picking up broadcast position information on the ground
instead of using ground radar stations, ADS-B represents
a significant advancement over the existing ATC system
by providing increased accuracy and safety. This is possible because ADS-B addresses the major deficiency of
TCAS - accuracy. In the TCAS system, aircraft positions
are only accurate to a few degrees; thus, the accuracy of
TCAS decreases with distance. Moreover, the reliance on
transmission timing for range data in TCAS is errorprone. The method used by ADS-B avoids this problem.
In addition to the broadcast of position to the ground,
ADS-B can be used to enable a new collection of aircraft-based applications. Unlike conventional radar,
ADS-B works at low altitudes and on the ground. It is
　
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