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equipped aircraft. Depending on the location of the
ground based transmitters (GBT), ADS-B has the
potential to work well at low altitudes, in remote locations, and mountainous terrain where little or no radar
coverage exists.
1-20
COMMUNICATIONS
Most air traffic control communications between pilots
and controllers today are conducted via voice. Each air
traffic controller uses a radio frequency different from the
ones used by surrounding controllers to communicate
with the aircraft under his or her jurisdiction. With the
increased traffic, more and more controllers have been
added to maintain safe separation between aircraft. While
this has not diminished safety, there is a limit to the
number of control sectors created in any given region
to handle the traffic. The availability of radio frequencies
for controller-pilot communications is one limiting factor.
Some busy portions of the U.S., such as the Boston-
Chicago-Washington triangle are reaching toward the
limit. Frequencies are congested and new frequencies are
not available, which limits traffic growth to those aircraft
that can be safely handled.
DATA LINK
The CAASD is working with the FAA and the airlines
to define and test a controller-pilot data link communication (CPDLC), which provides the capability to
exchange information between air traffic controllers
and flight crews through digital text instead of voice
messages. With CPDLC, communications between the
ground and the air would take less time, and would
convey more information (and more complex information) than by voice alone. Communications would
become more accurate as up-linked information would
be collected, its accuracy established, and then displayed for the pilot in a consistent fashion.
By using digital data messages to replace conventional
voice communications (except during landing and departure phases and in emergencies) CPDLC is forecast to
increase airspace capacity and reduce delays. Today the
average pilot/controller voice exchange takes around 20
seconds, compared to one or two seconds with CPDLC.
In FAA simulations, air traffic controllers indicated that
CPDLC could increase their productivity by 40 percent
without increasing workload. Airline cost/benefit studies
indicate average annual savings that are significant in the
terminal and en route phases, due to CPDLC-related
delay reductions.
CPDLC for routine ATC messages, initially offered in
Miami Center, will be implemented via satellite at all
oceanic sectors. Communications between aircraft and
FAA oceanic facilities will be available through satellite
data link, high frequency data link (HFDL), or other
subnetworks, with voice via HF and satellite communications remaining as backup. Eventually, the service will
be expanded to include clearances for altitude, speed,
heading, and route, with pilot initiated downlink capability added later.
MODE S
The first comprehensive proposal and design for the
Mode S system was delivered to the FAA in 1975.
However, due to design and manufacturing setbacks,
few Mode S ground sensors and no commercial Mode S
transponders were made available before 1980. Then, a
tragic mid-air collision over California in 1986
prompted a dramatic change. The accident that claimed
the lives of 67 passengers aboard the two planes and
fifteen people on the ground was blamed on inadequate
automatic conflict alert systems and surveillance
equipment. A law enacted by Congress in 1987
required all air carrier airplanes operating within U.S.
airspace with more than 30 passenger seats to be
equipped with Traffic Alert and Collision Avoidance
System (TCAS II) by December 1993. Airplanes with
10 to 30 seats were required to employ TCAS I by
December 1995.
Due to the congressional mandate, TCAS II became a
pervasive system for air traffic control centers around
the world. Because TCAS II uses Mode S as the standard air-ground communication datalink, the widespread international use of TCAS II has helped Mode S
become an integral part of air traffic control systems
all over the world. The datalink capacity of Mode S has
spawned the development of a number of different
services that take advantage of the two-way link
between air and ground. By relying on the Mode S
datalink, these services can be inexpensively deployed
to serve both the commercial transport aircraft and
general aviation communities. Using Mode S makes
not only TCAS II, but also other services available to
the general aviation community that were previously
accessible only to commercial aircraft. These Mode
　
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