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The radar service provided by a tower controller is not,
nor was it ever intended to be, the same thing as radar
service provided by an approach control or Center. The
primary duty of tower controllers is to separate airplanes
operating on runways, which means controllers spend
most of their time looking out the window, not staring at
a radar scope.
RADAR COVERAGE
A full approach is a staple of instrument flying, yet
some pilots rarely, if ever, have to fly one other than
during initial or recurrency or proficiency training,
because a full approach usually is required only when
radar service is not available, and radar is available at
most larger and busier instrument airports. Pilots come
to expect radar vectors to final approach courses and
that ATC will keep an electronic eye on them all the
way to a successful conclusion of every approach. In
addition, most en route flights are tracked by radar
along their entire route in the 48 contiguous states,
with essentially total radar coverage of all instrument
flight routes except in the mountainous West. Lack of
radar coverage may be due to terrain, cost, or physical
limitations.
New developing technologies, like ADS-B, may offer
ATC a method of accurately tracking aircraft in nonradar environments. ADS-B is a satellite-based air
traffic tracking system enabling pilots and air traffic
controllers to share and display the same information.
ADS-B relies on the Global Positioning System (GPS)
to determine an aircraft’s position. The aircraft’s precise location, along with other data such as airspeed,
altitude, and aircraft identification, then is instantly
relayed via digital datalink to ground stations and other
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.
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Instrument Procedures Handbook (IPH)仪表程序手册下(24)
