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NAVIGATION SYSTEMS
Navigation systems are the basis for pilots to get from
one place to another and know where they are and what
course to follow. Since the 1930s, aircraft have navigated by means of a set of ground-based NAVAIDs.
Today, pilots have access to over 2,000 such NAVAIDs
within the continental U.S., but the system has its
limitations:
• Constrained to fly from one NAVAID to the
next, aircraft route planners need to identify a
beacon-based path that closely resembles the
path the aircraft needs to take to get from origin
to destination. Such a path will always be
greater in distance than a great circle route
between the two points.
• Because the NAVAIDs are ground-based, navigation across the ocean is problematic, as is
navigation in some mountainous regions.
• NAVAIDs are also expensive to maintain.
Since the 1980s, aircraft systems have evolved towards
the use of SATNAV. Based on the GPS satellite constellation, SATNAV may provide better position information
than a ground-based navigation system. GPS is universal
so there are no areas without satellite signals. Moreover, a
space-based system allows “off airway” navigation so that
the efficiencies in aircraft route determination can be
exacted. SATNAV is revolutionizing navigation for airlines and other aircraft owners and operators. A drawback
of the satellite system, though, is the integrity and availability of the signal, especially during electromagnetic
and other events that distort the Earth’s atmosphere. In
addition, the signal from space needs to be augmented,
especially in traffic-dense terminal areas, to guarantee the
necessary levels of accuracy and availability.
The CAASD is helping the navigation system of the
U.S. to evolve toward a satellite-based system. The
CAASD analysts are providing the modeling necessary
to understand the effects of atmospheric phenomena on
the GPS signal from space, while the CAASD is providing the architecture of the future navigation system and
writing the requirements (and computer algorithms) to
ensure the navigation system’s integrity. Moving toward
a satellite-based navigation system allows aircraft to
divorce themselves from the constraints of ground-based
NAVAIDs and formulate and fly those routes that aircraft route planners deem most in line with their own
cost objectives.
With the advent of SATNAV, there are a number of
applications that can be piggybacked to increase capacity in the NAS. Enhanced navigation systems will be
capable of “random navigation,” that is, capable of
Figure 1-15. FMS Control Display Unit. This depicts an aircraft
established on the Atlantic City, NJ, RNAV (GPS) Rwy 13
instrument approach procedure at the Atlantic City
International Airport, KACY. The aircraft is positioned at the
intermediate fix UNAYY inbound on the 128 degree magnetic
course, 5.5 nautical miles from PVIGY, the final approach fix.
treating any latitude-longitude point as a radio navigation fix, and being able to fly toward it with the
accuracy we see today, or better. New routes into and
out of the terminal areas are being implemented that
are navigable by on-board systems. Properly
equipped aircraft are being segregated from other aircraft streams with the potential to increase volume at
the nation’s busy airports by keeping the arrival and
departure queues full and fully operating.
The CAASD is working with the FAA to define the
nation’s future navigation system architecture. By itself,
the GPS satellite constellation is inadequate to serve all
the system’s needs. Augmentation of the GPS signal via
WAAS and LAAS is a necessary part of that new architecture. The CAASD is developing the requirements
based on the results of sophisticated models to ensure the
system’s integrity, security, and availability.
SURVEILLANCE SYSTEMS
Surveillance systems are set up to enable the ATC system to know the location of an aircraft and where it is
heading. Position information from the surveillance
system supports many different ATC functions. Aircraft
positions are displayed for controllers as they watch
over the traffic to ensure that aircraft do not violate separation criteria. In the current NAS, surveillance is
achieved through the use of long-range and terminal
radars. Scanning the skies, these radars return azimuth
and slant range for each aircraft that, when combined
with the altitude of the aircraft broadcast to the ground
via a transceiver, is transformed mathematically into a
position. The system maintains a list of these positions
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Instrument Procedures Handbook (IPH)仪表程序手册下(27)
