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effective in remote areas or in mountainous terrain
where there is no radar coverage, or where radar coverage is limited. One of the greatest benefits of ADS-B is
Figure 6-9. ACARS Communications Display.
6-10
its ability to provide the same real-time information to
pilots in the aircraft cockpit and to ground controllers,
so that for the first time, both can view the same data.
ADS-B will also enable aircraft to send messages to
each other to provide surveillance and collision avoidance through data link. Other aircraft in the immediate
vicinity can pick up position information broadcasts
from equipped aircraft. This enables equipped aircraft to
formulate a display of nearby aircraft for the pilot; the
pilot’s awareness of the current situation is enhanced.
Combined with databases of current maps and charts,
the onboard displays can show terrain as well as proximate aircraft. This is a powerful inducement for change.
The heightened situational awareness offered by satellite navigation in conjunction with modern database
applications and map displays, combined with the position of proximate aircraft, builds a picture in the cockpit
equivalent to that on the ground used by the controller.
This is particularly important in places like Alaska
where aviation is vital, NAS infrastructure is minimal
(because of the harsh conditions), and weather changes
quickly and in unpredictable fashions.
Eventually, as the fleets equip, it may be possible to save
money by retiring expensive long-range radars.
Identified by the FAA as the future model for ATC,
ADS-B is a major step in the direction of free flight.
While ADS-B shows great promise for both air-to-air
and air-to-ground surveillance, current aircraft transponders will continue to support surveillance operations in
the NAS for the foreseeable future. If enough users
equip with ADS-B avionics, the FAA will install a
compatible ADS ground system to provide more accurate surveillance information to ATC compared to
radar-based surveillance.
In the United States, two different data links have been
adopted for use with ADS-B: 1090 MHz Extended
Squitter (1090 ES) and the Universal Access Transceiver
(UAT). The 1090 ES link is intended for aircraft that primarily operate at FL180 and above, whereas the UAT
link is intended for use by aircraft that primarily operate
at 18,000 feet and below. From a pilot's standpoint, the
two links operate similarly and both support ADS-B and
TIS-B. The UAT link additionally supports Flight
Information Service-Broadcast (FIS-B) at any altitude
when within ground based transmitter (GBT) coverage.
FIS-B is the weather information component, and
provides displays of graphical and textual weather
information. Areas of approved use for the UAT
include the United States (including oceanic airspace
where air traffic services are provided), Guam, Puerto
Rico, American Samoa, and the U.S. Virgin Islands.
The UAT is approved for both air and airport surface
use. ADS-B broadcast over the 1090 MHz data link
has been approved for global use.
REDUCING VERTICAL SEPARATION
Current vertical separation minima (2,000 feet) were
created more than 40 years ago when altimeters were
not very accurate above FL 290. With better flight and
navigation instruments, vertical separation has been
safely reduced to 1,000 feet in most parts of the world,
except Africa and China.
RVSM airspace has already been implemented over the
Atlantic and Pacific Oceans, South China Sea, Australia,
Europe, the Middle East and Asia south of the
Himalayas. Domestic RVSM (DRVSM) in the United
States was implemented in January 2005 when FL 300,
320, 340, 360, 380, and 400 were added to the existing
structure. To fly at any of the flight levels from FL 290
to FL 410, aircraft and operator must be RVSMapproved. [Figure 6-10]
REDUCING HORIZONTAL SEPARATION
The current oceanic air traffic control system uses filed
flight plans and position reports to track an aircraft’s
progress and ensure separation. Pilots send position
reports by high frequency (HF) radio through a private
radio service that then relays the messages to the air traffic control system. Position reports are made at intervals
of approximately one hour. HF radio communication is
subject to interference and disruption. Further delay is
added as radio operators relay messages between pilots
and controllers. These deficiencies in communications
and surveillance have necessitated larger horizontal separation minimums when flying over the ocean out of
radar range.
As a result of improved navigational capabilities made
　
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