Automatic dependent surveillance – broadcast

Automatic dependent surveillance—broadcast (ADS–

B) is a surveillance technology in which an aircraft determines
its position via satellite navigation and periodically broadcasts
it, enabling it to be tracked. The information can be received
by air traffic control ground stations as a replacement for
secondary surveillance radar, as no interrogation signal is
needed from the ground. It can also be received by other
aircraft to provide situational awareness and allow self-
separation.

ADS–B is "automatic" in that it requires no pilot or

external input. It is "dependent" in that it depends on data
from the aircraft's navigation system.

Description

ADS-B, which consists of two different services, "ADS-

B Out" and "ADS-B In", could replace radar as the primary
surveillance method for controlling aircraft worldwide. In the United States, ADS-B is an integral
component of the NextGen national airspace strategy for upgrading and enhancing aviation
infrastructure and operations.

Using "ADS-B Out" each aircraft periodically broadcasts information about itself, such as
identification, current position, altitude, and velocity, through an onboard transmitter. ADS-B Out
provides air traffic controllers with real-time position information that is, in most cases, more accurate
than the information available with current radar-based systems. With more accurate information, ATC
will be able to position and separate aircraft with improved precision and timing.

"ADS-B In" is the reception by aircraft of FIS-B and TIS-B data and other ADS-B data such as
direct communication from nearby aircraft. The ground station broadcast data is typically only made
available in the presence of an ADS-B Out broadcasting aircraft, limiting the usefulness of purely ADS-B
In devices.

Benefits

Traffic — When using an ADS-B In system, a pilot is able to view traffic information about surrounding
aircraft if those aircraft are equipped with ADS-B out. This information includes altitude, heading, speed,
and distance to aircraft. In addition to receiving position reports from ADS-B out participants, TIS-B
[USA-only] can provide position reports on non ADS-B out-equipped aircraft if suitable ground
equipment and ground radar exist. ADS-R re-transmits ADS-B position reports between UAT and 1090
MHz frequency bands.
Weather — Aircraft equipped with universal access transceiver (UAT) ADS-B In technology will be able to
receive weather reports, and weather radar through flight information service-broadcast (FIS-B). [USA-
only]

Flight information — Flight information service-broadcast (FIS-B) also transmits readable flight
information such as temporary flight restrictions (TFRs) and NOTAMs to aircraft equipped with UAT.
[USA-only]

Expense — ADS-B ground stations are significantly cheaper to install and operate compared to primary
and secondary radar systems used by ATC for aircraft separation and control.

Safety

Situational awareness

ADS-B makes flying significantly safer for the aviation community by providing pilots with
improved situational awareness. Pilots in an ADS-B In equipped cockpit will have the ability to see, on
their in-cockpit flight display, other traffic operating in the airspace as well as access to clear and
detailed weather information. They will also be able to receive pertinent updates ranging from
temporary flight restrictions to runway closings.

Improved visibility

Even aircraft only equipped with ADS-B Out will benefit from air traffic controllers' ability to
more accurately and reliably monitor their position. When using this system both pilots and controllers
will see the same radar picture.

ADS-B enables improved safety by providing:

 Radar-like IFR separation in non-radar airspace

 Increased VFR flight following coverage
 ATC final approach and runway occupancy, reducing runway incursions on the ground
 More accurate search and rescue response — although ADS-B can transmit "aircraft down" data, the
FAA has stated that there is no intention to perform even a study of ADS-B's effectiveness in an
"aircraft down" situation, simply based on the fact that ADS-B equipment has no requirement to be
crashworthy, as compared to the current "black box" recorder. ADS-B was demonstrated to the Civil
Air Patrol (CAP) in March 2003 by AOPA via flight demonstrations for possible integration of the
technology in CAP activities.
 Helps pilots to see and avoid other aircraft
 Cockpit final approach and runway occupancy
 Visual separation in VFR and MVFR conditions
 VFR-like separation in all weather conditions
 Real-time cockpit weather display
 Real-time cockpit airspace display
Efficiency

Reduced environmental impact

ADS-B technology provides a more accurate report of an aircraft's position.[18] This allows
controllers to guide aircraft into and out of crowded airspace with smaller separation standards than it
was previously possible to do safely. This reduces the amount of time aircraft must spend waiting for
clearances, being vectored for spacing and holding. Estimates show that this is already having a
beneficial impact by reducing pollution and fuel consumption.[19]

Traffic capacity improvement

ADS-B enables increased capacity and efficiency by supporting:

 Better ATC traffic flow management

 Merging and spacing
 Self-separation or station keeping
 Enhanced visual approaches;
 Closely spaced parallel approaches;
 Reduced spacing on final approach;
 Reduced aircraft separations;
 Enhanced operations in high altitude airspace for the incremental evolution of the "free flight"
concept;
 Surface operations in lower visibility conditions;
 Near visual meteorological conditions (VMC) capacities throughout the airspace in most weather
conditions;
 Improved air traffic control services in non-radar airspace;
 Trajectory-based operations providing a gently ascending and descending gradient with no step-
downs or holding patterns needed. This will produce optimal trajectories with each aircraft
becoming one node within a system wide information management network connecting all
equipped parties in the air and on the ground. With all parties equipped with NextGen equipage,
benefits will include reduced gate-to-gate travel times, increased runway utilization capacity, and
increased efficiency with carbon conservation.
 Use of ADS-B and CDTI may allow decreased approach spacing at certain airports to improve
capacity during reduced-visibility operations when visual approach operations would normally be
terminated (e.g., ceilings less than MVA +500).

Theory of operation

The ADS-B system has three main components: 1) ground infrastructure, 2) airborne component, and 3)
operating procedures.

1. A transmitting subsystem that includes message generation and transmission functions at the
source; e.g., aircraft.
2. The transport protocol; e.g., VHF (VDL mode 2 or 4), 1090ES, or 978 MHz UAT.
3. A receiving subsystem that includes message reception and report assembly functions at the
receiving destination; e.g., other aircraft, vehicle or ground system.
Traffic collision avoidance system
A traffic collision avoidance system or traffic alert and collision avoidance system (both abbreviated as
TCAS, and pronounced /tiːkæs/; TEE-kas) is an aircraft collision avoidance system designed to reduce the
incidence of mid-air collisions between aircraft. It monitors the airspace around an aircraft for other
aircraft equipped with a corresponding active transponder, independent of air traffic control, and warns
pilots of the presence of other transponder-equipped aircraft which may present a threat of mid-air
collision (MAC).

System description
TCAS involves communication
between all aircraft equipped with an
appropriate transponder (provided the
transponder is enabled and set up
properly). Each TCAS-equipped aircraft
interrogates all other aircraft in a
determined range about their position
(via the 1.03 GHz radio frequency),
and all other aircraft reply to other
interrogations (via 1.09 GHz). This
interrogation-and-response cycle may
occur several times per second.

The TCAS system builds a

three dimensional map of aircraft in
the airspace, incorporating their range
(garnered from the interrogation and
response round trip time), altitude (as reported by the interrogated aircraft), and bearing (by the
directional antenna from the response). Then, by extrapolating current range and altitude difference to
anticipated future values, it determines if a potential collision threat exists.

TCAS and its variants are only able to interact with aircraft that have a correctly operating mode
C or mode S transponder. A unique 24-bit identifier is assigned to each aircraft that has a mode S
transponder.

The next step beyond identifying potential collisions is automatically negotiating a mutual
avoidance manoeuver (currently, manoeuvers are restricted to changes in altitude and modification of
climb/sink rates) between the two (or more) conflicting aircraft. These avoidance manoeuvers are
communicated to the flight crew by a cockpit display and by synthesized voice instructions.
 A protected volume of airspace surrounds each TCAS equipped aircraft. The size of the
protected volume depends on the altitude, speed, and heading of the aircraft involved in the encounter.
The illustration below gives an example of a typical TCAS protection volume.

System components
1. TCAS computer unit

Performs airspace surveillance, intruder tracking, its own aircraft altitude tracking, threat detection,
resolution advisory (RA) manoeuvre determination and selection, and generation of advisories. The
TCAS Processor uses pressure altitude, radar altitude, and discrete aircraft status inputs from its
own aircraft to control the collision avoidance logic parameters that determine the protection
volume around the TCAS aircraft.

2. Antennas

The antennas used by TCAS II include a directional antenna that is mounted on the top of the
aircraft and either an omnidirectional or a directional antenna mounted on the bottom of the
aircraft. Most installations use the optional directional antenna on the bottom of the aircraft. In
addition to the two TCAS antennas, two antennas are also required for the Mode S transponder.
One antenna is mounted on the top of the aircraft while the other is mounted on the bottom. These
antennas enable the Mode S transponder to receive interrogations at 1030 MHz and reply to the
received interrogations at 1090 MHz.

3. Cockpit presentation

The TCAS interface with the pilots is provided by two displays: the traffic display and the RA display.
These two displays can be implemented in a number of ways, including displays that incorporate
both displays into a single, physical unit. Regardless of the implementation, the information
displayed is identical. The standards for both the traffic display and the RA display are defined in DO-
185A.

Relationship to automatic dependent surveillance – broadcast (ADS–B)

Automatic dependent surveillance – broadcast (ADS–B) messages are transmitted from aircraft
equipped with suitable transponders, containing information such as identity, location, and velocity. The
signals are broadcast on the 1090 MHz radio frequency. ADS-B messages are also carried on a Universal
Access Transceiver (UAT) in the 978 MHz band.

TCAS equipment which is capable of processing ADS–B messages may use this information to
enhance the performance of TCAS, using techniques known as "hybrid surveillance". As currently
implemented, hybrid surveillance uses reception of ADS–B messages from an aircraft to reduce the rate
at which the TCAS equipment interrogates that aircraft. This reduction in interrogations reduces the use
of the 1030/1090 MHz radio channel, and will over time extend the operationally useful life of TCAS
technology. The ADS–B messages will also allow low cost (for aircraft) technology to provide real time
traffic in the cockpit for small aircraft. Currently UAT based traffic uplinks are provided in Alaska and in
regions of the East coast of the USA.

Hybrid surveillance does not make use of ADS–B's aircraft flight information in the TCAS conflict
detection algorithms; ADS–B is used only to identify aircraft that can safely be interrogated at a lower
rate.

Reduced vertical separation minima

Reduced vertical separation minima or minimum (RVSM) is the reduction, from 2,000 feet to
1,000 feet, of the standard vertical separation required between aircraft flying between flight level 290
(29,000 ft) and flight level 410 (41,000 ft). Expressed in the International System of Units (SI), it is the
reduction, from 600 m to 300 m, of required vertical separation of aircraft between altitudes 8,850 and
12,500 m. This reduction in vertical separation minima therefore increases the number of aircraft that
can fly in a particular volume of controlled airspace.