ACNS UNIT-5.pdf

AIR TRAFFIC CONTROL SYSTEM AND
TRAFFIC ALERT AND COLLISION
AVOIDANCE SYSTEM
UNIT – V

Air traffic control system and traffic
alert and collision avoidance system
ATC overview, ATC transponder modes, Airborne
equipment System operation, Automatic dependent
surveillance-broadcast, Communications, navigation
and surveillance/air traffic management, Airborne
collision avoidance systems, TCAS overview, TCAS
equipment System operation.

ATC Overview
Air Traffic Controller is an important part of the Aviation
industry. Air Traffic Controllers are trained to continue the safety,
orderly flow of air traffic in the air traffic control system globally.
ATC’s primary radar system places the target(s) on a plan
position indicator (PPI). With primary radar, high energy is
directed via an antenna to illuminate a ‘target’
Primary surveillance radar (PSR), has its disadvantages; one of
which is that the amount of energy being transmitted is very large
compared with the amount of energy reflected from the target
Secondary surveillance radar (SSR) transmits a specific low
energy signal (the interrogation) to a known target. This signal is
analysed by a transponder and a new (or secondary) signal, i.e.
not a reflected signal, is sent back (the reply) to the origin

Primary Surveillance Radar (PSR)

Secondary Surveillance Radar (SSR)

ATC Overview
The ATC system operates on two frequencies within
the L-band of radar:
1. Interrogation codes on a 1030Mhz carrier wave
2. Reply codes on a 1090Mhz carrier wave
3. By implementing the SSR transponder system, each icon
can be identified via a unique four-digit code (allocated
by ATC for eachflight).
4. Developments of the ATC transponder system have
provided additional functionality allowing details such
as flight number and altitude to be displayed on the
controller’s screen

ATC Transponder Modes
SSR systems have been developed for both military and
commercial aircraft applications
Mode A : In this transponder system, the pilot selects the four-
digit code on the ATC control panel prior to each flight.
Since each aircraft is allocated with a unique code, only one
icon per aircraft will be highlighted; this unique identification
is referred to as a squawk code.
Each of the four digits ranges from 0 to 7, these are then coded
as octal numbers for use by the transponder. (This system is
called Mode 3 for military users.)

Mode C : Azimuth is now augmented by pressure altitude; this
is displayed on the controller’s screen, adjacent to the aircraft
icon thereby providing three dimensional information.
Altitude can be taken from the pilot’s altimeter from an encoder
that sends parallel data (in Gillham/Gray code) to the
transponder.
This coded data is in 100-foot increments.
Aircraft with air data computers will send altitude to the
transponder in serial data form, typically ARINC 429.

MODE S : In addition to the basic identification and altitude
information, Mode S includes a data linking capability to provide
a cooperative surveillance and communication system.
Aircraft equipped with Mode S transponders allow specific
aircraft to be interrogated
This increases the efficiency of the ATC resources.
This exchange occurs each time an interrogation signal is
transmitted.
Mode S transponders only send a reply to the first interrogation
signal; the ground station logs this aircraft’s address code for
future reference.

Airborne equipment System operation
Commercial transport aircraft are installed with two ATC antennas,
a control panel and two transponders

Control panel
This is often a combined air traffic control and traffic alert and
collision avoidance system (TCAS)
The four-digit aircraft identification code is selected by either
rotary switches or push buttons, and displayed in a window.
Altitude reporting for Mode C transponders can be selected on
or off.
When requested by ATC, a momentary make switch is
pressed; this transmits the selected code for a period of
approximately 15 to 20 seconds.

Transponder
The aircraft transponder provides the link between the aircraft and
ground stations.
General aviation products have a combined panel and transponder
to save space and weight.
These can be Mode S capable for IFR operations.
If the transponder is interrogated by a TCAS II equipped aircraft, it
will select the appropriate antenna to transmit the reply.
This technique is called antenna diversity

GA Transponder
General Aviation (GA) transponders are typically self-
contained within a single panel mounted unit.
The Avidyne AXP340 transponder, is a Mode S transponder,
with support for 1090MHz Extended Squitter (ES), that meets
all the current requirements for Mode S elementary surveillance
transponders for both IFR and VFR flight.
This unit has additional functionality, e.g. pressure altitude and
GPS readout, Flight ID entry, one-touch VFR code entry, stop
watch timer, flight timer, and altitude alerter

Automatic dependent surveillance-
broadcast
ADS-B is an emerging technology for air traffic management
(ATM) that is intended to replace conventional secondary
surveillance radar.
The system is automatic in that no interrogation is required to
initiate a transponder broadcast from the aircraft. This type of
unsolicited transmission is known as a squitter.
ADS-B utilises conventional global navigation satellite system
(GNSS) and onboard broadcast equipment for communication
via satellites making it dependent.
Air traffic coordination is thereby provided though surveillance
between aircraft; the system has a range of approximately 150
nm.

Automatic dependent surveillance-
broadcast
The system provides real-time data for both flight crews and air
traffic controllers.
Data is exchanged between aircraft and can be independent of
ground equipment.
ADS-B is being proposed with three methods of exchanging data:
Mode S transponder extended squitter (ES)
VHF digital link (VDL)
Universal access transceiver (UAT).

Extended squitter (ES)
The Mode S method extends the information already described
above for enhanced surveillance.
Extended squitter (ES) messages include aircraft position and other
status information.
The advantage of using ES is that the infrastructure exists via
Mode S ground stations and TCAS equipped aircraft.
Note that Mode S provides only unidirectional communications

VHF Digital Link (VDL)
VDL utilizes the existing aeronautical VHF frequencies to provide
bi-directional communications; digital data is within a 25kHz
bandwidth.
This protocol is based on a technique called ‘self organizing time
division multiple access’ (STDMA).
VDL is suited for short message transmissions from a large number
of users over longitude range.
The system utilizes conventional global navigation satellite system
(GNSS) to send messages of up to 32 bytes at 9.6 kbps.
The system can manage 9,000 32-byte messages per minute and is
self-organising, therefore no master ground station is required.

Universal Access Transceivers (UAT)
UAT uses conventional global navigation satellite system
(GNSS) technology and a relatively simple broadcast
communications link
The 978 MHz universal access transceiver (UAT) receives
inputs from a global navigation satellite system (GNSS),
It combines this data with other parameters e.g. airspeed,
heading, altitude and aircraft identity, to facilitate the air traffic
management
This data is transmitted to aircraft in the surrounding area, and
to ground receivers that distribute the data in real time via
existing communication infrastructures.

Communications, navigation and
surveillance/air traffic management
Air traffic management addresses traffic flow at the optimum
speed, height and route to minimize fuel consumption.
Numerous enabling factors for CNS/ATM will lead to higher
navigation accuracy at lower cost (not just the cost of fuel, but
also the impact of air travel on the environment).
Area navigation already provides a flexible and efficient means
of en-route and terminal area operations in place of airway
routings.

Controller pilot datalink communications
(CPDLC)
A critical factor in CNS/ATM is the saturation and congestion of
voice communication channels.
Controller pilot datalink communications (CPDLC) helps alleviate
this situation by providing an additional communications medium
for aircrews and controllers.
CPDLC allows air traffic controllers to communicate with pilots
over a datalink system, e.g. ATN, FANS, or VDL Mode 2.
CPDLC is being globally implemented, and is in different
implementation stages.
European airspace will require CPDLC for operations above
FL285.

Controller pilot datalink communications
(CPDLC)
CPDLC is an air/ground datalink application which enables the
exchange of datalink messages between controllers and pilots,
thereby complementing traditional voice communications.
The objective of CPDLC is to improve the safety and efficiency
of air traffic management, enabling routine, non-time-critical
communications between pilots and controllers.
Examples of CPDLC message include
➢ changes SSR code/squawk ident
➢ change of communications frequency
➢ ATC clearances, e.g. level changes, vectoring, direct routing speed
control, etc.

Airborne collision avoidance systems
There are various collision avoidance technologies in use, or
being planned:
Passive receivers :
➢ They monitor ATC transponder signals in the immediate area,
and provide visual or audible signals to warn of nearby traffic.
➢ They have a range of approximately six miles and monitor 2,500
feet above or below the host aircraft.
➢ The receiver is normally located on the aircraft’s glare-shield.
➢ It has an internal antenna, which can lead to intermittent
coverage depending on how and where the unit is positioned.
➢ Some passive devices provide vertical trend information, e.g.
indicating if the other aircraft is climbing or descending.

Traffic information system (TIS) :
➢ This uses the host aircraft’s Mode S transponder to communicate
with the ground-based secondary surveillance radar (SSR)
network.
➢ Traffic information can be obtained within a five-mile radius,
1,200 feet above or below the host aircraft.
➢ This traffic information is provided on a (near) real-time basis.
➢ The attraction of TIS is that aircraft hardware and software are
minimal since the system ‘feeds’ off ground station
computations.

FLARM:
➢ FLARM is a traffic awareness and collision avoidance
technology for General Aviation (GA) light aircraft, helicopters,
gliders and unmanned aerial vehicles (UAVs).
➢ With FLARM installed, the pilot is alerted of both traffic and
imminent collisions with other aircraft.
➢ The system calculates and broadcasts its own future flight path to
nearby FLARM equipped aircraft.
➢ At the same time, it receives the future flight path from
equipped surrounding aircraft.

ACNS UNIT-5.pdf

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