atc

Air Traffic ControlAir Traffic Control

ATCATC

OutlinesOutlines Function.Function. Operation.Operation. Modes Of Operation.Modes Of Operation. How a transponder works?How a transponder works? System Components.System Components.

FunctionFunction Air traffic control system allows controllers Air traffic control system allows controllers

to track airplane movement on ground to track airplane movement on ground radar displays (GND controller scope).radar displays (GND controller scope).

It is a service provided by ground-based It is a service provided by ground-based controllers who direct aircraft on the ground controllers who direct aircraft on the ground and in the air.and in the air.

The GND station monitor the A/Cs The GND station monitor the A/Cs identification and altitude and computes its identification and altitude and computes its range, bearing and airspeed. range, bearing and airspeed.

AIR TRAFFIC CONTROL (ATC) TRANSPONDERS

Types Of RadarTypes Of Radar PSR (Primary Surveillance Radar):PSR (Primary Surveillance Radar): Operates on the normal radar principle in Operates on the normal radar principle in

which any aircraft appears as lighting point which any aircraft appears as lighting point on the radar display with no other on the radar display with no other information than distance and bearing. It information than distance and bearing. It transmits a narrow RF beam through a transmits a narrow RF beam through a rotating antenna and receives its reflection rotating antenna and receives its reflection by any target in its path. from the elapsed by any target in its path. from the elapsed time, the distance to the A/C is computed.time, the distance to the A/C is computed.

It is not requiring any response from the It is not requiring any response from the aircraft.aircraft.

Types Of RadarTypes Of Radar SSR(Secondary Surveillance Radar):SSR(Secondary Surveillance Radar): Uses what is called an “ATC transponder” Uses what is called an “ATC transponder”

in the aircraft .in the aircraft . It provides the communication capabilities It provides the communication capabilities

between the GND station and the A/C.between the GND station and the A/C. The primary and secondary surveillance The primary and secondary surveillance

antennas are mounted on the same antennas are mounted on the same rotating mounting, therefore both always rotating mounting, therefore both always look in same direction at the same time. look in same direction at the same time.

ATC COMMUNICATION

ATC ComponentsATC Components ATC system consists of :ATC system consists of :

Ground station PSR & SSR.Ground station PSR & SSR.

Airborne ATC transponder is a Airborne ATC transponder is a receiver/transmitter which transmits in receiver/transmitter which transmits in response to an interrogation from the response to an interrogation from the ground station secondary surveillance radar ground station secondary surveillance radar system.system.

OperationOperation The ground station SSR transmits The ground station SSR transmits

interrogation signals that is received by the interrogation signals that is received by the aircraft.aircraft.

ATC transponder replies to this interrogation ATC transponder replies to this interrogation by transmitting a reply signal consisting of a by transmitting a reply signal consisting of a special code which will be displayed on the special code which will be displayed on the ground radar unit.ground radar unit.

Modes Of OperationModes Of Operation1.1. Mode AMode A:: It is used to give the It is used to give the

identification code of the A/C, delay = 8 identification code of the A/C, delay = 8 μs.μs.

2.2. Mode BMode B: Optional mode for identification : Optional mode for identification transmitting, delay = 17 μs.transmitting, delay = 17 μs.

3.3. Mode CMode C: Interrogates the A/C about its : Interrogates the A/C about its altitude, delay = altitude, delay = 2121 μs.μs.

4.4. Mode DMode D: Optional mode for altitude : Optional mode for altitude transmitting, delay = 25 μs.transmitting, delay = 25 μs.

How a transponder How a transponder worksworks??

The ground SSR interrogates the airborne The ground SSR interrogates the airborne transponder in one of two modes A and C. transponder in one of two modes A and C.

The type of interrogation is determined by The type of interrogation is determined by the spacing between between two pulses the spacing between between two pulses (P1 and P3) transmitted on a carrier (P1 and P3) transmitted on a carrier frequency of frequency of 10301030 MHZ. MHZ.


Mode AMode A

Mode Mode CC


Each interrogation contains a third pulse Each interrogation contains a third pulse P2 which is transmitted by an omni-P2 which is transmitted by an omni-directional antenna located with SSR. The directional antenna located with SSR. The pulse P2 is transmitted 2 pulse P2 is transmitted 2 μsμs after P1 pulse. after P1 pulse. P2 has lower amplitude than P1. P2 has lower amplitude than P1.

* P2 (SLS) : side lobe suppression* P2 (SLS) : side lobe suppression


By comparing the relative amplitude of the By comparing the relative amplitude of the P1 and P2 pulses, the airborne P1 and P2 pulses, the airborne transponder can determine whether or not transponder can determine whether or not the airplane is in the main lobe of the SSR.the airplane is in the main lobe of the SSR.


The transponder replies to mode A or C The transponder replies to mode A or C interrogations with coded pulses on a carrier interrogations with coded pulses on a carrier frequency of frequency of 10901090 MHZ. MHZ.

Mode A reply : the coded pulses represent 4 Mode A reply : the coded pulses represent 4 digit octal code entered on the ATC control digit octal code entered on the ATC control panel by the pilot from 0000 7777 panel by the pilot from 0000 7777 (8^4=4096 combinations).(8^4=4096 combinations).

An emergency identification code (7700 or An emergency identification code (7700 or 7777) causes aural warning to the controller.7777) causes aural warning to the controller.


Mode C reply : the coded pulse represent Mode C reply : the coded pulse represent altitude data from CADC.altitude data from CADC.


F1 and F2 (first and last framing pulses) are always F1 and F2 (first and last framing pulses) are always present and separated by 20.3 μs. present and separated by 20.3 μs.

The encoding of the reply is done by means of the The encoding of the reply is done by means of the presence (1) or absence (0) of the 12 reply pulses.presence (1) or absence (0) of the 12 reply pulses.


This pulse is initiated when the IDENT This pulse is initiated when the IDENT pushbutton switch is pushed on the pushbutton switch is pushed on the ATC/TCAS control unit. The IP follows the ATC/TCAS control unit. The IP follows the last framing pulse by 4.35 μs. last framing pulse by 4.35 μs.


This causes the display on the radar scope to change This causes the display on the radar scope to change so the ground station operator can be positive of his so the ground station operator can be positive of his particular location on the radar scope.particular location on the radar scope.

System ComponentSystem Component Transponder.Transponder. 2 antenna for each system.2 antenna for each system. Control Panel.Control Panel.

System ComponentSystem Component

RECEIVER

DUPLEXER

TRANSMITTER MODULATOR

ENCODER

DECODER

CONTROL PANEL

AIR DATA COMPUTER

SUPPRESSOR BUS

Transponder (Aircraft)

Code/ altitude information

System ComponentSystem Component The air data computer supplies coded The air data computer supplies coded

altitude to the encoder. altitude to the encoder.

The control panel supplies selected code The control panel supplies selected code information to the encoder.information to the encoder.

The two transponders and the two DME The two transponders and the two DME interrogators are interconnected by a interrogators are interconnected by a suppressor bus which prevents transmission suppressor bus which prevents transmission from more than one system at a time. from more than one system at a time.

System ComponentSystem Component Transponder.Transponder. 2 antenna for each system.2 antenna for each system. Control Panel.Control Panel.

System ComponentSystem Component According to the aircraft configuration, the According to the aircraft configuration, the

transponder selects the antennas (lower or transponder selects the antennas (lower or upper) which receive the best transmission upper) which receive the best transmission signal from the ground ATC secondary signal from the ground ATC secondary radar this allows better radar cover.radar this allows better radar cover.

The reply message is transmitted through The reply message is transmitted through the same antenna from which the selected the same antenna from which the selected received signal originated.received signal originated.

ATC antenna

Control PanelControl Panel

Control PanelControl Panel System selector switch: selects ATC

system desired for use with the knob in “standby” position Transmission and answers to ground station or other aircraft are inhibited (electrically supplied but not operating).

IDENT: by pressing “ident” button an IP (identification pulse) is transmitted for (18 ~ 20 sec) the ground radar operator will see an enhanced target on the radar scope.

Control PanelControl Panel Two sets of knobs of two below the

code digital readout : each knob controls one digit to enable the selection of the ATC transmission code.

Mode select switch : to select the desired mode of operation.

Control PanelControl Panel ALT RPTG SELECTOR : when on enables

coded altitude information to be sent to the appropriate ATC system.

ALT SOURCE : to select NO.1 or NO.2 central air data computer to give altitude information to the ATC transponder.

Control Panel (Recently)Control Panel (Recently)

Mode SMode S In a mode S environment, both

ATCRBS and mode S equipped airplanes can operate at the same time. Mode S, TCAS and ATCRBS interrogators may also be operating simultaneously in the same airspace.

Mode S interrogators use “all call” interrogations and discrete interrogations.

Mode SMode S There are two types of all call

interrogations.

The mode S/ATCRBS all call : is used to interrogate ATCRBS transponders and to elicit the unique mode S address from mode S transponders.

Mode SMode S The mode S only all call: elicits the mode

S address - an ATCRBS airplane will not reply.

After gaining the mode S address from a transponder, the interrogator will “lock out” the transponder from replying to later all call interrogations and interrogate the transponder using discrete interrogations.

Mode SMode S An additional signal, the ATCRBS all

call, is used to interrogate ATCRBS transponders only; mode S transponders will not reply.

Discrete interrogations are used to interrogate mode S transponders only; ATCRBS transponders will not reply.

Mode SMode S The mode S/ATCRBS or ATCRBS only all call:The mode S/ATCRBS or ATCRBS only all call:

The additional P4 pulse is ignored by an ATCRBS The additional P4 pulse is ignored by an ATCRBS transponder and an ATCRBS transponder will transponder and an ATCRBS transponder will respond based on normal criteria. respond based on normal criteria.

Mode SMode S The mode S transponder evaluates the P4 The mode S transponder evaluates the P4

pulse, if the P4 pulse is 1.6 µs, the mode pulse, if the P4 pulse is 1.6 µs, the mode S transponder identifies the interrogation S transponder identifies the interrogation as a mode S/ATCRBS all call and the as a mode S/ATCRBS all call and the transponder will respond (if not previously transponder will respond (if not previously locked out) with its unique 24 bit address.locked out) with its unique 24 bit address.

Mode SMode S If the P4 pulse is 0.8 If the P4 pulse is 0.8 µsµs, then the , then the

mode S transponder identifies the mode S transponder identifies the interrogation as an ATCRBS only interrogation as an ATCRBS only interrogation and will not respond. interrogation and will not respond.

Mode SMode S If the P4 pulse is absent, then the If the P4 pulse is absent, then the

mode S transponder will reply with mode S transponder will reply with the appropriated ATCRBS response.the appropriated ATCRBS response.

Mode SMode S The mode S only all call is sent in the The mode S only all call is sent in the

same format as the mode S discrete same format as the mode S discrete interrogations except that the address interrogations except that the address being interrogated is all logic ones.being interrogated is all logic ones.

Mode SMode S Mode S discrete interrogation: Discrete interrogations are called uplink formats Pulses P1 and P2 are of the same amplitude

ATCRBS transponders will not reply.

Mode SMode S 1.5 µsµs after the beginning of the P2 pulse, the

mode S interrogator begins the P6 pulse. The P6 pulse is a 4 MHz continuous wave.

Mode SMode S 2.75 2.75 µsµs after the Beginning of the P2 pulse, the after the Beginning of the P2 pulse, the

P6 signal reverses the phase of the 4 MHz wave. P6 signal reverses the phase of the 4 MHz wave. This reversal is called the sync phase reversal. This reversal is called the sync phase reversal.

Mode SMode S Another pulse, P5 is in phase with the Another pulse, P5 is in phase with the

beginning P6 wave and is transmitted from beginning P6 wave and is transmitted from an omni-directional antenna at the same an omni-directional antenna at the same time as the sync phase reversal occurs.time as the sync phase reversal occurs.

Mode SMode S If the mode S transponder senses the sync phase If the mode S transponder senses the sync phase

reversal, the transponder will evaluate the address. reversal, the transponder will evaluate the address. If the interrogation is addressed to that If the interrogation is addressed to that transponder, the transponder will reply.transponder, the transponder will reply.

Mode SMode S If the mode S transponder is in the side If the mode S transponder is in the side

lobe of the interrogator, the P5 pulse lobe of the interrogator, the P5 pulse will mask the sync phase reversal and will mask the sync phase reversal and the transponder will not respond.the transponder will not respond.

Mode SMode S The 4 MHz P6 signal after the sync phase The 4 MHz P6 signal after the sync phase

reversal is divided into either 112 or 56 bit reversal is divided into either 112 or 56 bit times. If during a bit time the phase of the times. If during a bit time the phase of the signal is reversed, that bit is a logic one. If the signal is reversed, that bit is a logic one. If the phase of the signal is not reversed, the bit is a phase of the signal is not reversed, the bit is a logic zero (DPSK).logic zero (DPSK).

Mode SMode S Mode S repliesMode S replies

Responses are called downlink formats and the Responses are called downlink formats and the downlink format used by the transponder is downlink format used by the transponder is based on the uplink based on the uplink formatformat received from the received from the interrogator. interrogator.

The data bits are transmitted using pulse The data bits are transmitted using pulse position modulation (PPM). position modulation (PPM).

PPM sends a logic one if the pulse is in the first PPM sends a logic one if the pulse is in the first portion of the bit time and a logic zero if the portion of the bit time and a logic zero if the pulse is in the last portion of the bit time. Bit pulse is in the last portion of the bit time. Bit time is 1 time is 1 µsµs. .

Mode SMode S Mode S transponder is designed to Mode S transponder is designed to

reply to one all call every 4 or 8 or 16 reply to one all call every 4 or 8 or 16 all call interrogators (random) so all call interrogators (random) so there is a better chance to receive a there is a better chance to receive a response from only airplane at a time.response from only airplane at a time.

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