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Air Traffic Control Radar Beacon System

(ATC RBS)

The antenna system of a typical ground radar. The ladder-like top section is the

SSR directional antenna, and the remainder of the assembly makes up the PSR

antenna

In the name of ALLAH, Most Gracious, Most Merciful

Copyright Pakistan International Airlines Training Center Karachi

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Introduction

• The air traffic control radar beacon system (ATCRBS) isa system used in air traffic control (ATC) to enhancesurveillance radar monitoring and separation of airtraffic.

• ATCRBS assists ATC surveillance radars by acquiringinformation about the aircraft being monitored, andproviding this information to the radar controllers.

• The controllers can use the information to identifyradar returns from aircraft (known as targets) and todistinguish those returns from ground clutter.

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Primary Surveillance Radar

• Primary Surveillance Radar (PSR)

 – The target is not actively involved.

 – Displays the range, bearing on a plan position

indicator (PPI) display on the ground. – A lot of energy has to be transmitted.

 – Clutter will be displayed.

 – Individual aircraft could not be displayed.

 – Altitude could not be calculated.

 – A data link could not be set up.

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Secondary Surveillance Radar

• Secondary Surveillance Radar (SSR) – The target is actively involved (airborne

transponders/Ground interrogators).

 – Displays the range, bearing on a plan position

indicator (PPI) display on the ground. – Less energy has to be transmitted.

 – Clutter will be eliminated.

 – Individual aircraft could be displayed & Identified

(Mode-A). – Altitude can be communicated and displayed.

 – A data link can be set up.

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ATCRBS BLOCK DIAGRAM

P

S

R

Timing Tx

Display Rx

Rx

Tx Coding

Airborne transponderGround Interrogator

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How it works

Interrogation-1030 MHz 

Reply-1090 MHz. 

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The Side Lobe Problem

ground

station 

aircraft 1 (should reply) 

aircraft 3 (should NOT reply) 

aircraft 2 (should NOT reply) 

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Side Lobe Suppression

P2

- Side lobe suppression-send anotherpulse from an omni-directional antennaco-located with the main one.

-

ground

station

aircraft 1 

aircraft 2 

aircraft 3 

P1 P2

A2

P2P1

A1

P1-P2>=9 db

P2>P1

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Interrogation Formats-Mode-S

8/21 micro sec

2 micro sec

0.8 micro sec

0.8 micro sec0.8 micro sec

P 1

P 2

P 3Modes-A+C+S

All Call

P 4

1.6 micro sec

2 micro sec

3.75 micro sec

2 micro sec

P 1 P 3

1.26 micro sec

P 2

P 5

Sync Phase Reversal

Mode-S data word starts-DBPSK

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Differential Binary Phase Shift Keying

(DBPSK)

Data 0 1 1 10 0 0

+5 V

0 V

DBPSK or Manchester encoding

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Exercise

• The code 0077 is selected on the ATC Control

panel:

 – What does it mean?

 – Draw the bit pattern for this code and give the

number of 1 s in each pattern:

• With SPI.

•

Without SPI.

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Altitude Encoding 

• Uses Gray code.0: 0000

1: 0001

2: 0011

3: 0010

4: 01105: 0111

6: 0101

7: 0100

8: 1100

9: 1101

A: 1111B: 1110

C: 1010

D: 1001

E: 1000

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A-310-300 ATC Mode-S Installation

ControlPanel

MODE-STransponder ATC

# 1

MODE-S

Transponder ATC #

2

DME 1 TCAS DME 2

Address

CodingAddress

Coding

FMC # 1

TCAS

ADC # 1

FMC # 2

ADC # 2

Air/ground

relay# 1

Air/ground

relay# 2

Flt ID

Baro-Altitude

Suppression Pulse -30 micro sec

c

cc

cc

Top Antenna Sys 1Top Antenna Sys 2

Bot Antenna Sys 1 Bot Antenna Sys 2Back Plug pinprogrammed

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Calculation Of Mode-S Address And Back Pin

programming

• Covered in ICAO Annex 10, Vol-III, Part-I, Chapter-9.

• Total Mode-S Address = 24 bits.

 – State allocation block =4-14 bits (Pakistan=9 bits).

 – By state registration authority =20-10 bits.

•

Pakistan’s code -0111 01 100.• Exercise : develop the Mode-S Address for the registration: AP-BCA.

A : 00001 G: 00111 M : 01101 S: 10011 Y: 11001

B: 00010 H : 01000 N: 01110 T: 10100 Z: 11010

C: 00011 I : 01001 O: 01111 U: 10101

D: 00100 J: 01010 P: 10000 V: 10110

E: 00101 K : 01011 Q: 10001 W: 10111

F: 00110 L : 01100 R: 10010 X: 11000

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Solution

• Pakistan’s code -0111 01 100.

• Exercise : develop the Mode-S Address for the registration: AP-BCA.

 – B: 00010.

 – C: 00011.

 – A : 00001.

 – Complete Address: 0111 01 100 00010 00011 00001

 – Now pin program on the back plug as per AMM.

A : 00001 G: 00111 M : 01101 S: 10011 Y: 11001

B: 00010 H : 01000 N: 01110 T: 10100 Z: 11010

C: 00011 I : 01001 O: 01111 U: 10101

D: 00100 J: 01010 P: 10000 V: 10110

E: 00101 K : 01011 Q: 10001 W: 10111

F: 00110 L : 01100 R: 10010 X: 11000

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Recap• What is the function of ATC RBS?

• How does the ATC RBS work?

• What its’ interrogation and reply frequencies? 

• Which band does it lie in & what precaution is taken to ensure thatmutual interference does not result when one system istransmitting?

• What is Side Lobe Suppression (SLS)?• What are the interrogation and reply modes in Modes-A & C and

how are they differentiated?

• What is Mode-S and how is it ensured that only a Mode-S equippedstation responds to a Mode-S interrogation?

• How is “ident” communicated via Mode-A?• How is Altitude reported via Mode-C?

• What is the format of a Mode-S address and how is it developedand incorporated on the Mode-S transponder on the aircraft?