Lecture 11: Secondary Surveillance Radar (SSR)

Introduction

• Presenter: Grant
• Topic: Secondary Surveillance Radar in radio navigation
• Key Points:
  • SSR is an advanced radar system used alongside primary radar by Air Traffic Control (ATC).
  • Facilitates safer navigation by providing more detailed data than primary radar.

How SSR Works

• Basic Mechanism:
  • Sends interrogation wave from ground; aircraft responds based on transponder mode.
  • Signal travels one direction, reducing size, cost, and erroneous reflections.
• Signal Details:
  • Ground sends pulses at 1030 MHz; aircraft responds at 1090 MHz.
  • Signals in all directions; uses time and direction for locating aircraft similar to primary radar.

Transponder Modes

Mode A

• Features:
  • 4-digit squawk code (0-7, octal system).
  • 4096 unique codes for aircraft identification.
  • Squawk codes given during departure or leaving an airport.
• Unique Emergency Codes:
  • 7500: Hijack
  • 7600: Communication failure
  • 7700: Emergency

Mode C

• Features:
  • Provides altitude information along with Mode A features.
  • Altitude readout based on standard pressure (1013 hPa), rounded to nearest 100 ft.
  • Used to check aircraft's altitude accuracy.

Mode S

• Features:
  • Unique 24-bit address, enabling over 16 million codes.
  • Allows selective interrogation and complex data exchange.
  • Compatible with Mode A and C.
  • Shares data like speed, heading, registration, and more.
  • TCAS System:
    • Traffic Collision Avoidance System.
    • Aircraft communicate to avoid collisions by coordinating maneuvers.

Errors and Solutions

• Fruiting:
  • Occurs when multiple radars pick up a signal meant for one.
  • Causes confusion in bearing and distance; resolved by unique Mode S codes.
• Garbling:
  • Happens when replies from aligned aircraft overlap.
  • Causes confusion in identifying aircraft position.

Summary

• SSR provides more detailed and reliable navigation support compared to primary radar.
• Different modes (A, C, S) offer various functionalities, with Mode S being the most advanced.
• Errors such as fruiting and garbling are minimized by using Mode S.
• SSR, especially with TCAS, enhances aviation safety by preventing collisions.