Transponder Basics

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Transponder Basics

• Phil King,
• Herefordshire Gliding Club

2
Overview

• Secondary Surveillance Radar
• Mode A
• Mode A/C
• Mode S (selective)
• Mode S/ES (extended squitter)
• ADS-B (Automatic Dependent Surveillance
  Broadcast)
• STCA (Short Term Conflict Alert)
• TCAS (Traffic alert Collision Avoidance System)
• Flarm

3
Acronyms

• ACAS Airborne Collision Avoidance System
• ADS-B Automatic Dependent Surveillance
  Broadcast
• CAT Commercial Air Traffic
• EHS (Mode S) Enhanced Surveillance
• ELS (Mode S) Elementary Surveillance
• ES Extended Squitter
• GA General Aviation
• LAST Light Aviation SSR Transponder
• LPST Low Power SSR Mode-S Transponder
• SSR Secondary Surveillance Radar
• STCA Short Term Conflict Alert
• TCAS Traffic alert Collision Avoidance System
• TMZ Transponder Mandatory Zone
• TRA(G) Temporary Reserved Areas (Gliders)
• UAV Unmanned Aerial Vehicle

4
Secondary Surveillance Radar

• Primary radar relies on passive reflection
• Secondary radar co-located with primary
• SSR beam interrogates transponders
• Transponder reply depends on its mode
• ATC computers use SSR to tag primary echoes with
  identifier and height

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Mode A

• Developed from 2nd World War IFF (Identification
  Friend or Foe)
• Limited number of squawk codes (4096)
• Non-selective all transponders respond to every
  interrogation broadcast
• Typically high power requirements
• Frequent transmissions
• Power hungry transmitter

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Mode A/C

• Incorporates mode A plus
• Altitude (i.e. Flight Level) as well as
  identifier
• Extra power required to heat the altimeter to a
  constant temperature

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Mode S (selective)

• Incorporates mode A/C ability plus
• Less power required because
• Transmitter is typically more efficient
• Altimeter does not need a heater
• Can respond selectively
• If the SSR interrogates selectively
• Can reduce power even more
• Two levels of function (and price!)
• ELS Elementary Surveillance (gliders light
  aircraft)
• EHS Enhanced Surveillance (for CAT)

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Mode S/ES (extended squitter)

• As for mode S plus
• Periodically broadcasts GPS position velocity
• Potentially provides one half of an airborne
  collision avoidance system
• Provides some interoperability with ADS-B

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ADS-B

• Broadly equivalent to Mode S/ES
• 150 mile range
• Does not need a ground-based radar system
• Already in use in Alaska Australia
• Arguably the system of the future

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STCA (Short Term Conflict Alert)

• Automated system which alerts ATC
• Requires transponders on both aircraft
• Requires at least one aircraft to be in contact
  with ATC

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TCAS (Traffic alert Collision Avoidance System)

• Is an Airborne Collision Avoidance System
• Receives transponder signals
• Works with mode A/C, S, or S/ES
• Mandatory for CAT
• Useless at detecting non-transponder aircraft

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Flarm

• Good value collision avoidance
• 11,000 in use
• High take up in the Alps and Germany
• Growing take up in UK
• Short range (a few kilometres)
• Ideal for gliders and slow speed aircraft
• May be unsuitable for ATC or CAT
• Unacceptable as an alternative to mode S

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The End