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Transponder Basics

Transponder Basics. Phil King, Herefordshire Gliding Club. 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)

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Transponder Basics

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  1. 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

  5. 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

  6. 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

  7. 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)

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. The End

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