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)• TCAS (Traffic alert Collision Avoidance System)• Flarm

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

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

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

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

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)

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

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

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

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

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

The End