Modern aircraft do not rely only on pilots talking by radio to be seen by air traffic control. They also use electronic systems that report who they are, how high they are, and where they are. Two of the most important systems are the transponder and ADS-B.
These systems help controllers separate traffic and help pilots avoid other aircraft.
Key Facts
- A transponder replies when it is interrogated by radar, while ADS-B broadcasts automatically.
- Mode A transponders send an identification code called a squawk code.
- Mode C transponders add pressure altitude so controllers can see height information.
- Mode S transponders can send a unique aircraft address and more detailed data.
- ADS-B Out sends GPS-based position, altitude, speed, and aircraft identity to ground stations and nearby aircraft.
- Distance = speed x time, so a signal travel time can be used to estimate range in radar systems.
Vocabulary
- Transponder
- An aircraft radio device that replies to radar signals with information such as an identity code and altitude.
- ADS-B
- Automatic Dependent Surveillance-Broadcast is a system that automatically broadcasts an aircraft's GPS-based position and flight data.
- Squawk code
- A four-digit code assigned by air traffic control that helps identify an aircraft on radar displays.
- Interrogation
- A radar signal sent from the ground or another aircraft that asks a transponder to reply.
- Pressure altitude
- Altitude calculated from air pressure using a standard reference setting, used by aircraft systems for consistent reporting.
Common Mistakes to Avoid
- Thinking a transponder sends GPS position by default is wrong because basic Mode A and Mode C transponders reply with code and altitude, not latitude and longitude.
- Confusing ADS-B Out with ADS-B In is wrong because ADS-B Out broadcasts your aircraft's data, while ADS-B In receives traffic and weather information.
- Assuming radar and ADS-B are the same system is wrong because radar measures aircraft using reflected or replied signals, while ADS-B depends on the aircraft broadcasting its own GPS-based data.
- Forgetting that altitude reports may be pressure altitude is wrong because the value sent electronically may not match the pilot's indicated altitude until corrected by ATC systems.
Practice Questions
- 1 A radar signal travels at about 300,000,000 m/s. If a radar interrogation and reply take a total round-trip time of 0.0002 s, about how far away is the aircraft? Use distance = speed x time and remember the time is for the round trip.
- 2 An aircraft broadcasts an ADS-B message once every second. How many ADS-B position messages does it send in 5 minutes?
- 3 Explain why ADS-B can show an aircraft's position more directly than a basic Mode C transponder, and describe one reason transponders are still useful.