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.
Understanding Aviation: The Transponder and ADS-B
A radar antenna does more than draw a dot on a screen. Secondary surveillance radar sends out a coded radio request while the antenna points in a known direction. An aircraft transponder receives that request and sends a reply after a carefully controlled delay.
The ground system measures the total travel time of the radio signal. Since radio waves move at the speed of light, the time gives the aircraft range from the antenna. The antenna direction gives bearing.
A controller can then place the reply at a particular location on the display. This is different from primary radar, which depends on a weak echo reflected from the aircraft body.
The code entered by pilots matters because it links a radar target to a flight plan. Controllers assign most codes for normal flights. Some codes have urgent meanings.
Code seven five zero zero indicates unlawful interference. Code seven six zero zero indicates a radio communication failure. Code seven seven zero zero indicates a general emergency.
Pilots use these only when appropriate because they immediately draw controller attention. Height reports need care too. The transponder normally reports pressure altitude, based on the standard pressure setting rather than the local air pressure.
Controllers use this common reference so aircraft at high altitude can maintain reliable vertical separation. The altitude shown to a pilot can differ slightly because the pilot uses a local pressure setting for landing.
Mode S was designed to make a busy airspace problem less severe. When many aircraft reply at nearly the same moment, their signals can overlap. This is called garbling.
A Mode S radar can address one aircraft more selectively, using its unique address. Its replies can carry information needed by surveillance systems without every nearby aircraft replying to every request. ADS-B works from a different starting point.
A navigation receiver on board determines position from satellite signals, then the aircraft transmits that position repeatedly. Nearby aircraft with suitable equipment can receive the broadcast directly.
Ground stations can receive it too. In some regions, a second type of ADS-B service can send traffic and weather information back to cockpit displays.
Students should treat every displayed position as a measurement with limits, not as perfect truth. Satellite navigation needs a good view of the sky. Buildings, mountains, antenna placement, equipment faults, or poor signal quality can reduce accuracy.
An ADS-B message includes information about how reliable the position is, so automated systems can judge whether it is suitable for a task. A transponder can fail, be set to the wrong code, or transmit an incorrect altitude after maintenance or setup errors. Controllers compare several sources when possible, including radar, ADS-B, flight plans, and pilot reports.
Pilots still scan outside, listen carefully, and follow controller instructions. Electronic surveillance greatly improves awareness, but safe separation depends on equipment, procedures, and people working together.
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.