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2 Answers

Transponders and ADS-B

Asked by: 4873 views Aircraft Systems

I understand that transponders use secondary radar and ADS-B uses GPS position, which is more accurate. But I don’t understand why both are required. On ATC’s radar, do they see your position given from your transponder or from ADS-B?

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2 Answers



  1. awair on Jul 24, 2022

    Great question!

    I think that it’s really a legacy issue:
    Originally the radar scope only displayed echo returns from primary radar. Later ATC devices had increasing computer integration which would show both primary and secondary (SSR) returns.

    Only the newest equipment would be capable of adding the ADS input, and until all aircraft, and all ATC units are equipped with new tech, then we will have both the legacy transponder and newer ADS requirement.

    In other parts of the world, especially Oceanic, ADS is used as aircraft are beyond effective radar coverage.

    I’m not 100% certain, but believe there can be a significant (in terms of collision risk) delay using satellite communication, and also the requirement for (2-way) confirmation that signals have been correctly received.

    Finally, as a guess, once the satellites are in place, ADS should be significantly cheaper to operate, so there is a balance between (ATC) costs, and reliability and effectiveness.

    Having used both ADS & CPDLC for the last 20+ years, this is my understanding. Hope it helps.

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  2. John D Collins on Jul 25, 2022

    ATC sees a “Fused” position that will include input from multiple sources, including primary radar, secondary radar from your transponder, and if ADS-B is available, There are other less used surveillance systems that are also integrated. These are combined to show a single position. Not all airspace requires the use of transponders or ADS-B and not all aircraft are equipped. ADS-B is used to enhance the position accuracy in areas where this is needed for efficient spacing of aircraft. ADS-B in the US is supported using two independent frequencies and systems. One system (1090ES) is built on a Mode S transponder base and is part of the transponder. In this case, both secondary radar replies and ADS-B share the transponder transmitter, even though they are received by different systems on the ground. Secondary radar is received at the radar site and ADS-B broadcasts are received by ground stations. The other system is called UAT and broadcasts on 978 MHz and is not a part of a transponder. The UAT is cheaper and may be used in the US below 18000. Many GA piston aircraft use UAT and a separate mode A/C transponder. UAT also has a capability to add FISB weather information to the pilot in the cockpit.

    As far as which is more accurate, radar has a slower position update rate, between one update per 5 to 12 seconds. The accuracy varies with distance from the radar site. ADS-B broadcasts the aircraft position once per second and the accuracy depends on how accurate the GPS position source is at the time. Usually ADS-B will have an accuracy better than radar, but it also provides the accuracy data each second, so when the accuracy drops below a standard of .05 NM, it may no longer be used for surveillance. Even where ADS-B is required, secondary radar and primary radar are backup systems.

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