Welcome Guest. Sign in or Signup

2 Answers

Transponders and ADS-B

Asked by: 1423 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?

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

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.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  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.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes


The following terms have been auto-detected the question above and any answers or discussion provided. Click on a term to see its definition from the Dauntless Aviation JargonBuster Glossary.

Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.