ADS-B traffic explained

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Portable ADS-B receivers for the iPad (like the Garmin GDL 50 and the Stratus 3) can receive ADS-B traffic in addition to weather. But unlike weather, which is broadcast continuously, traffic is only transmitted in response to specific prompts. This can make ADS-B traffic very confusing – when does it work and when doesn’t it work?

To help, we’ve created this series of graphics, which shows three common scenarios:

ADS-B Scenario 1

Graphic 1: The worst scenario, where you are flying with a portable ADS-B receiver, but do not have an ADS-B Out transponder installed in your panel. Here, you’ll receive any airplane that is transmitting ADS-B Out via air-to-air (no ground station required). many airplanes do not have ADS-B Out, so this is fairly limited. You will not see regular, Mode C targets.

ADS-B Scenario 2Graphic 2: In this case, you are still flying with a portable ADS-B receiver and no ADS-B Out in your airplane, but you are close to another aircraft that is ADS-B Out equipped. In this case, that ADS-B Out airplane is waking up the ground station and is receiving a custom traffic picture for a 30-mile “hockey puck” around that airplane. If you are close enough to that airplane, your portable receiver can listen in on that traffic message. While you won’t get a complete traffic picture, you will get a better one, since the ground station transmits Mode C targets in addition to ADS-B targets. Just remember that you don’t know what you don’t know – there could be a target just outside the hockey puck that’s not showing up.

ADS-B Scenario 3Graphic 3: This is the best possible case. You have an ADS-B Out transponder in your airplane, so you are transmitting out to the ground stations and creating your own “hockey puck” of traffic information. You’ll see all traffic within a 30 mile diameter and +/- 3500 ft.

Here’s a helpful video showing ADS-B traffic in action in the ForeFlight app:

For more information:

Detailed ADS-B technical review

ForeFlight Traffic Tips: How to get the most out of ADS-B traffic

ADS-B Webinar (video)

The complete graphic is below (click on the image for a larger view)

ADS-B Scenarios

10 COMMENTS

  1. How prevalent is the situation in the first graphic where there is no ground station? I’m guessing the number of ADS ground stations has increased in recent years but am curious where there are the most gaps – and if those coverage gaps will eventually be filled. Anybody know?

  2. In the graphic 3 scenario, you state that you will see all traffic if you have ADSB out. I was under the assumption that you only see aircraft with working transponders. Please clarify.

  3. In scenario 3, if I’m correct, you will see aircraft with a working transponder, also any aircraft that shows up on radar in a radar environments, ( and within the 15 miles of the ADSB equipped aircraft). Please verify… also, I would hope that you could add this information into your presentation so everyone viewing it would become fully informed. Thanks

  4. In Scenario 1, what is the vertical and horizontal range of the air-to-air transmission from the ADS-B equipped aircraft? In scenario 2, is the 30-mi hockey puck centered around the ADS-B equipped aircraft or centered around the ground station? And, can I assume that one has to be within 3500′ vertical range from the ADS-B equipped aircraft to ‘listen in’? Finally, in scenario 3 if there happens to be no ground station nearby can I still track Mode C equipped traffic nearby? Thanks!

    • The range in scenario 1 depends on line of sight. I’ve seen air-to-air traffic from 70 miles away at altitude, so it’s generally quite good.

      In scenario 2, the hockey puck is centered on the ADS-B Out airplane. You do not need to be within the 3500’ vertical range of the ADS-B airplane to listen in – but that’s the only traffic you’ll receive. So you could be 10,000 feet above it and get the message, but you won’t see any traffic at your altitude.

      You cannot track Mode C targets without ADS-B ground stations. Those are sent up as a rebroadcast (TIS-B) from the ground stations. Without those, you’ll only see ADS-B Out equipped airplanes via air-to-Air transmissions.

  5. There are two ground base stations in the interior of Alaska a space the size of ten states in the lower 48. Nor do we have satilite radio. Ninth percent of the time we do not have ground based stations. Forget about any in cockpit weather.

  6. For context I have a ADS-B 1090 OUT and 978 IN. I also have a Stratus portable dual channel receiver in my plane, so I have the option of comparing the traffic between systems.

    What I’ve been seeing lots of 1090 targets that I can see on Stratus connected display, but I never ever see it on 978 IN. My understanding is that my setup should:

    A) Receive direct reports from aircraft equipped with UAT/978 OUT hardware. (Air-to-Air) (Good)
    B) Receive ADS-R rebroadcast traffic from ground stations, for 1090 targets. (Not seems to work)
    C) Receive TIS-B traffic from ground stations, for Mode-C targets that aren’t participating in ADS-B. (Good)

    So my question is, why do scenario B seem to not be working for me?

    I pulled ADS-B Performance Report for my flights and I see:

    • OUT capability is correctly listed as 1090 and IN capability is listed as UAT. This should indicate Ground Stations that I need 1090 targets rebroadcast (ADS-R) to me.

    Any thoughts?

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