ADS-B Traffic 101


Portable ADS-B receivers for the iPad (like the Garmin GDL 39 and the Stratus 2S) 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 most likely 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). Most 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.

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

Flying with ADS-B receivers – a real world scenario

Flying in an ADS-B Out airplane

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


  1. If I have ADS-B in and out but am not near a ground tower or not near another ADS-B out aircraft in contact with a ground tower, will I be able to see mere Mode C traffic?

    • If there is no ADS-B ground station in the area then no Mode C equipped planes will be visible on an ADS-B receiver, whether there is ADS-B out nearby or not. I fly to the Outer Banks of NC regularly and over much of the area radar coverage is not available below 3500 ft MSL Since ATC can’t see those aircraft below 3500 ft they can’t rebroadcast their position over the ADS-B system. I have ADS-B In & Out but below 3500 ft the only aircraft I see are those with ADS-B out. The only aircraft that can see Mode C equipped aircraft would be those equipped with TCAS or a TIS system that detect Mode C transponders directly (or those with aircraft-detecting radar.)

    • DB:
      You will NOT see any traffic in that scenario. You must remember to maintain a visual outlook at all times, even with ADSB in & out for exactly that reason.

  2. Unfortunately, this tutorial neglects one very important class of traffic that never shows up: Traffic not using a transponder, either because it doesn’t have one installed, isn’t turned on, or isn’t working properly. Old biplanes without an electrical system, for example. Gliders. Drones. Some military. Bottom line: ADS-B greatly assists — but does not replace — see and avoid.

    • In scenario #1 there is no ADS-B ground station available to re-broadcast the Mode C traffic position to ADS-B receivers. This little detail is indicated in the bottom left of the slide.

  3. A 4th scenario that is not discussed is when one is flying in an area where radar coverage is limited. If another aircraft is Mode C equipped but flying below radar coverage then even ADS-B out aircraft will not see it. This scenario should go away once everyone has ADS-B out in 2020.

  4. This is largely incorrect. It implies that ADS-B receivers pick up Mode C equipped aircraft, when that is ONLY true when the Mode C aircraft is under ATC Radar and the ATC image is being retransmitted through ADS-B towers. ADS-B receivers do not “hear” Mode C transmissions directly and implying they do could get a person in hot water.

  5. Remember all weather received on a iPad weather thought ADSB or ap is advisory only.
    Not to be used as primary info! Which means to be legally use needs to be received directly though a FSS or DUATs

  6. what is the 4th scenario that I fly in most of the time?

    ADSB out with stratus, no tower within reach,the case in most of north west Ca below 3500 AGL

    • It will be very similar to the first scenario – you’ll only see airplanes via air-to-air ADS-B Out transmissions.

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