Understanding ADS-B traffic: when can you trust it?

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Portable ADS-B receivers for the iPad (like the Sentry, Garmin GDL 50 and Stratus 3) can receive ADS-B traffic in addition to weather. But unlike weather, which is broadcast continuously, traffic is only transmitted in certain cases. This can make ADS-B traffic very confusing—when does it work and when does it not work?

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

ADS-B Scenario 1

Graphic 1: The worst case scenario, where you are flying with a portable ADS-B receiver, but you do not have an ADS-B Out transponder installed in your panel and there are no ADS-B ground stations in range. Here, you’ll receive any airplane that is transmitting ADS-B Out via air-to-air transmission (no ground station required, so the lack of coverage does not matter). Not all airplanes have ADS-B Out, though, so you will see some traffic but definitely not all. In particular, you will not see 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 and there is a ground station in range. 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 diameter “hockey puck” around that airplane. If you are close enough to that airplane, your portable receiver can listen in on that traffic message and display targets.

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 (the red airplane in the graphic above) because the hockey puck is not centered on you.

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. This includes all ADS-B Out traffic and Mode C traffic. You’ll also see ADS-B Out traffic beyond the hockey puck, via a direct, air-to-air transmission. The only traffic not visible would be airplanes without any type of transponder. That means you still need to look outside, but it’s highly likely that you’ll see all the airplanes nearby.

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

If you have ADS-B Out and you feel like you’re not seeing a full traffic picture, there are some options for troubleshooting. Read this article for detailed steps.

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

ADS-B Scenarios

5 COMMENTS

  1. And then there’s Scenario 4: Flying with Stratus and ABS-B Out Transponder but not within line of sight of a tower OR flying in an area where ATC can’t see everyone. In that case, back to Square 1. Use your eyeballs plus do all the right stuff like make good CTAF calls and fly appropriate altitudes.

    ABS-B is a helpful tool, but it’s not foolproof.

  2. I fly under a class bravo a lot and I always see traffic whether they have ADSB-OUT or not (because of ground radar). But the other day I was flying 40 miles away from the class bravo where there is no ground radar coverage and came within a mile of another small high wing aircraft that obviously had no ADSB-OUT and he was nowhere to be seen on my gps or foreflight. So the traffic alerts are terrific but if you’re flying out in the county not under radar coverage, better use the old fashioned method of heads up and scanning.

    And for those of you who don’t want ADSB-OUT, please go out and install on your airplane no matter if your plane is grandfathered in or not. The life you save might be your own.

  3. ADS-B is sort of a suckers game. You think that you are “covered” but the reality is that there is much out there that you will not see. It is helpful but only gives part of the picture. Using flight following and your Mk 20-20 eyeballs is much safer. As we all know, there are many situations where the Mk20-20 eyeballs also fail us.
    I see too many people who accept the ADS-B display as reality. Sort of like asking “anyone in the area please advise.” You are setting yourself up for failure. No answer, therefore, nobody in the area, except the NORDO Cub or Champ doing touch and goes. Then there are the high performance composite gliders that may not have a transponder. ATC can’t see them, ADS-B is not aware of them but they are out there.
    Lets all be careful up there.

  4. Not ignoring anyone Mark, and see and avoid is all of our responsibility. However, if the Aeroncas, Cubs, and anyone else out there knows they’re vulnerable and therefore exposing others to unnecessary risk, why not be part of the safety solution and buy a battery operated portable ADS-B unit and an iPad. Taking a victim’s approach here when you don’t have to modify your airplane to improve safety, and for less than $1500 is one thought.

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