ADS-B traffic – when does it work, when does it not?

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Portable ADS-B receivers for the iPad (like the Sentry, 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 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, 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.

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

For more information:

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

13 COMMENTS

  1. Your Scenario 3 has me wondering is that 30 nm from the antenna on the ground – or 30 nm from my airplane (equipt with ADS-B out)?

  2. Scenario 2 and 3 indicates you will see all aircraft. I was understanding that military aircraft will not be seen. Is that correct?

  3. You don’t mention that depending upon where you mount your Stratus or other portable receiver it is likely to be screened from seeing at least some aircraft by the airframe of your airplane or maybe even the other airplane.

    This happens even if you have ADS-B out because ground stations do not rebroadcast the location of ADS-B out equipped traffic you should be seeing air-to-air (your ADS-B out system tells the ground system what type of ADS-B traffic your receiver is capable of seeing directly). In my case with an 978 only MHz ADS-B receiver on the glareshield of my ADS-B out equipped low wing airplane, I never see aircraft with 978 MHz ADS-B out systems when they are located at my 5-7 O’Clock directions and below my altitude. Those directions are blocked by my fuselage so I can’t see them air-to-air and their positions are not rebroadcast from the ground because the ground stations “think” that I am seeing them air-to-air. This happens despite rarely being in contact with fewer than 3 ground stations, and often receiving as many as 6 or more (in well served metropolitan airspace).

    So, just because one has ADS-B out, and one is in communication with ground stations, it is not certain that all traffic will be seen via ADS-B in.

  4. …implied, but may not be obvious, scenario 2 and 3 also dependent on reception from ground stations…no ground station reception (due to low altitude terrain blockage or out of service towers) and best case you’re at scenario 1, regardless of your equipment.

  5. Military aircraft have combat control modes that will not appear on civilian IFF, but they MUST have Modes 3, C and S (including ADS-B) when transiting civil airspace, like anyone else. In combat zones they make sure to turn on the fancy stuff and turn off the usual stuff, lest an enemy use it to Target them.

  6. These scenarios are actually just what is hoped for by 2020. They are not currently true. I have ADS-B in and out and I frequently see (visually), and hear on the radio, aircraft which do not show up on my electronics even in a strong radar environment. It’s a great pleasure and comfort to have ADS-B and know where nearly all the traffic is before the controller tells me. But we need to keep looking outside and hope that everyone complies in the new year.

  7. To be technically correct, the slide for Scenario 2 has an incorrect, misleading statement. It says “You will see ALL aircraft within 30 NM of the ADSB-Out equipped aircraft”. It later states, and the drawing depicts, a 30 NM diameter (not radius), with the ADSB-Out equipped aircraft in the center of the “puck”. This means that all aircraft seen will be 15 NM, not 30, from the subject aircraft.

  8. Sorry for asking questions that are probably blatantly obvious to others. But what your saying is if I don’t have ADS-B out and there is a plane in that does have a ADS-B out in a 30 nm radius that I will be able to see any aircraft within that radius? Oh, and also has a ground station in range.

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