ADS-B traffic - shows at distance, not nearby

[bkspero]

I recently started to use a portable UAT ADS-B in device in my non-ADS-B out airplane at the edge of the 30 mile Mode C veil around New York City. By 1000 ft I am receiving several ground stations and see a hornets nest of traffic around the City and the NYC metro airports (about 25-45 nm from my location). Sometimes I see traffic nearby, and sometimes not.

I thought that the only way I could see traffic would be if either there was an airplane with UAT ADS-B out (such that I could receive its ADS-B signal air to air) or if I was near an airplane that triggered ADS-B ground rebroadcast (and I received the signal being sent to them).

But that can't be right because it doesn't explain how I am seeing all the airliner traffic around NYC (that is either 1090es out or simply Mode C) without any nearby-to-me traffic to trigger its broadcast to me.

As an example, today there was a 737 climbing out about 3000 ft above and ahead of me and not showing up on my tablet's ADS-B display while, at the same time, there were dozens of planes showing up 35 miles away around NYC. See the attached screenshot. Why is this happening?

I can only think of 2 explanations. One is that at least around NYC, local ADS-B traffic is being broadcast continuously for just about the whole Mode C veil. The other explanation is that ADS-B rebroadcast can be triggered by any airplane with ADS-B out no matter how far away it is from my airplane. And that I will see traffic in that airplane's vicinity...but only in that vicinity. Meaning that just because I am seeing traffic somewhere, it does not mean that TIS-B rebroadcast is sending me information about nearby traffic.

 

[chartbundle]

The other explanation is that ADS-B rebroadcast can be triggered by any airplane with ADS-B out no matter how far away it is from my airplane.

That's usually it, you're seeing traffic being rebroadcast for one or more ADS-B equipped aircraft in that other location.

 

[Shawn]

The other explanation is that ADS-B rebroadcast can be triggered by any airplane with ADS-B out no matter how far away it is from my airplane. And that I will see traffic in that airplane's vicinity...but only in that vicinity.

That is exactly what is happening. A "hockey puck" of traffic info is sent out centered around each ADS-B OUT aircraft. You are seeing their hockey puck of traffic information and no one near you is triggering the ground stations to see your nearby traffic.

That is why they say that the traffic picture being seeing by non ADS-B planes is not a complete or reliable source of information because you may see the edge of another traffic picture being broadcast but there still may be a 737 barreling down on you that you don't see!

 

[airdale]

The way I explain it to people is to say: "If you're not participating (ADS-B Out), then the traffic you see is somebody else's traffic.

What gets to the display depends on the software you are running too. In most cases the software should probably suppress targets that are not threats, but in your case it sounds like that is not happening.

 

[Shawn]

In most cases the software should probably suppress targets that are not threats, but in your case it sounds like that is not happening.

Depends on what app or device you are viewing it on. Many have changeable settings to filter the traffic both by distance and altitude from you.

 

[John Collins]

All ADS-B traffic is line of sight reception, either from an ADS-B Out equipped aircraft or a ground station providing TISB services to a ADS-B Out equipped aircraft. When your aircraft does not have ADS-B Out, you will still receive a fair amount of traffic, some over 100 NM from your location, but you won't see most of the traffic that is relevant as a mid-air threat. As was previously stated, you are mostly seeing traffic for other aircraft. On a typical flight, you will be lucky to see a few percent of the traffic that you care about.