ADS-B Traffic Anomalies

[FlyingMonkey]

I was out flying yesterday with some buddies to the local grass strips, and I noticed something slightly weird.  I have ADS-B IN/OUT installed, using a Garmin GDL-82 OUT device and a Stratux IN device feeding my iFly 740b as a traffic/weather display. 

The issue is that for most of the flight I did not see the other two airplanes in our flight as traffic targets.  Later in the flight I did get a single target, but never saw them both.  Both are mode-C transponder equipped, and I confirmed both had the transponders on and operating normally.  We were operating at low level, probably 500-1000ft AGL for almost the entire flight.  The only thing I can can figure is that we were low enough that their transponders were not getting picked up by ATC.  But it seems that they should have at *least* seen the airplanes on primary radar, and IIRC that information is relayed to the ADS-B network.  

Any thoughts?    

[WmInce]

1 hour ago, FlyingMonkey said:

I was out flying yesterday with some buddies to the local grass strips, and I noticed something slightly weird.  I have ADS-B IN/OUT installed, using a Garmin GDL-82 OUT device and a Stratux IN device feeding my iFly 740b as a traffic/weather display. 

The issue is that for most of the flight I did not see the other two airplanes in our flight as traffic targets.  Later in the flight I did get a single target, but never saw them both.  Both are mode-C transponder equipped, and I confirmed both had the transponders on and operating normally.  We were operating at low level, probably 500-1000ft AGL for almost the entire flight.  The only thing I can can figure is that we were low enough that their transponders were not getting picked up by ATC.  But it seems that they should have at *least* seen the airplanes on primary radar, and IIRC that information is relayed to the ADS-B network.  

Any thoughts?    

Perhaps, at those altitudes, one or more of the aircraft were not communicating with an ADS-B ground station . . .  or at least, the communications were intermittant.

[FlyingMonkey]

11 minutes ago, WmInce said:

Perhaps, at those altitudes, one or more of the aircraft were not communicating with an ADS-B ground station . . .  or at least, the communications were intermittant.

Right...BUT, shouldn't I have at least gotten position data (if not reliable altitude) from the primary radar returns?  My understanding is with ADS-B IN/OUT we should see anything the ATC guys can see that has a transponder.

[WmInce]

27 minutes ago, FlyingMonkey said:

Right...BUT, shouldn't I have at least gotten position data (if not reliable altitude) from the primary radar returns?  My understanding is with ADS-B IN/OUT we should see anything the ATC guys can see that has a transponder.

True, but only if YOU have reliable communication with a ground station. With intermittent communication, there would be dropouts.

What’s more, at those low altitudes, even primary radar returns may be a stretch. Any rising terrain (curvature of earth), between target and antenna would block the line of sight, which is required for radar detection.

[Ed Cesnalis]

welcome to my world.   nothing to see here below 18,000'

[Warmi]

Next time have the entire party climb up to say 3000-4000 feet and see if everyone magically starts showing up.:-) 

That would rule out any possibly issues with onboard equipment.

[SportFlyer1]

That would be a good test. I think you were seeing what ATC saw, just the occasional low level blip.