FlightAware Discussions

Duplicate returns?


#1

Ever since setting up my PiAware at PHNL, I’ve been seeing multiple cases of duplicate returns from aircraft. Is this an issue with my setup, or what is going on? These symbols mimic each other extremely closely as the positions update.


#2

Clicking them so you can see the according entry highlighted in the right hand side table would be useful.

As you can see you get data from multiple data sources.
Maybe one is ADS-B and the other position is TIS-B ?


#3

Yes, one shows as ADS-B (or MLAT), and the other is TIS-B. Is that expected?


#4

I don’t have TIS-B in my area so i’m not sure.

Just look up TIS-B on wikipedia? Seems like some aircraft at HNL are running dual transponders somehow.
Can’t imagine how it could be a problem with your setup.


#5

Oh and regarding the gain in the other thread, does your antenna have a direct view of the horizon looking towards the pacific?

If it has not then it does not really matter what you do with the gain, the range is not going to get much better.
But reducing the gain can still help to see very close signals if they are too strong before.
(And obviously signals too strong to be received can’t be listed in the table so you have to take that into account and test if with lower gain you can receive more close in messages)


#6

My understanding with TIS-B was that it was essentially ground based radar type system, with no transponder onboard the aircraft? I may be totally off on that though.

As for the gain, I don’t quite have a direct line of sight to the Pacific - it’s obstructed by a few buildings there at the airport. If I could get another 20ft of antenna mast, I could probably see it.


#7

Looking far out onto the pacific is not that interesting anyway i would say :slight_smile:

Regarding TIS-B: Your are correct. It’s just a traffic service for planes with ADS-B IN so for example GA planes are receiving traffic information broadcast from the airport.

So you are receiving that signal being broadcast from ATC at the airport would be my assumption. But you are also receiving the regulare ADS-B transponder from the aircraft.


#8

This is normal if you are in range of an airport or otherwise near an ATC ground station that is transmitting TIS-B. TIS-B provides track information from ATC based on their position sources, such as radar.