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This question is asked in the context of small certified GA aircraft in the US.

The TIS-B traffic that is displayed on board is a subset of all the TIS-B traffic in the area. It's often described as a hockey puck surrounding your aircraft where other traffic that is outside the puck is not displayed.

My question is whether the algorithm that filters what does or doesn't get displayed resides on the onboard box or the ground station.

  • Does the ground station (which knows your position because of your ADS-B out) send a custom set of traffic suited for your hockey puck? OR...
  • Does the ground station send out all traffic it is aware of and your on-board box filter only the ones in your hockey puck and send them onto your display? OR...
  • Something else?
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    $\begingroup$ Since B stands for broadcast, I surmise the on-board receiver does the filtering. It makes sense to divide the work up that way, apart from anything else it makes more efficient use of radio bandwidth in the highly congested airspace where TIS-B is most useful. $\endgroup$ – RedGrittyBrick Aug 17 '15 at 14:41
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TIS-B is part of the ADS-B system that will be required in class B and C airspace come 2020. The processing of traffic relevant to your aircraft is done on the ground to keep the airwaves "clean" as it would require transmission of much more data to show all aircraft and then have your local device filter it. In order for you to take advantage of your own "hockey puck", you first need to have installed in your aircraft the appropriate avionics, namely a GPS with WAAS and an ADS-B out or Mode S transponder. Your transponder will broadcast your location and altitude, then the ground station will process which traffic is relevant to you without the 3D boundaries of the "hockey puck" surrounding your aircraft, then it will broadcast that response out to the air. The key is that not only your aircraft can pick up the response - any aircraft with an operable ADS-B In system can read it and thus display traffic/give traffic alerts even if you don't have an ADS-B out. The key though is that without an ADS-B out the traffic information is not customized for you so critical information/traffic may be missing (what is transmitted is might not be in YOUR hockey puck).

Note: It is also possible for you to pick up signals directly from other ADS-B out equipped aircraft in your proximity without an ADS-B ground station to relay that information and they will display directly on your ADS-B in equipped system.

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TIS-B transmissions are broadcast (the -B) which mean they are not addressed to individual aircraft. The TIS-B transmitter will not select traffic of interest for your specific aircraft, as it provides the data simultaneously to many TIS-B receiving aircraft.

If the display system in the aircraft only shows a 'hockey puck' of TIS-B traffic, then some filtering must be done in the ADS-B / TIS-B receiver on board of the aircraft.

The information contained in TIS-B transmissions covers a wide area. That is usually the area that the transmissions can be received in and then some buffer. To achieve this, some geographical filtering is done in the TIS-B server which provides data to the TIS-B transmitters. This is needed to protect the radio spectrum from unnecessary transmissions; it is useless to report the position of an aircraft that is a couple of states away. To make efficient use of the radio frequency, TIS-B transmitters may use multiple directional antennas,each broadcasting only the traffic that is relevant to its covered airspace.

Summarizing: there is filtering done on both the transmitter side and the receiver side, but the selection of which traffic is relevant to your aircraft is done on board.

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  • $\begingroup$ If there are a number of aircraft in one of the directional antenna's covered airspace but none of them have ADS-B out, is there going to be any TIS-B broadcasting going on in that coverage area? $\endgroup$ – Steve H Aug 17 '15 at 19:04
  • $\begingroup$ Yes the information is provided regardless whether there are ADS-B transmissions from that area. Otherwise a malfunctioning ADS-B transmitter could result in loss of TIS-B traffic as well. It is possible to have a TIS-B receiver without transmitting ADS-B. $\endgroup$ – DeltaLima Aug 17 '15 at 19:07
  • $\begingroup$ @DeltaLima that's only partially right because ADS-B data packets only supply information for a limited number of ADS-B targets... if the number of aircraft is too high it can't transmit info about all of them in a single transmission. $\endgroup$ – Pugz Aug 17 '15 at 20:09
  • $\begingroup$ @Pugz ADS-B data packets supply information for at most a single ADS-B target. If the number of aircraft is greater than one, more packets are needed, i.e. more transmissions are needed. That holds for both 1090ES and 978UAT... $\endgroup$ – DeltaLima Aug 17 '15 at 21:38
  • $\begingroup$ @DeltaLima That's exactly right, but when there are "many" targets (I forget what the number is) the ground station won't necessarily broadcast all of them to prevent congestion, hence the "customized" traffic selection transmitted by the ground station for aircraft with mode S / ADS-B out. $\endgroup$ – Pugz Aug 17 '15 at 21:42

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