ADS-B "Partial" Traffic

[Bruce]

I am trying to understand ADS-B traffic grading, specifically the difference between the "Full" and "Partial" indications.  I have a SV system updated to v.10.1 with a SV-261 xpdr and SV-470 rcvr. 

This morning while flying over flat central Florida, I was at 1000' AGL about 6 miles from the KGNV ground based transceiver.  ADS-B status indicated 4 GBTs being received.  The  xpdr reply light was active.  I saw non-ADS-B-out traffic (no N number shown) on the display, yet for two minutes I had a "Partial" traffic indication.  Then it returned to "Full."  Can you explain?

Also, out of curiosity, is reception of one GBT enough to provide "Full" traffic?

I have seen fewer similar instances of seemingly false "Partial" indications after upgrading from v.10.0, yet the sensitivity still feels too high.

TIA.

 

[dynonsupport]

Page 7-31 of the pilot's guide:

You may see additional status information for some traffic devices. These include:

FULL: ADS-B devices only - This state signifies that the transponder is communicating with the US-based ADS-B ground stations; that those ground stations are replying to the SV-ADSB-470; and that your aircraft is in a known radar service volume. This allows the ADS-B ground stations to report radar-only targets that are not yet ADS-B out equipped, thus providing a “full” traffic picture.

PARTIAL: You will also see PARTIAL when an ADS-B system is reporting that it does not know about radar traffic targets, or when you are not receiving ADS-B ground station replies (in this case, you may still see aircraft that are equipped with 978 MHz ADS-B out devices via the SV-ADSB-470). This means that, although you are receiving ADS-B traffic from other ADS-B equipped aircraft, you are not receiving non-ADS-B targets because you are not currently in a radar service volume and/or ADS-B service volume. Therefore, the traffic portrait may be incomplete.

Partial has nothing to do with the reception of a ground station. However, you can't have FULL without a ground station. FULL means the ground station can see you on radar- thus they have faith they can warn you of a Mode-C target that is near you. Dynon doesn't tune this- the ADS-B system sends us a message saying "you have full traffic coverage" and we display that.

You can absolutely still see other Mode-C targets at times when you are partial- Imagine you are at 1,000 AGL and the radar volume starts at 1,500 AGL at your location. ADS-B will still send those Mode-C targets, so you can see a plane 500' above you. However, you can't see the plane 100' below. Thus, partial.

 

[Bruce]

Thanks for the helpful explanation.  I thought that SkyView algorithms determined the "Partial" warning, but now understand that the GBTs make that determination.  Learning the ins-and-outs of ADS-B is an on-going process.