ADS-B Ground Station Wakeup

I'm working on getting traffic info into the cockpit. My system is a Trig Mode S transponder (ADS-B Out), a Garmin GDL39 (ADS-B In), and a Garmin 796 (GPS location and weather/traffic display). I know that the 796 doesn't meet the 2020 specs for GPS position source, but it appears that the FAA is being lenient on this point during the ADS-B rollout.

I'm receiving air-to-air traffic info, but nothing from the nearby ground station, so it appears that I'm not lighting up the ground station for traffic. I'm receiving weather info, and the transponder is working for normal detection by ATC.

Can someone tell me what info the ground station requires for traffic wakeup? The NMEA choices on the 796 are FAST, which sends position, velocity and nav data to the transponder at 1-sec intervals; and NORMAL, which adds satellite status and flight plan data, at 2-sec intervals. Using NORMAL, the Trig gives an occasional error message that the GPS input isn't valid. No error message when using FAST.

Trying to eliminate one potential issue at a time. If you have successfully integrated a similar system, could you share the interface parameters?

You need to go into Trig's settings and configure it properly so it broadcasts your "IN" capabilities. Since you have a GDL-39, you should set up both 1090 In and UAT In as enabled (both)... Trig will then broadcast this every so often to the GBT so GBT knows what to send you. It won't send traffic if you're saying you are not capable of receiving it.

Looks like my Trig needs a software update. Current software on the unit asks whether an ADS-B receiver is installed (Yes/No) - currently set to "Yes". The new software version asks whether a 1090Mhz receiver is installed and also whether a UAT receiver is installed - the GDL39 satisfies both.

Makes sense. If you have both set to Yes (and you should) then GBT won't bother sending you ADS-R, since you're capable of directly receiving that traffic (with your dual-freq GDL-39).. it'll only send TIS-B which is all you need from it anyways.

Just to follow up, the software update to the Trig transponder solved the problem. And I had to set the Garmin 796 NMEA stream to "FAST" to prevent the transponder from occasionally missing GPS location info.