Hi Alan,
I think I see where we differ.
I was referring to the fact that Lee was concerned that the PING Device was only transmitting GPS altitude and no Baro therefore any device comparing altitude from a Ping Device with GPs Altitude and a Device with Baro only altitude would have a significant difference when it comes to alerting especially with the differences in pressure we have been experiencing lately. The device alerting the pilot to a possible target in an relative altitude band would have to resolve and difference between Baro and GPS and therefore detect which reference the transmitting device is using.
Maybe I wasn't clear, my concern is unrelated to whether PING transmist GNSS or BARO Altitude, I think this is fine.
In the case of PilotAware we decode and compare the Altitude based upon its reference format, so if it is GNSS, we compare to the local GNSS, if it is BARO, we compare to the local BARO.
My concern was on the Nav display, eg skydemon.
Skydemon is receiving the Ownship message (ie me) altitude in GNSS
but it being given traffic reports in BARO.
So now there is an inconsistency, my own altitude is reported in GNSS, but everybody else in BARO, and as Tim Dawson quite rightly said in his posting
https://forums.flyer.co.uk/viewtopic.php?f=1&t=102806&hilit=PilotAware&start=60#p1512134This is a disadvantage of devices without a barometric sensor which I have not managed to figure out a good workaround for.
Without a barometric sensor you are left trying to compare pressure altitudes (from other devices) with GPS altitude (from the onboard device) in order to give the user the relative height of other traffic. And you just can't do it.
This is an onerous task to place on the NAV software, because in GDL90 the altitudes are reported in absolute, in a different reference format.
PAW reports the altitude differences as relative, so this is already resloved prior to dispatch to the NAV device
Sorry for any confusion I may have caused, I hope this is now clear.
Thx
Lee