Quote Originally Posted by PaulSS View Post
Transponders work off 29.92 and then 'fudge' the result to get altitude. It would be no good providing a GPS altitude because (a)they're not as accurate as you think and (b)they're not working off a standard pressure setting, so everyone can be off by varying amounts. So you definitely need to provide your transponder with a pressure-based altitude, instead of GPS-based. Since the EFIS is plumber for pitot and static pressure it's quite normal to just have the transponder 'talk' to the EFIS over the CAN bus and gain the information it needs from there.
Thanks for your response. Just for the sake of discussion I thought the WAAS GPS was more accurate then anything? It gives you true altitude very accurate. For example I can get lower on the approach on most WAAS gps LPV approaches than i can via ILS. Vertical guidance is provided by gps alone. Maybe garmin protocol is not there yet. Or the other idea is maybe they are running two sources of altitude on the garmin transponder for redundancy?