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?