You'll need to read about map datums for the full picture, but basically, GPS altitude and baro-corrected altitude aren't strictly equivalent, and the synthetic vision data is based on GPS altitude. The amount of customers that have devices that don't have a GPS that provides altitude that ALSO don't have an SV-GPS-250 (which provides altitude) is very very small, and so this hasn't been a technical issue that has needed solving. We generally recommend the SV-GPS-250 to everyone because it gets you both 5hz position data and a robust 3d position lock, but it's not strictly required if you have another device that outputs altitude.