terrain height at GPS coordinates

Hi all. I was wondering how I'd go about extracting the terrain height at a GPS coordinate I specify. From a search of the forums, I know that as of FS2002 there was no way to do this, but I also know that people have done terrain following radars since then, and I don't see how that could be done without somehow knowing the outlay of the terrain in front of you. What is the secret here? This is for my Hornet Heads Up Display. I've fixed things and added some very cool features - energy caret, HUD steering (an HSI-like display tacked onto the velocity vector), etc... and I'm also adding a "Mark" feature that lets you put your plane's current coordinates into a GPS waypoint, and I then display a "target" diamond in the HUD at that point. It's no problem getting and storing the terrain altitude just underneath your plane, but I'd like to expand it to include waypoints not marked in this way. Possible? Any help appreciated!