Originally Posted by
snippercr
Yeah thats how I figured it. It knows you are 2.34567 nm from the missed approach point and AT 2.34567nm you need to be at 784 feet and that is how it determines the glide path information. But my question was how does it compute deviance? But lets say you are at 850 feet when you should be at 784, how does the GPS know your actual altitude and thus give you a deviance indication (fly down needle)?
Like any other FMS approach, the waypoints are in the Db. It compares your position to the Db, compares your GPS altitude with where you SHOULD be at the point on the course, and generates a deviation signal for the needle.
GPS can determine your altitude above the plane of the earth's surface, just like it can determine your lateral position. In fact GPS could be used in space also as long as you are within range of the signal. You would probably need some special software for that though, normal receivers automatically throw out position solutions which show you to be in outer space (this is the equivalent of the old ADF reciprocal bearing).
Originally Posted by
snippercr
LAAS will be cool when it starts to become implemented - am I correct that there is no civilian LAAS approaches in use publicly?
I think they have some in place for testing in Alaska, not sure if they restricted to certain operators.