GPS coordinates are calculated off what are known as psudoranges: http://en.wikipedia.org/wiki/Pseudorange http://en.wikipedia.org/wiki/Pseudorange. You can im

GPS coordinates are calculated off what are known as psudoranges: http://en.wikipedia.org/wiki/Pseudorange http://en.wikipedia.org/wiki/Pseudorange. You can imagine these 'ranges' as a calculated distance to a given satellite. Knowing the altitude ahead of time allows you to calculate your 3D position faster because you don't need as many range samples, or satellites, to do accurate estimates of position.