Name: Navstar 66
Satellite number: 37753
Launch date: 2011
Type of orbit: medium-Earth orbit
Period: 12 hours
Perigee: 20,148 km
Apogee: 20,217 km
Dr Allan McInnes
GPS navigation satellites are in a medium-Earth orbit at about 20,000 kilometres altitude above the Earth. It’s specially designed to enable GPS satellites to see a lot of the Earth all at one time without having to build lots and lots of satellites.
These GPS satellites broadcast a special radio signal to the Earth. It’s what’s called a navigation signal and it carries information about the time at which it was transmitted and which satellite it came from and things of that nature. What happens is that your GPS receiver on the ground receives that signal, not just from one satellite but from several of them and can use that information along with information about what orbits the satellites are in to figure out how far away it is from each satellite.
And once it knows how far it is from each satellite, if it’s got more than one measurement – in fact you need at least four to get a good fix – it can figure out where on the Earth it is. And that’s essentially how GPS works, is by measuring the distance to several different GPS satellites and saying OK, well the only place that could be that distance from all of these satellites is this place right here.