How a GPS works

[LandSailor]

For anyone who is interested...a short 3 min movie from Catalyst on how a GPS works.

[ninjapuppet]

Thats interesting.

I knew that 3 satellites were required for triangulation but i always wondered why my Garmin would not give me a location once 3 satellite signals were locked.
It always had to wait for the 4th one.

[LandSailor]

Thats interesting.

I knew that 3 satellites were required for triangulation but i always wondered why my Garmin would not give me a location once 3 satellite signals were locked.
It always had to wait for the 4th one.

Yeah I wondered how the satellites know their positions so accurately but I guess they must use the same process in reverse.
Triangulating of multiple signals from fixed ground stations.

[michael_p]

Yeah I wondered how the satellites know their positions so accurately but I guess they must use the same process in reverse.
Triangulating of multiple signals from fixed ground stations.

My understanding is the satellite positions are refined using ground based radar. Good explanation here: http://www.trimble.com/gps_tutorial/howgps-positions.aspx

[Rob A]

Im a bit dim. Whats the fourth satalite for? How come the clock in the GPS cant take the time data from any of the other three satelites?

[Rob A]

Actually the catalyst explanation seems to be wrong? Its nothing to do with the intersection (distance or angle) to a point on earth, as implied by the video, but trilateration of the distances from the GPS (which is resting on the ground) to the satelites. Yes? No?
Untill someone brighter comes along I'll run with Michaels trimble reference.

[Rob A]

Bingo, with the fourth the "receiver looks for a single correction factor that it can subtract from all its timing measurements that would cause them all to intersect at a single point." Trimble ref up^
Trimble relegates Catalyst art departments obtuse grasp on the subject to the waste bin.

[Rob A]

Anyone watch SBS? Apparently the mass drag bends time a billionth of a second every day at the altitude of GPS satelites orbit, which uncorrected would be worth about six miles on a GPS fix. There are some smart cookies programming to get us the right stuff.

[madmacca]

Can you imagine. There are a bunch of computer programmers and trigonometry PhD's who have been working on coming up with the coding for GPS for 18 months. And then an astrophysicists walks in and tells them they have forgotten to allow for mass drag and the bending of space time.

"And that's when I shot him, your Honour".

[LandSailor]

a billionth of a second every day

..so if my GPS time is out by a *full* second does that mean my location fix is only accurate to within 6 billion miles. Hmmm...that explains a few things on a recent trip : )

[Rob A]

..