How does a gps satellite know exactly where it is in space when it transmits its position?

Question

How does a gps satellite know exactly where it is in space when it transmits its position?

Answer 1

the gps networks work by transmitting time pulse between themselves and the ground.  by very accurate measurement of the time delays, the ground based gps can calculate it's position.

Answer 2

It doesn't.

But the ground unit knows how FAR AWAY it is supposed to be.  the ground unit measure the time diffence between the signal of two or three different satallites, and uses this difference to triangulate it's own position.

The GPS satellites are geo-synchronous.  That is, they are always over the same spot on the earth.  The effects of gravity (that casue a satellite to orbit) are so well known that this position CAN be predicted and used.

Answer 3

Very precise orbital models, calibrated against ground stations.

Answer 4

GPS satellites know where they are by transmitting signals to known ground stations.  Timing of these signals helps pin down where they are, which helps refine what their orbit is, which helps them determine where they will be in the near future.  They have to know quite a bit about the atmosphere and General Relativity to make this work.

The real question is, how do we know where the ground stations are?  After all, the continents are floating around on a sea of magma.  And if you're using the ground stations to calibrate the GPS systems, you can hardly use GPS to figure out where the ground stations are.  What do they do?  The pick a spot, and call it the reference spot.  It's totally arbitrary.  So GPS only tells you where you are with respect to a totally arbitrary spot.  Usually you want to know your location with respect to somewhere else.  Like, how far is it from home?  For that, you can use GPS for both locations, and then you can do the easy math for your real answer.

I've really just glossed over the details.  There are more details on the wiki page.  But even that glosses over the real details.  Certified smart people are employed who sweat these details.