24 GPS satellites carry atomic clocks

[PPonte]

Quoted from http://hyperphysics.phy-astr.gsu.edu/hbase/gps.html#c1"

Adding a third as in the GPS system locates it at one of two discrete points where the three spheres intersect. The correct one is easily chosen to get a precise location. The 24 GPS satellites carry atomic clocks to give them the accuracy necessary for position measurement.

How do they know which of the points is the correct one? (see sentence in blod)

Thank you!

 

[George Jones]

I took a guess that one of the points doesn't lie on surface the Earth, and it seems that I was partially correct - see http://www.trimble.com/gps/how.html" .