The thick circles in the drawing show the "pseudo-ranges" caused by our clock's error. The word "pseudo-range" is used in the GPS world to describe measurements that contain errors.

Notice that while Sat A and B's pseudo ranges intersect at point XX, Sat C's pseudo range cannot go through that point. This discrepancy alerts the receiver's computer that there is a clock error.

Since any clock error or offset would affect all measurements, the computer looks for a single correction factor that would allow all the measurements to intersect at one point.

In our example, it would discover that by subtracting a second from each measurement the ranges would all intersect at one point.

With that correction factor determined, the receiver can then apply the correction to all measurements from then on.

And from then on its clock is synced to universal time. Of course this correction process would have to be repeated constantly to make sure the receiver's clocks stay synced. But with it, even the lowliest GPS receiver turns into an atomic-accuracy clock.