Question

Explain why Einsteins general relativity theory is important to the makers of GPS system

Answer 1

The general theory is important to the GPS system due to two effects.  The one usually ascribed to the general theory is that clocks run more slowly in a gravity well.  That is, clocks closer to the earth (or any gravitational body) run more slowly the closer they are to the center of mass of the object.
The other effect can be understood in terms of the special theory of relativity (which is technically encompassed by the general theory and so also applies) and that is time dilation due to relative motion.  The fact that the GPS satellites are in motion as seen from the earth's frame of reference means the clocks on board are moving slower compared to our clocks from our frame of reference.
Note the relative motion effect is just that, relative.  From the satellite's frame of reference the surface of the earth is what is moving and therefore it sees our clocks as moving slower than its own.  There is no paradox here since if either of us accelerated to the other frame of reference to compare them, they would then be running at the same rate.
The general relativity effect, however, is not relative.  The clock near the earth is running more slowly in all frames of reference. 

Edit:
I guess I should point out that timing is everything in GPS, since the satellites are moving very fast around the earth compared to cars and people on the surface.  So without exact timing, the system would lose precision quickly.  After about a day or so it would have our positions off by several kilometers but it's supposed to be accurate within a few meters.

Answer 2

Albert Einstein's general theory of relativity is one of the towering achievements of 20th-century physics. Published in 1916, it explains that what we perceive as the force of gravity in fact arises from the curvature of space and time. Einstein proposed that objects such as the sun and the Earth change this geometry.