# How does GPS work?

Tracking something in 510 million square km on earth surface very accurately is unimaginable.

Helps to GPS technology we could do that it now in very less expensive smart phones. U.S. Department of Defense launched set of satellites in to orbit provide this technology called The Global Positioning System (GPS).

Let’s think about a satellite in space above the earth it knows exactly where it is in space. It has a very accurate clock. It sends out a signal with its position at a particular time. Your GPS receives a signal and knows that the signal travels at the speed of light. So we can work out how far it is from the point in space where the satellite was, when it’s in out the message. So the GPS is that it lies somewhere on a certain size fear centered on with the satellite was. Now let’s use a second satellite, now the gps knows that it falls on the intersection of two particular Spears this means that the gps lies somewhere on a circle in space. Now let’s use a third satellite now the gps nice that it is at one of two particular points on a circle. Not good enough a person needs to know whether he’s inside New York or Boston particularly if he’s a Red Sox fan so we need a fourth satellite then we can determine exactly where we are. So far so good but it doesn’t work. Everyday the gps would accumulate an error of about ten kilometers. Which is over six miles. You see, Einstein’s theories of relativity and not just of academic interest because of special relativity, the fast speed of the satellite means that its clock run slow. Because of general relativity, the satellites increased distance from the massive object that is Earth means that its clock runs fast. Unfortunately the two effects don’t perfectly offset. So engineers adjust the satellites clock to compensate for these effects.