If I tell you that I am 3 miles away from home, I could be potentially anywhere in the circumference of a circle of radius 3 miles from home. Not enough information? If I also tell you that I am 1 mile away from my workplace (MindTree Consulting), does it help you? Yes, but not fully.

I could be at point A or point B. To resolve this ambiguity, we could use one more landmark. Thus, we see that to locate a place on earth, we can use landmarks. To improve the accuracy, we can specify as many landmarks as we please. How does GPS use this principle? GPS uses satellites as landmarks in the place of my home and MindTree to specify my location. Since satellites are thousands of miles above the earth, we can "see" them from a lot of places on earth. It's not enough if we are able to see just one satellite due to the reason we saw above. GPS uses 24 operational satellites for position determination. These are positioned at 6 different planes, four in each plane.

Satellites offer us both advantages and disadvantages. The advantages are that they are high up in space enabling us to see them from several vantage points either on earth or above (We can't use it below earth because electromagnetic waves don't travel below. The same is true under water.) They give us lot of information and they can be queried from earth. Unfortunately the advantages end here. The disadvantages are that they lead to a lot of errors in calculation due to various reasons. We will examine them below.

Hey, I missed out something. How do I determine the distance between the GPS receiver and the satellite? The same principle used by RADAR is used here. From the time taken by the signal to travel between the satellite and the receiver and the speed of the wave (c=300,000 km/s), we can determine the distance. The same principle is applied with three satellites independently to determine the location. Simple? Not quite. That is what is discussed in the next section.

The works!
The above explanation is a very simplistic view of what really happens.

This is where life gets a bit complicated. The vital parameters to determine are the time taken for the signal to traverse the distance between the satellite and the GPS receiver. The time needs to be more accurate than we can imagine because a billionth of a second's error leads to an error of 300 centimeters! This can get compounded, as we need to take at least three measurements to determine the position. The satellite's position is continuously changing in predictable ways as they orbit the earth, but there are extraneous influences like the gravitational pull of the sun and the moon, which can skew the satellite's real orbit.