Most of the technology and circuitry revolving around GPS receivers is in making sure that we get the time spent by the radio wave between the satellite and the receiver correct to the accuracy of nanoseconds. This is what is discussed below.

To identify a point uniquely in a three dimensional space, we usually have three coordinates, X, Y and Z which are mutually perpendicular axes. As in the circle denoting a constant distance of three miles above, in three-dimensional space, a point that is farther by 3 miles lies anywhere in a sphere of radius three miles. We have to find the point of intersection of three spheres to correctly identify an arbitrary point in space.

Small errors in time calculation result in huge distance errors due to the very high speed of light. What are the problems with time calculation? How do you find the time delay between the satellite and the receiver?

Satellites have on board atomic clocks, cesium clocks that weigh 20 kgs and cost $30,000. In spite of having the most accurate timekeeper invented by man, these clocks lose a nanosecond every three hours. The corrections are monitored by satellite earth stations and other parameters like drift in orbit is transmitted back to the satellite. This is included in the information transmitted to the GPS receivers. This way, they can get an accurate time.

The main source of error is the receiver clock. It is anything but accurate. To make up for the huge inaccuracy of the receiver, we use that as an unknown parameter T, the receiver time to our position calculations. So now, instead of three parameters, X, Y and Z coordinates, we have four unknowns, X, Y, Z and T. Since we have four unknowns, we need four equations. Thus, by including another satellite in our position calculations, we can make up for the error in time, because the spheres corresponding to the distance will not intersect at the same point unless the receiver clock error is corrected.

GPS also suffers from all other radio wave propagation problems with obstructions like walls, trees and other vegetation, interference from other transmissions, power lines, etc. The main problem that frustrates proper signal reception is fading due to multipath effect. Signals arriving out of phase after bouncing off from obstacles add up thus leading to interference, which increase noise. The antenna can be constructed in such a way as to minimise these well-known problems

Radio waves don't travel at the speed of light in all media. It varies depending on the refractive index of the medium. The path between the satellite and the receiver is a distance of roughly 20,000 miles. Most of this distance has things that don't matter to us. But, the troposphere and ionosphere reduce its speed. The troposphere is the region right above us and is full of clouds, rain, temperature variations, pressure, etc. In fact, the majority of the atmosphere is here. These errors are very difficult to eliminate. Satellites transmit the composition of the troposphere at frequent intervals.

Ionosphere is full of charged particles and is open to cosmic rays from the sun. In other words, it is full of ions. Ionosphere has had a profound impact on radio transmission. The sun is mostly responsible for the activity and hence, radio propagation models vary widely between day and night. Sunspots, periods of intense solar magnetic activity also impact ionosphere. Using mathematical models, we can account for 50% of the speed variation (speed reduction). The rest 50% is still significant. This can be removed by having two signals of different frequencies compared for the delays. Only advanced GPS receivers have two signals called L1 and L2. These are more accurate.