GPS satellites orbit the earth at an altitude of about 20,200 kilometers. They transmit signals to GPS receivers on the ground, which use those signals to calculate their location. The signal from a satellite can take up to 0.24 seconds to reach a GPS receiver on the ground.
The accuracy of the GPS system depends on several factors, including satellite geometry, number of visible satellites, atmospheric conditions, and receiver quality. Good satellite geometry means that the satellites are evenly distributed around the sky and that there is a clear line of sight between four or more satellites and the receiver. The more satellites a receiver can see, the better its position fix will be. Atmospheric conditions can cause errors in the signal by delayings its arrival time at the receiver or by refracting (bending) the signal as it passes through layers of ionized gas in Earth's upper atmosphere. Finally, poor-quality receivers may have clock inaccuracies or other problems that degrade their performance.
Most consumer-grade GPS receivers have an accuracy of 3-5 meters under good conditions. However, this accuracy can degraded quickly under poor conditions (e.g., if there are few visible satellites or if atmospheric conditions are unfavorable).