I’m sure that you can guess that the accuracy of a GPS unit is dependant on the time piece in the unit that is sending the signal. If the time in GPS unit is off it can impact the accuracy of the positioning information. Clock errors are not the number one source of inaccurate information in GPS units. The number one source of inaccurate information come from the atmosphere. When the signal passes through the atmosphere to the satellites, it can encounter conditions that can speed it up or slow it down, thus affecting the time and the calculations that are given.Inaccuracies occur when a signal bounces off of a skyscrapers, mountains, or other such terrain. This will then cause distortions that are similar to those that are caused by atmospheric conditions.
Another problem occurs when the satellite updates its position, this takes place every twelve minutes. If a signal reaches a satellite near its update time, the satellite doesn’t know its correct position, and it makes a calculation that is based on bad information. These type of errors are unavoidable with the GPS systems that are being sold to consumers. However for most uses, these problems don’t amount to much.