There are many factors which can contribute to the decrease of the accuracy of the GPS.
Ionosphere and troposphere delays
The satellite’s speed decreases as it passes through the atmosphere. The GPS system tried to correct this error by automatically calculating an average amount of delay.
Signal multi-path
This causes errors by increasing the time the signal takes to travel between satellite and receiver when the signal is reflected off of interfering objects.
Reciever Clock Errors
Since a receiver’s clock is not as accurate as the satellite clocks, there may be some error in calculating a signal’s travel time. This can cause slight errors in the calculation of the position of the user.
Orbital errors
Also known as, ephemeris errors, these are when there is an error in the satellite’s reported location.
The number of satellites in sight
Since the accuracy increases with the number of satellites visible to the receiver, when satellites are blocked by buildings, mountains, electronic interference, etc., the position reading may be inaccurate, if one is given at all.
Intentional Degradation
Sometimes, the signal is degraded intentionally, such as is the case with Selective Availability, or SA. SA was once used by the US Department of Defense to prevent enemies from using the GPS signals. When the government turned off SA in May of 2000, the accuracy of civilians’ receivers increased greatly.
