What Are the Sources Of Inaccuracy in a GPS?

Possibly the best way to find any given location on the Earth, is with the use of a GPS.  Global Positioning System, GPS, refers to a satellite network that encircles the Earth.  These satellites provide information to GPS receivers that guide us to our destination.  While GPS navigation uses the most advanced technology available, it still has a small margin of error.

The GPS navigation system and GPS receivers are highly accurate, but there are still issues that cause inaccuracies.

1. Atmospheric delays are caused because of signal speed.  GPS signals travel through space at a different rate than they do when they pass through the ionosphere and the troposphere.  


2. Signal redirection, or multipath, causes errors when items such as buildings or rocks reflect the signal, lengthening its path to the receiver.


3. The receiver clocks are less accurate than the satellites' atomic clocks. The discrepancy between the clocks causes minimal errors and is known as clock drift.


4. Orbital or ephemeris errors are caused when the receiver misinterprets the exact position of the satellite.


5. Satellite visibility affects the data the receiver generates.  When the receiver can locate multiple satellites it provides greater accuracy.  Many factors can affect the number of satellites a receiver can locate.  Buildings, geographic features and plant material can all act as blinders to the receiver.


6. Since GPS satellites orbit the Earth twice each day, they are continually in motion.  A combination of the Earth's shape and the satellites' orbits causes satellite shading or satellite geometry.  At different times during their orbit, the satellites will end up in a bunch or in a line.  During these times satellite signal is inhibited.  When the satellites occupy a more openly spaced geometric net, satellite signal is at its best.


7. Prior to May, 2000 GPS receivers experienced intentional signal distortion called Selective Availability, or SA.  The Department of Defense instituted SA as a protective intervention.  Errors caused by SA could affect positioning by a much as 70 meters.  SA was discontinued so commercial and civilian GPS users could obtain more accurate data.