# Accuracy calculated by GPS

I found a lot of pieces of an answer to this question, but no complete and good one.

How does a GPS System calculate it's own accuracy at the moment?

Where does it get the known position to compare measured positions to?

There are many many factors that affect the accuracy of the GPS. However they all go down to there is no "correct" but a more accurate value. Of course you might add some more accurate values to compare your results to - e.g. by using differential GPS which uses a base-station of a very accurate measued (e.g by geodetic information) location.

However a usual GPS calculates its own accuracy based on usual statistics. It gets a number of locations for one single point and calculates its arithmetic mean as well its the standard deviation and many other parameters.

Having said this the accuracy of a GPS-device is mostly determinded by the number of signals (sensors) that measure a given location. On most deviced you have therefor the number of satellites displayed which are currently used/can be seen by your device.

EDIT: Unfortunetaly the US-military (which invented GPS) has a disturbing signal which reduces accuracy from 20cm to 1-2meter.

• Finally a answer that explains it nicely. Thank you very much :) Additional to that, is there a guideline of how many points you need to give a good accuracy? Dec 1, 2015 at 14:09
• No, there is none (as far as I know). It´s as simple as: the more signals the higher the accuracy. However as far as GPS is considdered the current maximum is something above 30. However there is a minum of 4 satelites to get the x, y, z and time-information. This might be different for Glonass or Galileo however. Dec 1, 2015 at 14:15
• Military & civilian is the same gps.gov/systems/gps/performance/accuracy with the right equipment (Professional grade) you will find high accuracy with GPS augmentation gps.gov/systems/augmentations
– Mapperz
Dec 1, 2015 at 14:58
• @Mapperz I already mentioned the differential GPS, however the others were new to me. Thanks for the link. Dec 1, 2015 at 15:00
• Thanks Guys for your answers! Mapperz, that is a very good web page, thanks for that as well. Really amazing how many different elements there are to GPS :) Dec 2, 2015 at 6:58