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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?

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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? – Sanny Dec 1 '15 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. – HimBromBeere Dec 1 '15 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 '15 at 14:58
  • @Mapperz I already mentioned the differential GPS, however the others were new to me. Thanks for the link. – HimBromBeere Dec 1 '15 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 :) – Sanny Dec 2 '15 at 6:58

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