i need to know what is the accuracy of the GPS sensor for Samsung Galaxy S 3 and Nexus 4 .
i need to know how much its accuracy, I didn't find the information which i need, ex: how many meters the accuracy will be ?
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As a generic information, the German Wikipedia GPS page states an accuracy of 10m or better for civilian use. While this does not explicitly state anything on Android's implementation, you can find some test results on urremote confirming this. How accurate exactly the data are, very much depends on the device: according to those tests, accuracy varied between 2.5 m and 6.3 m with the tested devices.
A further reading of interest is How accurate is Android GPS?, which might add some more background information. Further you might want to check into Improve Android GPS Accuracy By Having The Accurate Time, or test your own device using the GPS Test app.
According to Qualcomm (who makes the SoCs used by the GS3 i747 (also the CDMA and T999 versions.) and the Nexus 4), under optimal conditions, the combined GPS/GLONASS lock is capable of being accurate to 2 metres.
To determine actual possible accuracy in use is more complex and depends on time and location, as the number of satellites visible varies due to their orbits. This Silverlight app will allow you to determine this (Be sure to only select GPS and GLONASS under the satellite library. The GS3 and Nexus 4 are not capable of using the other constellations.). The important bit is the DOP (Dilution of precision). Multiply the DOP reported by the app (if greater than 1) for the relevant time by that 2 metres to determine the actual possible accuracy.
Note that this is only the possible accuracy. Obstructions (trees, buildings, etc.) that block the view of the sky may also block the view of some satellites. If you know the obstructions, you can enter them into the app and it will include them in its work.
Also, some apps, like GPS Status & Toolbox, will give you your error on the fly.