The position accuracy is derived from the hdop (Horizontal Dilution of Precision) parameter calculated by your phone’s GPS chip.
With our cellphones, in open sea, the accuracy is generally around 5 meters.
If you have a phone that picks up the Galileo constellation, you will have an accuracy to the order of the meter. You can view the list of Galileo compatible phones here.
As of Q3 2018, the new cellphones should be equipped with precise GPS, with an accuracy around 30cm…
In the meantime, you can use an ultra-precise Bluetooth GPS receiver for Galileo satellites and supporting EGNOS technology in Europe or WAAS in the United States. You will obtain an accuracy well below the meter. The best today is the GNS2000 Plus.
To use it, you need to:
- load an application such as  Bluetooth GPS
- Allow the Developer options: in the Android menu, go to: Settings / System / About the device / Tap up to seven times the build number to unlock the Developer options.
- In Developer options / Select mock location app menu, select Bluetooth GPS as the mock location provider.