detailed instructions depend on the type of phone and the version of Android - someone fix this ?!
Calibrate/document your phone's capabilities
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Install the "GPS Test" app, this one: GPS TEST and check whether your phone is using SBAS "Satellite-based Augmentation Services", which is another name for EGNOS, the differential GPS system.
You need to record if SBAS is in operation whenever you record the position of a cave entrance (and ideally whenever you take a photo too, but that would be unrealistic).