If there's a GPS signal then it may be possible to use the differential phase satellite between two phones to get relative position down to a meter or so (kind

If there's a GPS signal then it may be possible to use the differential phase satellite between two phones to get relative position down to a meter or so (kind of like a super crappy rtk taking advantage of the pps stabilițy). Not sure the gps devices on phones expose enough information for this. Differential rssi of cell/wifi networks is another indicator but probably not very accurate.