Does anyone know a good method, or library to match 2 gps locations based on proximity to each-other?

while being fed a few 100 GPS locations per device per hour.

I currently do it by having the client send their Geohash to a database, then request all neighboring hashes around it and all other clients in those hashes, then get their GPS to check for distance.

But this does not seem to work flawlessly, it works on tests and emulators (its for phones) but on real phones it seems to be hit an miss, but no errors, the event just does not seem to fire.