You don't need a local atomic clock for fairly accurate positioning using 3 satellites it just takes longer. Basically, what you need to know is distance from each satellite and an atomic clock let's you find that directly. However, GPS satellites are moving so much faster than you that at over time you narrow down the possible distances involved fairly quickly. Though this does take a lot of processing and is not on any consumer device I know of.

Also of note, phones get a reasonable fix using base towers which can speed up GPS calculations though I don't know of any phone that does this.