I wonder how many implementations of the calculations there are. Do all the commercial GPS and phone manufacturers write this on their own, or is there a tested version they can license?
A significant portion of the calculation is usually done in hardware (less 'written' more 'designed'), and yes these things are licensable. Most, though, have no interest in fabricating their own chips (and can't get much out of customizing one) and buy chips off the shelf. There certainly are several independent implementations.
So even a few individuals are able to wrap their heads around the problem (I guess commercial projects would also tend to have one or two people doing significant amounts of the work).
