At on etime almost all GPS' used the same IC's and even circuit boards. They'd repackage it with their own I/O and exterior. Kind of like electronic watches mostly sharing the same guts. I don't know if that's still the case.

For example, I'm pricing upgrading our sub-meter backpack units and all the company is going to do is shift the logic and timing stuff into upgraded package, which includes Blue Tooth and an additional I/O port. Chips are still the same after 7 years.