[michaelt]

Right now, the GPS receivers that support RTK corrections charge a substantial premium for the capability, so you're unlikely to see this in a mobile phone without the phone makers intending it.

Also, you only get ~3cm precision while you have direct line of sight to a reasonable number of satellites, including some that are closer to the horizon. So if you're close to a building, or in a car with a metal roof - the accuracy might not turn out that great anyway.

People doing fancy surveys use a special antenna that rejects reflections from near ground level, but has to be pointed towards the sky. That's undesirable for a phone, where the user might want it in portrait or landscape, might hold it at an angle for readability, and so on. I have no idea if you could achieve respectable RTK performance with an omnidirectional antenna.

So even if you were the iPhone GPS chip selector, and you had the market leverage to get RTK support cheaply and get a killer deal on network NTRIP corrections, you might think the benefits aren't worth it.

I suspect costs will fall precipitously in the next few decades, though, because this sort of tech is useful for self-driving cars, and automotive order volumes tend to make chip prices fall a great deal.