> Also, the GNSS software in most phones is sadly unable to accept the correction data from any of these systems, regardless of whether it's a nationwide network or your personal setup. This is purely a software limitation on the vendor GNSS stack, but sadly there is not enough demand for this. (An app will not fix this, we're talking vendor specific low level system code here.)

I don't think that's true. Android surfaces raw GNSS measurements including carrier phase (sub wavelength measurements) to do centimeter level positioning through the raw measurements API [0].

There's even an API to specify the phone antenna phase pattern to correct the carrier phase measurements (source: I implemented it [1]). For those that aren't familiar, the idea is that the antenna pattern on phones isn't perfectly symmetrical, and depending on the direction of the incoming signal, it may appear longer. Knowing the antenna pattern, you can correct for this.

[0] https://developer.android.com/reference/android/location/Gns...

[1] https://developer.android.com/reference/android/location/Gns...