GPS has the advantage of seeing multiple satellites and working with that by design since its one way. Despite that, Japan launched its own satellite to supplement GPS because it had so many issues. I think we’re spoiled by our phones that supplement GPS with cellular signals, wifi and inertial references.
Existing systems also supplement GPS network data with russia's GLONASS, as well as sometimes WAAS if the satellite gets picked up (WAAS is GEO system to provide for additional accuracy, since it's further and requires a path facing south, it's not often picked up on phones)
Can we really do inertial reference with current phone hardware? I would imagine our usecases don't really even give inertial reference a nice playing field.
My old Garmin wouldn’t realize I did a u-turn until I have half way back down the street.
Yes, check the specs for any recent (past 3 years) qualcomm chipset, initia sensors will be included. You can play around with this sensor (and others) using the Phyphox app for ios and android.