You may be interested in knowing that Android has exposed raw GNSS measurements through public APIs [0,1] so anybody can build their own raw processing engines using techniques like the ones described in the article
Is anyone actually doing that? I see a build of RTKLIB but it's not actually using the Android GNSS API, it's just using your Android phone as a processor to handle data coming from an external Ublox receiver. Aww.
The android API exposes basically the same data that would come down the serial link from a 90's era GPS receiver plugged into a PC.
That's the position estimate, information about satellite orbits, signal strengths and pseudoranges.
The data doesn't include the raw baseband information necessary to, for example, do carrier phase locking or to do a fully software implementation of GPS decoding.
[0] https://developer.android.com/guide/topics/sensors/gnss
[1] https://developer.android.com/reference/android/location/Gns...