Great project, thanks for posting it!

It just so happens I've got an RTL-SDR, a GPS receiver that outputs raw pseudoranges, and a signal splitter that lets me put the signal from one antenna into two receivers.

So if you like I can get the pseudoranges out of a commercial GPS receiver, and the raw signal from an RTL-SDR at the same time, which might help you pinpoint your last bit of location inaccuracy.

Would you be interested in that? Or do you consider this project complete?