And somehow GPS is surprisingly stable along the I93 when it goes thorough The Big Dig. Thankfully - it’s a warren.
It might be dead reckoning working remarkably well I suppose.
The built in navigation system in my car seems to work in tunnels, presumably because it has data from the car itself to use for dead reckoning, but I haven’t found that to be the case with navigation apps or standalone GPS units.
It would require reading data out of the OBD2 - greatly useful, but a Bluetooth dongle needs to be installed in the engine, first. Hence, quite rare: https://github.com/osmandapp/Osmand/issues/3204