"*and a freakin' barometer?!" Wow, that makes it really interesting. The huge problem with GPS is height information. My GPS devices have commonly 10-15 m prac

"*and a freakin' barometer?!" Wow, that makes it really interesting. The huge problem with GPS is height information. My GPS devices have commonly 10-15 m practical error in height on cities (urban canyon effect). This could be very interesting for tunnel inertial navigation as makes the inertial problem a 2d issue from 3d.