[kika]

There's such thing as inertial navigation (that's how missiles fly even with GPS assistance), and potentially you can use a phone's accelerometer for that. We have a datacenter in downtown SJ and I was amazed when first parked there on the -3 floor underground. My car's in-dash nav works perfectly there :-) There's no even remote chance that GPS can work 3 stories down in a solid concrete building.