How Satellite Navigation Works in Tunnels and Canyons

Satellite navigation is a marvel until you drive into a tunnel, and the problem is that the signal your navigation depends on cannot pass through solid rock or reach into a deep canyon. Understanding how the system behaves in these places explains both the little glitches you have noticed and the bigger failures that happen exactly where a wrong turn is costliest.The system works by your receiver listening to signals from a constellation of satellites and calculating your position from the tiny time differences between them. This requires a clear line of sight to several satellites at once, and anything that blocks that line of sight, a tunnel roof, a canyon wall, a dense row of tall buildings, degrades the fix. In a tunnel the signal vanishes entirely, and the navigation holds your last known position, then snaps back when you emerge, which is why the map sometimes freezes and then jumps.Modern systems paper over this with prediction, dead reckoning from your last known speed and heading, and with map data that knows where the tunnels are, so the arrow keeps moving even while the signal is gone.

But in a long tunnel with junctions, or a deep canyon with