Why Your GPS Works in a Forest But Fails in a Tunnel

Your phone finds you in a forest. But drive into a tunnel — and it goes blind.

Leaves, clouds, light rain — GPS gets through it all. So why does a concrete tube stop it cold?

GPS rides a microwave signal — the L1 band, 1575 megahertz, wavelength about 19 centimetres. That long wavelength bends around small obstacles, so in a forest it slips past leaves and through canopy gaps — weakened, but readable. Your phone needs only four satellites; open sky gives it a dozen. Lose a few to leaves — no problem. A tunnel is different. Reinforced concrete is full of steel rebar — a conductor that reflects microwaves like a mirror. Nothing reaches you. Zero satellites, no fix.

But your phone doesn't go fully dumb. It switches to dead reckoning — using its accelerometer and gyroscope to estimate movement and keep your dot going, briefly. It's the same trick sailors used for centuries before GPS. For a moment, your phone navigates like an 18th-century ship captain.

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