VaneWhy Your Phone's Speedometer Stops in a Tunnel
Satellite signals do not go through rock, concrete or much steel. Here is what happens to a phone's speed reading when they disappear, and why carrying on without them is harder than it sounds.
Updated 1 October 2026
The signal simply is not there
GNSS satellites (GPS, Galileo, GLONASS, BeiDou) orbit about 20,000 km up and transmit at very low power. By the time a signal reaches the ground it is far weaker than the background radio noise, and a receiver finds it only by knowing exactly what pattern to listen for. Tree canopy weakens it, a car roof weakens it a little more, and earth, concrete and rock stop it entirely. Inside a tunnel or an underground car park there is nothing to receive.
What phone apps do when it goes
A phone keeps its last fix, then reports nothing new. Apps handle that in one of three ways, and you have probably seen all three:
- Freeze: the last speed stays on screen, so you appear to drive through the tunnel at a constant 87 km/h whatever you actually do.
- Drop to zero: the app treats no new position as no movement.
- Jump: as the signal fades at the entrance, a few poor fixes arrive with large errors, and position-based speed spikes or swings.
A frozen or zero reading is wrong in a way that looks right, which is the worst kind of wrong for an instrument. Showing no reading until the signal returns is the honest option. That is what our online speedometer does, and what Vane does.
Coming out the other side
When the sky comes back, the receiver has to reacquire satellites. Because it still knows roughly where it is and what time it is, this usually takes only a few seconds, much less than a cold start. The first fixes can be rough, especially in a city where buildings reflect the signal, so the speed may wobble briefly before it settles.
What cars do instead
Built-in car navigation keeps your position moving in a tunnel by dead reckoning: it counts the wheels turning, from the same sensors the anti-lock brakes use, and takes the turn rate from a gyroscope, then advances the last known position along that path. The wheel sensors are the key part. They measure distance directly, so the error grows slowly, and a tunnel of a few kilometres is easy.
Why a phone cannot simply do the same
A phone has no connection to the wheels. Its only motion sensors are an accelerometer and a gyroscope, so to get speed it has to integrate acceleration, adding up tiny changes over time. Every small error in the sensor, or in working out which way is down, is added up as well, and the speed error grows steadily with time. Position, which integrates speed again, drifts faster still. A phone in a cup holder or a pocket also moves relative to the car. Bumps, turns and a hand picking it up all look like vehicle motion.
Research systems can narrow the gap with careful constraints and learned models, but on consumer phone sensors the drift over a minute or two in a vehicle is still large enough that a displayed speed would often be wrong by more than a car speedometer's tolerance. For a speed you are going to read while driving, that is not good enough, so the sound choice is to stop and say so.
What does keep working underground
- The compass, which reads the Earth's magnetic field, although the steel in tunnels and cars can bend it.
- The barometric altimeter, which reads air pressure, so it still shows a climb or a descent. Tunnel ventilation and air pressure waves from trains can nudge it.
- Walking pace, which a phone can estimate from steps on foot, because each step has a roughly known length.
Sources
- US Government, GPS Standard Positioning Service Performance Standard, 5th ed. (2020): minimum received L1 C/A power −158.5 dBW.
- US Government, GPS accuracy: blockage by buildings, bridges and trees, and indoor or underground use.
- u-blox, NEO-6V automotive dead reckoning: GPS combined with wheel ticks and a gyroscope for positioning in tunnels.