When satellite signals disappear in tunnels and parking garages, dead reckoning keeps your position alive. Here’s how it works and how to integrate it.

The problem: GNSS doesn’t work everywhere

City navigation is exactly where positioning matters most — and exactly where GNSS struggles. Tunnels, underground parking, elevated roads and dense high-rise canyons all block or corrupt satellite signals. For insurance telematics, fleet management and ride-hailing, a position that stops updating in a tunnel is a serious product flaw.

What dead reckoning does

Dead reckoning (DR) fuses GNSS with inertial measurement. A 6-axis IMU (accelerometer + gyroscope) continuously tracks motion, so when GNSS drops, the module keeps computing position from the last known fix plus measured movement. The two inputs correct each other: GNSS anchors the drift of the IMU, and the IMU bridges the gaps where GNSS is blind.

The key spec: INS error

The metric to check is INS positioning error — typically expressed as a percentage of distance travelled. A module quoting “less than 2% of distance” stays within about 20 meters after a 1 km tunnel, which is plenty for most telematics. Raw IMU data at 50 Hz is also useful if you want to run your own sensor-fusion on the host.

Integration considerations

  • Mounting: choose a module that supports “free installation” — no precise orientation required — to avoid tight assembly tolerances.
  • Calibration: most modern DR engines self-calibrate in motion; avoid modules that demand a manual calibration routine.
  • Power: DR adds little — a fused module like TAU951M-K2 tracks at ~35 mA, comparable to RTK-only parts.
  • Data output: confirm the module outputs continuous position during outages, not just raw IMU samples.
“For vehicles that spend real time underground or between towers, a DR-capable module is no longer optional — it is the difference between a broken trace and a continuous one.”

The TAU951M-K2 combines dual-band RTK with a built-in 6-axis IMU and free-installation mounting — designed for exactly these urban cases. See the RTK + DR module.

Dead ReckoningIMUUrban Navigation