DR - dead reckoning

Dead reckoning is a “backup” feature that allows GPS navigation systems to estimate your location in situations where it’s unable to receive healthy signals from GPS satellites. The location calculated by dead reckoning depends on the last known location and usually becomes less accurate the longer you go without GPS reception - say a long long tunnel. This feature is not usually available in entry level models.

The technology varies but dead reckoning usually relies on an accelerometer, 3D gyroscope, and/or your vehicles speed sensor.