GNSS technology determines your position by receiving signals from multiple GPS/GNSS satellites at the same time. These signals are then compared and used to calculate your position. However there may be cases when the satellite signal is lost or degraded, for example when you enter a tunnel or where multipath propagation occurs.

Untethered Dead Reckoning is a method of estimating ones position by using sensors to calculate the distance traveled from a known position. When a GNSS system like automotive navigation stops receiving satellite signals, they can use gyro sensors, accelerometers, speed pulse sensors and other sensors to estimate the distance and direction of movement from the last known location. This way a the GPS solution in a car can continue to track your position in a tunnel even when the satellite signal is weak or not available. This is known as Untethered Dead Reckoning.