Abstract: Carrier signal cycle slip errors occurring at a receiver in a satellite-based navigation system, such as NAVSTAR GPS, are detected. A continuous navigation signal is received from a satellite. Cycles of the carrier wave of the navigation signal are counted over a time &Dgr;t. The cycle count and the time &Dgr;t are used to compute a first velocity of the receiver along a vector between the receiver and the satellite. A second velocity of the receiver is received from an inertial reference unit associated with the receiver. A component of the second velocity along the vector to the satellite is computed. A difference between the first and second velocities is then computed and compared to predetermined value. If the difference exceeds the predetermined value, then a cycle slip error is indicated.