A Global Navigation Satellite System, GNSS/ inertial, INS, navigation system including: a GNSS subsystem including a GNSS antenna that receives GNSS satellite signals and a GNSS receiver that processes the GNSS satellite signals to produce GNSS measurement information; an inertial measurement unit, IMU, with a plurality of sensors including one or more gyroscopes that measure rotation about a z axis, the IMU being configured to produce IMU measurements for the respective sensors, an INS filter a yaw rate constraint subsystem configured to accumulate the z-axis measurements over mechanization update intervals to produce dead reckoning mechanization update values, accumulate the values over a turn rate accumulation period and produce a yaw rate, the yaw rate constraint subsystem being further configured to determine at the start of a next mechanization update interval if the yaw rate exceeds a turn rate threshold, direct the INS filter to perform a zero yaw rate update when the yaw rate does not exceed a turn rate threshold, and wherein the INS filter does not perform a zero yaw rate update and the sensor drift error remains unchecked for the start of the next mechanization update interval when the yaw rate exceeds the turn rate threshold; and the INS filter , when the GNSS measurements information is unavailable, being configured to: process the IMU measurements and perform the zero yaw rate update if directed to do so at the start of the next mechanization update interval, and process the IMU measurements over the next mechanization update interval to produce one or more of INS-based position, velocity and attitude. a GNSS subsystem including a GNSS antenna that receives GNSS satellite signals and a GNSS receiver that processes the GNSS satellite signals to produce GNSS measurement information; an inertial measurement unit, IMU, with a plurality of sensors including one or more gyroscopes that measure rotation about a z axis, the IMU being configured to produce IMU measurements for the respective sensors, an INS filter a yaw rate constraint subsystem configured to accumulate the z-axis measurements over mechanization update intervals to produce dead reckoning mechanization update values, accumulate the values over a turn rate accumulation period and produce a yaw rate, the yaw rate constraint subsystem being further configured to determine at the start of a next mechanization update interval if the yaw rate exceeds a turn rate threshold, direct the INS filter to perform a zero yaw rate update when the yaw rate does not exceed a turn rate threshold, and wherein the INS filter does not perform a zero yaw rate update and the sensor drift error remains unchecked for the start of the next mechanization update interval when the yaw rate exceeds the turn rate threshold; and the INS filter , when the GNSS measurements information is unavailable, being configured to: process the IMU measurements and perform the zero yaw rate update if directed to do so at the start of the next mechanization update interval, and process the IMU measurements over the next mechanization update interval to produce one or more of INS-based position, velocity and attitude. process the IMU measurements and perform the zero yaw rate update if directed to do so at the start of the next mechanization update interval, and process the IMU measurements over the next mechanization update interval to produce one or more of INS-based position, velocity and attitude. <EOS>