1. An inertial navigation system (INS)/global navigation satellite system (GNSS) navigation system comprising a GNSS subsystem including a GNSS receiver and a GNSS antenna, an INS subsystem including an inertial measurement unit and an INS filter, and a relative yaw subsystem configured to calculate relative yaw values based on phase windup measurements of GNSS satellite signals received by the GNSS antenna, the relative yaw subsystem providing the relative yaw values to the INS subsystem the INS subsystem operating the INS filter to determine an updated attitude based on using inertial measurements of the inertial measurement unit and the relative yaw values as an observable of the updated attitude.