Inertial navigation is going to be necessary for critical (propulsive) aspects of the flight.
The vehicle can cruise locked onto celestial objects (sun,star etc) and can be located via range-finding from ground based tracking stations. This will be the primary measurements and can be used to update the inertial models.
A basic inertial navigation system will consist of an attitude measuring platform and perhaps associated accelerometers. The accelerometers can be integrated to provide velocity and position changes, but may not be strictly necessary.
The attitude Platform' probably won't be a gyroscopic platform. The mechanical requirements and mass/power requirements are significantly larger than what can be accomplished with modern electronic strapdown platforms.
A Strapdown IMU uses fixed rotation sensors and mathematically integrates measured rotations into a model of the relative attitude of the vehicle with respect to some reference frame.
The accelerometer measurements can then be rotated into the reference frame and integrated into position and velocities.
The following sections will be a work in progress sensor corrections attitude representation attitude integration acceleration Rotation and Integration and Quaternion math
Everything here is based on the text <Amazon>Strapdown Inertial Navigation Technology</Amazon> on our Book List page.