A navigation system for a spinning projectile, the system comprising: inertial sensors ; a strapdown navigation processor ; a propagator-estimator filter , wherein the strapdown navigation processor is configured to receive as input inertial sensor data from the inertial sensors and navigation corrections from the propagator-estimator filter to generate a navigation solution, the navigation solution comprising at least a projectile velocity estimate and a projectile attitude estimate; an upfinding navigation aid configured to generate an angular attitude measurement indicative of a roll angle of the spinning projectile with respect to the Earth; and a spinning projectile physics model configured to perform calculations utilizing dynamics equations for a rigid body, wherein the spinning projectile physics model receives as input 1) one or more projectile state estimates from the navigation solution and 2) one or more platform inputs indicative of forces acting on a projectile platform, and the spinning projectile physics model outputs at least a set of three orthogonal predicted translational acceleration measurements based on the inputs; wherein the propagator-estimator filter comprises a measurement equation associated with the physics model and the upfinding navigation aid and is configured to calculate the navigation corrections as a function of the navigation solution, the set of predicted translational acceleration measurements, and angular attitude measurement indicative of a roll angle, wherein the measurement equation is computed by the propagator-estimator filter as a function of differences between spinning projectile physics model derived accelerations and inertial sensor derived accelerations, differences between upfinding navigation aid derived attitude measurements and strapdown navigation system derived attitude measurements, and navigation filter states of the propagator-estimator filter . inertial sensors ; a strapdown navigation processor ; a propagator-estimator filter , wherein the strapdown navigation processor is configured to receive as input inertial sensor data from the inertial sensors and navigation corrections from the propagator-estimator filter to generate a navigation solution, the navigation solution comprising at least a projectile velocity estimate and a projectile attitude estimate; an upfinding navigation aid configured to generate an angular attitude measurement indicative of a roll angle of the spinning projectile with respect to the Earth; and a spinning projectile physics model configured to perform calculations utilizing dynamics equations for a rigid body, wherein the spinning projectile physics model receives as input 1) one or more projectile state estimates from the navigation solution and 2) one or more platform inputs indicative of forces acting on a projectile platform, and the spinning projectile physics model outputs at least a set of three orthogonal predicted translational acceleration measurements based on the inputs; wherein the propagator-estimator filter comprises a measurement equation associated with the physics model and the upfinding navigation aid and is configured to calculate the navigation corrections as a function of the navigation solution, the set of predicted translational acceleration measurements, and angular attitude measurement indicative of a roll angle, wherein the measurement equation is computed by the propagator-estimator filter as a function of differences between spinning projectile physics model derived accelerations and inertial sensor derived accelerations, differences between upfinding navigation aid derived attitude measurements and strapdown navigation system derived attitude measurements, and navigation filter states of the propagator-estimator filter . <EOS>