Inertial navigation system with compensation of roll scale factor error

Abstract

An inertial measurement system () for a longitudinal projectile, comprising a first, roll gyro to be oriented substantially parallel to the longitudinal axis of the projectile; a second gyro and a third gyro with axes arranged with respect to the roll gyro such that they define a three dimensional coordinate system. The system further comprises a controller (), arranged: —to compute a current projectile attitude from the outputs of the first, second and third gyros, the computed attitude comprising a roll angle, a pitch angle and a yaw angle; —for at least two time points, to compare the computed pitch and yaw angles with expected values for the pitch and yaw angles; —for each of said at least two time points, to calculate a roll angle error based on the difference between the computed pitch and yaw angles and the expected pitch and yaw angles; —to calculate a roll angle error difference between said at least two time points; —to calculate the total roll angle subtended between said at least two time points; —to calculate a roll angle scale factor error based on said computed roll angle error difference and said total subtended roll angle and apply the calculated roll angle scale factor error to the output of the roll gyro.