A method for tracking an object moving relative to a moving platform, the moving platform moving relative to an external frame of reference, the method comprising: updating an inertial orientation estimate of the object relative to the external frame of reference using at least one angular rate sensor comprised in a sensor unit fixed to the object; obtaining measurements from a platform-mounted inertial navigation system of the orientation of a platform with respect to said external frame of reference; at certain measurement times, obtaining measurements from a non-inertial sensor of the orientation of the sensor unit relative to a measurement reference coordinate frame fixed with respect to the platform, and characterised by: combining said platform-referenced non-inertial orientation measurements with the INS measurement of platform orientation to form an indirect measurement of the orientation of said object with respect to said external frame of reference, and fusing said indirect measurement with said inertial orientation estimate of the object to form an improved estimate of object orientation relative to said external frame of reference; and combining said improved estimate of object orientation relative to said external frame of reference with said INS measurement of platform orientation relative to said external frame of reference to form an estimate of relative orientation of said object with respect to said moving platform. updating an inertial orientation estimate of the object relative to the external frame of reference using at least one angular rate sensor comprised in a sensor unit fixed to the object; obtaining measurements from a platform-mounted inertial navigation system of the orientation of a platform with respect to said external frame of reference; at certain measurement times, obtaining measurements from a non-inertial sensor of the orientation of the sensor unit relative to a measurement reference coordinate frame fixed with respect to the platform, and characterised by: combining said platform-referenced non-inertial orientation measurements with the INS measurement of platform orientation to form an indirect measurement of the orientation of said object with respect to said external frame of reference, and fusing said indirect measurement with said inertial orientation estimate of the object to form an improved estimate of object orientation relative to said external frame of reference; and combining said improved estimate of object orientation relative to said external frame of reference with said INS measurement of platform orientation relative to said external frame of reference to form an estimate of relative orientation of said object with respect to said moving platform. combining said platform-referenced non-inertial orientation measurements with the INS measurement of platform orientation to form an indirect measurement of the orientation of said object with respect to said external frame of reference, and fusing said indirect measurement with said inertial orientation estimate of the object to form an improved estimate of object orientation relative to said external frame of reference; and combining said improved estimate of object orientation relative to said external frame of reference with said INS measurement of platform orientation relative to said external frame of reference to form an estimate of relative orientation of said object with respect to said moving platform.