Abstract: A weapon system comprising a mobile platform (1) e.g. an aircraft incorporating a first three-axis attitude reference sub-system (13), and a guidable vehicle (3) launchable from the platform (1) and incorporating a guidance sub-system (15) incorporating gyros (17) wherein correction of gyro parameters, i.e. scale factor and zero offset, of the vehicle attitude reference sub-system (15) is effected prior to launch of the vehicle (3) on the basis of repetitive comparisons of attitude data as measured by the platform and vehicle sub-systems (13, 15). The vehicle (3) may comprise a dispenser of a number of munitions (5) each itself incorporating a guidance sub-system (25) incorporating gyros (27), in which case corrections of gyro parameters of the missile guidance sub-system (25) may be effected prior to missile launch on the basis of repetitive comparisons of attitude data as measured by the dispenser and munition sub-systems (15, 25).