An inertial measurement system for a longitudinal projectile comprising: a first, roll gyro to be oriented substantially parallel to a 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; a controller, arranged to: compute a current projectile attitude from outputs (210, 211, 212) of the first, second and third gyros, the computed attitude comprising a roll angle, a pitch angle and a yaw angle; characterised in that the controller is further arranged to: for at least two time points, compare the computed pitch and yaw angles with expected values for the pitch and yaw angles from an expected trajectory of the projectile; wherein the expected values for pitch and yaw angles as a function of flight time correspond to those expected from planar ballistic flight; for each of said at least two time points, calculate a roll angle error based on the difference between the computed pitch and yaw angles and the expected pitch and yaw angles; calculate a roll angle error difference between said at least two time points; calculate a total roll angle subtended between said at least two time points; 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. a first, roll gyro to be oriented substantially parallel to a 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; a controller, arranged to: compute a current projectile attitude from outputs (210, 211, 212) of the first, second and third gyros, the computed attitude comprising a roll angle, a pitch angle and a yaw angle; characterised in that the controller is further arranged to: for at least two time points, compare the computed pitch and yaw angles with expected values for the pitch and yaw angles from an expected trajectory of the projectile; wherein the expected values for pitch and yaw angles as a function of flight time correspond to those expected from planar ballistic flight; for each of said at least two time points, calculate a roll angle error based on the difference between the computed pitch and yaw angles and the expected pitch and yaw angles; calculate a roll angle error difference between said at least two time points; calculate a total roll angle subtended between said at least two time points; 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. compute a current projectile attitude from outputs (210, 211, 212) of the first, second and third gyros, the computed attitude comprising a roll angle, a pitch angle and a yaw angle; characterised in that the controller is further arranged to: for at least two time points, compare the computed pitch and yaw angles with expected values for the pitch and yaw angles from an expected trajectory of the projectile; wherein the expected values for pitch and yaw angles as a function of flight time correspond to those expected from planar ballistic flight; for each of said at least two time points, calculate a roll angle error based on the difference between the computed pitch and yaw angles and the expected pitch and yaw angles; calculate a roll angle error difference between said at least two time points; calculate a total roll angle subtended between said at least two time points; 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. for at least two time points, compare the computed pitch and yaw angles with expected values for the pitch and yaw angles from an expected trajectory of the projectile; wherein the expected values for pitch and yaw angles as a function of flight time correspond to those expected from planar ballistic flight; for each of said at least two time points, calculate a roll angle error based on the difference between the computed pitch and yaw angles and the expected pitch and yaw angles; calculate a roll angle error difference between said at least two time points; calculate a total roll angle subtended between said at least two time points; 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.