angle i as measured from axis y 1 is determined by determining whether a maximum or minimum value of the boresight angle b is measured the first derivative of the angle i then may be derived in any conventional way to determine the rate of rotation di dt of the engagement plane it will be observed that movement of the target along its course line t c and the range between the projectile and target are both factors contributing to the rotation of the engagement plane as the target moves from pt 1 to pt 2 this means that a the proper lead angle to effect impact does not lie in the engagement plane existing when the projectile is at point p 2 and b the trajectory of the projectile should be changed to compensate for the rotation of the engagement plane digressing for a moment it will be obvious to one of skill in the art that in theory at least a 34 pursuit 34 mode of guidance could be adopted here that is to say if forces were applied during flight to maneuver the projectile in successively existing engagement planes the 34 rotation of the engagement plane 34 could be ignored and the equipment needed to establish an inertial roll reference plane could be eliminated however there are practical considerations which make it here impractical to adopt a 34 pursuit 34 mode of guidance thus as is well known.