Regarding high latitude navigation. With two gyro compasses fitted there is no problem with polar flying. Gyro compass drift rates are montoired alternately during the north bound leg until passing 80 N when the magnetic coupling to both is switched off. The gyros are then corrected for the recorded drift rate every fifteen minutes until passing 80 N southbound again. RAF Britannias used the method on the aero-systems training courses well into the 1970s.