Hi BOAC,
Don't you use this system?
"Method for determining the heading of an aircraft.
United States Patent 4930085.
A method is disclosed for determining the heading of an aircraft which is equipped with both an inertial navigation system and a device capable of receiving navigation satellite signals and computing the aircraft's position, speed and acceleration therefrom. While the aircraft is still on the ground and before flight operations begin, an initial alignment of the inertial navigation system with an earth-fixed coordinate system is performed, and during this alignment, data corresponding to the initial attitude of an aircraft-fixed coordinate system relative to the earth-fixed coordinate system is also determined and stored. Thereafter, during flight operations, the acceleration vector of the aircraft relative to the aircraft-fixed coordinate system is measured by the inertial navigation system and then transposed into a coordinate system which has been horizontally aligned with the earth-fixed system using the initial attitude data. second acceleration vector, corresponding to the acceleration of the aircraft in the earth-fixed system, is computed using the navigation satellite system. The aircraft's heading is then determined by computing the difference, i.e., the angle between, the two acceleration vectors. The method is amenable to Kalman filtering techniques and, by eliminating the need for highly accurate gyroscopes or magnetic compasses, permits rapid and highly accurate heading determinations to be made at much less expense."