"Strapdown" induction compass transmitter with means to compensate for heading errors during turns and during dives and climbs due to the vertical component of the Earth's magnetic field and due to two cycle error

Claim 1

In a strapdown induction compass transmitter system compensated for errors due to the Earth's vertical field and for two cycle errors occurring during turns and during climbing and diving maneuvers, the combination comprising: (a) magnetic sensing means for producing a first modulated signal responsive to the strength of the Earth's magnetic field along one axis of an aircraft, and a second modulated signal responsive to the strength of the Earth's magnetic field along another axis of the aircraft, (b) means to compensate said signals for errors due to the Earth's vertical magnetic field during turns and while climbing and diving including: (1) means to generate a first correction signal proportional to the product of the Earth's magnetic field M, the magnetic dip D, and the roll angle φ, said first correction signal being equal to the vertical field error in the magnetic field sensed along one of the axes of the aircraft during a turn, (2) means to generate a second correction signal proportional to the product of the Earth's magnetic field M, the magnetic dip angle D and the pitch angle θ, said second correction signal being equal to the vertical field error in the magnetic field sensed along the other axis of the aircraft during climbing and diving maneuvers, (3) means responsive to said first and second correction signals to produce flux fields which cancel the flux sensed by said magnetic sensing means along the respective axes of the aircraft due to the Earth's vertical field during turns and while climbing or diving, (c) means for demodulating the compensated first and second modulated signals including, (1) first and second demodulators each having inputs and outputs associated with the magnetic sensing means to produce respectively, first and second demodulated signals from said compensated first and second modulated signals, (2) feedback paths from the outputs to the inputs of said first and second demodulators for producing negative feedback signals, (d) and means to correct said first and second signals for two cycle error including means to reduce the negative feedback in one of said paths by a factor proportional to the cosine of the bank angle 100 and the negative feedback in the remaining paths by a factor proportional to the pitch angle θ whereby the demodulated signals from the outputs of the demodulators are increased correspondingly to cancel two cycle error.