Compensated compass system

Claim 1

APPARATUS FOR DETERMINING THE DIRECTION OF THE EARTH''S MAGNETIC FIELD COMPRISING A PLURALITY OF SUBSTANTIALLY IDENTICAL MULTI-COIL MAGNETIC FIELD DETECTORS ADAPTED TO BE MOUNTED SEPARATELY IN SPACED LOCATIONS ON A VEHICLE AND EACH ADAPTED TO PROVIDE OUTPUT SIGNALS ON ITS RESPECTIVE COILS THAT DEFINE THE DIRECTION OF THE SUBSTANTIALLY HORIZONTAL COMPONENT OF THE EARTH''S MAGNETIC FIELD AT ITS RESPECTIVE LOCATION, SAID OUTPUT SIGNALS INCLUDING ERROR SIGNAL COMPONENTS PRODUCED BY SUBSTANTIALLY HORIZONTAL DEVIATION MAGNETIC FIELD COMPONENTS AT EACH LOCATION, THE DIRECTION OF SAID DEVIATION FIELD AT EACH LOCATION BEING THE SAME BUT THE MAGNITUDE THEREOF DIFFERING BY A KNOWN FACTOR, AND MEANS RESPONSIVE TO BOTH DETECTORS FOR PROVIDING A MEASURE OF THE TRUE MAGNETIC HEADING OF THE VEHICLE, SAID LAST-MENTIONED MEANS COMPRISING MEANS INTERCONNECTING CORRESPONDING COILS OF SAID DETECTORS TO COMBINE THE CORRESPONDING OUTPUT SIGNALS OF THE TWO DETECTORS IN A SUBSTRACTIVE MANNER, SIGNAL MODIFYING MEANS IN SAID INTERCONNECTION MEANS FOR SUBSTANTIALLY EQUALIZING THE DEVIATION SIGNAL COMPONENTS IN THE CORRESPONDING OUTPUT SIGNALS OF SAID TWO DETECTORS, THEREBY TO PRODUCE A PLURALITY OF RESULTANT SIGNALS DEFINING SUBSTANTIALLY ONLY THE HORIZONTAL COMPONENT OF THE EARTH''S FIELD, A CONTROL TRANSFORMER SYNCHRO HAVING COILS RESPECTIVELY COUPLED TO RECEIVE SAID RESULTANT SIGNALS AND ADAPTED TO PRODUCE AN OUTPUT SIGNAL WHENEVER THE AXIS OF THE ROTOR WINDING IS IN A DIRECTION DIFFERENT FROM THE DIRECTION NORMAL TO THE MAGNETIC FIELD ESTABLISHED IN SAID CONTROL TRANSFORMER BY SAID RESULTANT SIGNALS, AND DIRECTION INDICATING MEANS COUPLED TO SAID CONTROL TRANSFORMER AND RESPONSIVE TO THE OUTPUT SIGNAL THEREFROM TO PROVIDE AN INDICATION OF THE TRUE HEADING OF SIAD VEHICLE.