Calibration method for a relative heading sensor

A method of calibrating a direction sensor, said direction sensor having a relative heading sensor and a geomagnetic sensor, said geomagnetic sensor producing a first output signal proportional to the geomagnetic field in an X-direction and a second output signal proportional to the geomagnetic field in a Y-direction, said X and Y directions being perpendicular, the method comprising the steps of:

• rotating said sensors through an angle of at least 360.degree.;
• reading said geomagnetic sensor at each of a plurality of points about said angle;
• reading said relative heading sensor at each of said points;
• finding a first maximum and a first minimum heading in said first output signal;
• finding a second maximum and a second minimum heading in said second output signal;
• calculating a first difference between a first relative heading reading at a point corresponding to said first maximum and a second relative heading reading at a point corresponding to said first minimum;
• dividing said first difference by 180.degree. to obtain a first relative heading conversion constant;
• locating the center of a measurement curve of said geomagnetic sensor relative to a predetermined origin, said measurement curve being defined by said second output signal as a function of said first output signal when the geomagnetic sensor is rotated about 360.degree., said center being at the point (X.sub.0, Y.sub.0) wherein
• X.sub.0 =(X.sub.max +Y.sub.min)/2
• Y.sub.0 =(Y.sub.max +Y.sub.min)/2
• and wherein X.sub.0 is the first output signal at the center, Y.sub.0 is the second output signal at the center, X.sub.max is the first maximum, X.sub.min is the first minimum, Y.sub.max is second maximum and Y.sub.min the second minimum;
• determining a first and a second radius of said measurement curve, said first and second radii being defined as
• a=(X.sub.max -Y.sub.min)/2
• b=(Y.sub.max -Y.sub.min)/2
• wherein a is the first radius and b is the second radius;
• calibrating said relative heading sensor, using said first relative heading conversion constant with a third relative heading reading from said relative heading sensor to generate a calibrated relative heading;
• and
• employing said center of said measurement curve and said first and second radii with said first and second output signals to generate a calibrated absolute heading.