The compass system of the present invention utilizes an improved calibration routine in which a processing circuit of the compass recalibrates the compass each time three data points are obtained from a magnetic field sensor that meet predetermined criteria. The method includes steps of: determining an expected successful transmit time Ek, for packets of type k, for each of the set of packet types; choosing an optimum packet type for which the value EkLk is a minimum; and partitioning the payload data into packets of the optimum packet type. A first closed cable loop passes through both of the holes and a first hooking point, so that a first cable is movable within the holes. The data lines are intersected with the scan line groups.