A system is described for use with a magnetometer sensor to provide automatic cancellation of comparatively large steady state ambient magnetic field effects in order to sense the very small magnitude, very low frequency signal information. The system which is effective at the emplacement and initial actuation of the sensor as well as upon the subsequent occurrence of a gross ambient field change, causes a nulling voltage, electrically controlled in predetermined increments, to eliminate the ambient field effects. The resultant magnetometer sensor output responds to input information with full sensitivity and without the direct passband constraints introduced by other ambient nulling techniques.