Abstract: The invention contemplates use of a single magnetic-field (flux) sensor mounted within an elongate probe housing, to continuously generate telemetered electric signals from which the instantaneous orientation of the housing is at all times known and from which changes in orientation may be tracked as a function of probe depth within the borehole in which the probe housing may be displaced. The telemetered electric signals provide accurate and current indications of (a) magnitude of the horizontal component of the earth's magnetic-field lines, (b) direction and magnitude of probe-axis tilt with respect to the gravitational vertical, and the direction of the earth's magnetic field, and (c) a probe-housing frame-angle reference; provision being made within the probe housing for performing substantially all discriminating, detecting and other problem-solving functions, whereby the telemetered signals are directly utilizable by recording and/or display equipment at surface end of the borehole site.