Abstract

The separation between first and second points is determined. At the first point are first, second and third mutually orthogonal coils that are excited in sequence, so that first, second and third magnetic fields are sequentially derived. At the second point are first, second and third mutually orthogonal magnetic field sensors that respond to the first, second and third magnetic fields, respectively to derive responses indicative of the magnetic fields coupled to them. The derived responses are combined in accordance with: ##EQU1## to derive the separation magnitude, where:V11, V12 and V13 are responses of the first, second and third sensors while the first coil is excited;V21, V22 and V23 are responses of the first, second and third sensors while the second coil is excited;V31, V32 and V33 are responses of the first, second and third sensors while the third coil is excited.A variable gain element responds to the responses derived from the sensors for selectively modifying the amplitude of signals transduced by the sensors. An analog to digital converter responds to the selectively modified signals as derived by the variable gain element for deriving a multi-bit digital output signal having a predetermined optimum range. In response to the magnitude of the multi-bit digital output signal the gain of the variable gain element is controlled to maintain the multi-bit digital output signal in the range.