Patent ID: 8013595

Claim:
Magnetic locating apparatus comprising: (a) a transmitter including a plurality of co-located orthogonal transmitter coils and a transmitter circuit operative to apply a signal to each transmitter coil including a main sinusoidal component having a frequency f B , where B is an index denoting one of said transmitter coils, f B being different for different transmitter coils, the transmitter circuit also being operative to drive a particular one of the transmitter coils having a particular index B* with a sinusoidal marker component having a known amplitude and phase relationship to the main component transmitted by the same coil, the marker component having a frequency f′ B* such that the difference between f B* and f′ B* is smaller than the difference between any two of the main frequencies f B ; (b) a receiver including a plurality of co-located orthogonal sensor coils to yield a plurality of sensor signals responsive to magnetic fields sent by the transmitter, the receiver including a receiver circuit operative to process the sensor signals to derive separate components S AB in each sensor signal at each main frequency f B where A is an index denoting one of the sensor coils and to derive a separate component S′ AB* in at least one sensor signal at a marker frequency f′ B* , the receiver circuit also being operative to determine the phase of at least one component in at least one sensor signal from sensor signal components S A*B* and S′ A*B* , where A* is a particular value of A and to determine the position of the sensor coils relative to the transmitter coils based on the determined phase and on the components S AB .