an electromagnetic sensing apparatus comprising an electromagnetic transmitter , at least one receiver device , at least one calibration device , a switch and a processing means . the electromagnetic transmitter generates a transmitter signal . the receiver device is spaced from the transmitter , senses a receiver local electromagnetic field proximate thereto and generating a receiver signal . the calibration device senses a calibration local electromagnetic field proximate thereto , generates a calibration signal . the calibration device is positioned such that the distance between the transmitter and the receiver device is greater than the distance between the transmitter and the calibration device . the switch is connected between the receiver device and the calibration device . the processing means includes a first input for receiving a signal from one of the receiver signal and the calibration signal and a second input for receiving a signal from the switch . the processing means compares the first input and the second input and monitors distortions in the transmitter signal . the sensing apparatus may include a plurality of receiver devices in a variety of different configurations .