A system comprising: a transmitter that includes at least three coils, the transmitter configured to generate magnetic fields； a sensor that includes at least three coils, the sensor configured to provide sensor signals that correspond to the magnetic fields generated by the transmitter； and a computing device in communication with the transmitter and the sensor, the computing device configured to compare a first sensor signal and a second sensor signal, and based on the comparison, determine whether any of the sensor coils are locked to a corresponding frequency out-of-phase.
