A method performed in a medical navigation system includes driving a transmitter at a first frequency and a second frequency to generate first and second electromagnetic fields, wherein the first and second frequencies are sufficiently low such that the first and second electromagnetic fields are frequency independent receiving first and second distorted fields corresponding to the first and second electromagnetic fields, respectively, with each of at least two electromagnetic (EM) sensors attached to a surgical device generating first and second signals in response to receiving the first and second distorted fields, respectively, using each of the at least two EM sensors and determining a distortion in the first and second signals based at least on a distance between the at least two EM sensors and a difference between the first and second signals generated by each of the at least two EM sensors.
