A magnetic position tracking system for performing a medical procedure on a patient who is positioned on an upper surface of a table includes a location pad, which is positioned on the upper surface of the table beneath the patient. The location pad includes one or more field generators, which are operative to generate respective magnetic fields and are arranged so that a thickness dimension of the location pad is no greater than 3 centimeters. A position sensor is fixed to an invasive medical device for insertion into a body of the patient, and is arranged to sense the magnetic fields so as to measure a position of the medical device in the body.