A method and system for locating a source of a signal. The source may be a wireless marker that is implanted in an object, such as a human body. The signal generated by the marker is a magnetic field. The location system uses an array of sensors to measure the magnetic field of the marker at various sensor locations. The location system compares the set of actual measurements to sets of reference measurements for various known locations within a bounding volume. Based on the comparisons, the location system identifies the set of reference measurements that most closely matches the set of actual measurements. The location system then uses sets of reference measurements for known locations near the closest known location to more accurately determine the marker location when it is not actually at one of the known locations.