A method and system for using GPS signals and a magnetic field to track an underground magnetic field source. A tracker having an antenna for detecting the magnetic field and a GPS receiver is coupled to a processor. The magnetic field is used by the antenna to direct the tracker to a field null point. Once multiple measurements of the field are taken, the changes in signal strength as the absolute position of the tracker is changed, are used to determine whether the closest field null point is in front of or behind the underground beacon. The position and depth of the beacon can then be estimated.