The homing technique for directing an in-ground boring device through the ground from its particular location to a specific target point is disclosed herein. In accordance with this technique, an electromagnetic dipole field containing a predetermined homing signal is generated at the boring device and...http://www.google.co.uk/patents/US4881083?utm_source=gb-gplus-sharePatent US4881083 - Homing technique for an in-ground boring device
