Apparatus having a pair of magnetic field generating cables for measuring position of an underground excavator

A position measuring system for determining the position of an underground excavator in a horizontal plane, said system comprising:

• a first cable forming at least a substantial portion of a first loop and associated with the ground to produce a first alternating magnetic field in the ground upon the application of an alternating current to said first cable,
• a second cable forming at least a substantial portion of a second loop and associated with the ground to produce a second alternating magnetic field in the ground upon the application of an alternating current to said second cable, said first and second loops being positioned at least substantially perpendicular to each other,
• a magnetic field detector for detecting said first and second alternating magnetic fields and producing signals representative thereof, said magnetic field detector being associated with the position of said underground excavator, and
• a computing unit connected to receive said signals from said magnetic field detector and to use said signals to calculate the position of said underground excavator in said horizontal plane.