The location of underground objects such as pipes and conduits are located by detecting electrical signals emitted by the underground objects where the radiated signals include spread spectrum modulated RF signals. The system can operate in a passive or active mode, and in the active mode pseudo-noise...http://www.google.ca/patents/US6211807?utm_source=gb-gplus-sharePatent US6211807 - System using spread spectrum modulation for locating underground objects
