Radio acoustic position finding in hydrography

In hydroelectric surveys it is difficult to fix the positions of the soundings when the survey ship is beyond the range of visibility of shore objects; the radio-acoustic method is designed to help in meeting this difficulty. Temporary shore stations are set up, each equipped with a microphone placed in the water in a known position and connected with an amplifier ashore through a cable; the amplifier is in turn connected through a relay to an automatic key driven by clockwork, which causes a radio transmitter to send out a characteristic signal whenever the microphone is disturbed. When the position of the ship is wanted, a small bomb is fired in the water alongside the ship; the instant of the explosion is automatically recorded on a chronograph aboard the ship. The sound of the explosion disturbs the shore station microphone and the resulting characteristic radio signals of the shore stations are received on board the ship and also recorded on the chronograph. Thus the ship obtains the time of travel of sound in water from her position to each of the shore station microphones, which are in known positions. The speed of sound in sea water being known, the ship's position is thus fixed. The method has been used successfully in the U. S. Coast and Geodetic Survey with the ship 75 mi. or more away from the shore stations. A device for quick plotting on the field sheet is described. The accuracy of the method is discussed.