Direction-Finding

Radio Record, Volume II, Issue 24, 28 December 1928, Page 9

Direction-Finding

HE use of radio for direction and range finding has made great strides in the past few years, especially in the development of directive beacons for sea and air navigation. The United States Coast and Geodetic Survey has recentiy developed an _ interesting method of range-finding which makes use of both sound waves and radio waves. In making depth measurements off the coast it is frequently necessary for the survey ships to be out of sight of land, so that ordinary triangulation methods of accurately locating the position of the ship cannot be used. In such cases the position of the ship is determined by a method known as acoustic range-finding, in which the distance of the ship from shore is measured by the velocity of sound. When the surveying ship has made a depth measurement, or sounding, a bomb containing a pound or so of high explosive is dropped overboard and exploded twenty or more feet beneath the surface. "THE sound produced is picked up by a submerged microphone or hydrophone located on the ship, and the impulse transmitted throngh a three-stage audio amplifier to the pen-actuating magnet of a chronograph, making a mark on a paper recording the strip. The sound of the explosion also travels through the water in all directions, and is picked up by hydrophones anchored

in approximately fifty feet of water at two or three known points on the shore. Insulated cable connects these hydrophones to a three-stage amplifier at each shore station. The amplified signal.actuates a relay which sends a flash from a simple low-power radio transmitter. The radio signal is picked up by a tuned receiver on board ship and amplified, and this current also actuates the chronograph pen before mentioned. The paper strip or tape has been moving at a uniform rate during the time between the bomb explosion and the reception of the radio flash, and consequently the space between the two pen marks is an index of the time elapsed. Accurate measurements have determined that the velocity of sound through sea-water is approximately 4920 feet per second, varying somewhat with the water temperature. For example, if the elapsed time is 60 seconds the ship is consequently approximately 56 miles from the shore station. Two or more stations are required so that there may be no error. Tih system possesses certain difficulties of operation in some localities. EXxperience has shown that the apparatus works better where the bottom falls rapidly away from the shore, and where the water is cold and of fairly even temperature. Shoals also seem to present difficulties in the transmission of sound through water. The exact influence of each of these factors has not been fully determined, but active investigation is being carried on. On the Atlantic coast of the United States, where the continental shelf extends for a good many miles off shore, and also where the water is compara-

tively warm, considerable difficulty has been experienced in getting the apparatus to work satisfactorily over any great distance. On the other hand, on the west coast, where these conditions do not obtain, excellent results have been achieved over a distance of about two hundred miles, HE system has such attractive possibilities for the location of positions at sea rapidly and economically that development work will be rapidly earried on in an effort to perfect its use under all conditions.