Radio Broadcast (May 1928-Apr 1929)

THE U. S. COAST AND GEODETIC SURVEY SHIP SURVEYOR Radio Helps in the Coast Survey CT'HE method of acoustic range finding ■*■ described by Mr. Parkburst seems to have several advantages over the radio beacon system, when used for short distances over uniform masses of fairly deep water. The actual distance of the ship from shore stations is measured by the time taken for sound waves to pass through the water from ship to shore, whereas in the beacon system only the bearings of shore stations may be determined. Furthermore, the acoustic system is entirely automatic, the time of the signals being mechanically recorded on the .chronograph tape aboard the ship. With the beacon system there is always a possible error due to the operator. On the other hand, the acoustic method has proved itself useful only over short distances (200 miles at the most), and through water that is uniform in temperature and not broken up by shoals. These advantages indicate that it might have great usefulness as a guide to ships at the entrances of harbors. — The Editor. THE 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 U. S. Coast and Geodetic Survey has recently developed another interesting method of range finding, which makes use of both sound 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 deter By D. L. PARKHURST Chief, Instrument Division, U. S. Coast and Geodetic Survey mined by a method known as acoustic range finding, in which the distance of the ship from shore is measured by the velocity of sound waves through water, the recording being effected by means of radio. When the surveying ship has made a depth measurement, or sounding, a bomb containing a FIG. I. ONE OF THE AUTOMATIC TRANSMITTING KEYS 374 pound or so of high explosive, such as TNT, 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 through a three-stage audio amplifier to the pen-actuating magnet of a chronograph, making a mark on a paper recording 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 140-meter low-power radio transmitter. The radio signal is picked up by a tuned receiver on board the ship, and amplified, and this current also actuates the chronograph pen beforementioned. 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 elapsed time. 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 295,200 feet, or 55 10/11 miles, from the shore station. The information is not complete if only one shore station is in operation, as the ship may be anywhere on a circle whose radius equals the time multiplied by the velocity of sound in water; consequently, two or more stations are used and the crossing point of the arcs for each station in