Radio Broadcast (May 1928-Apr 1929)

OCTOBER. 1928 RADIO HELPS IN THE COAST SURVEY 375 dicates the ship's position. Fig. 3 illustrates this graphically. An automatic signaling device is attached to the transmitter of each shore station so that it sends out a characteristic signal and may be readily identified. HOW THE SYSTEM OPERATES IN GREATER detail, the opera1 tion of the apparatus is as follows: The bomb, a tin can or cast-iron container, is filled with loose TNT, and just before firing a No. 8 blasting cap, with a length of Bickford powder train blasting fuse crimped to it, is inserted, the joint being sealed with wax or other plastic. The fuse is anchored so that the cap will not pull out. At the proper moment, after a sounding has been completed, the fuse is lighted and the bomb thrown overboard. When the explosion occurs, the sound excites the carbon grain button in the ship's hydrophone, which is located in a small tank of water attached to the outer skin of the vessel. The ensuing current is amplified in a three-stage audio amplifier using two 20 1 a and one 171 tubes and 6 to 1 transformers. The amplified current passes through the coils of a pen-operating magnet on a chronograph. The chronograph, shown in Fig. 2, is of a commercial type, having two pens, and is driven by a 6-volt, storage battery shunt motor. Such a motor will run at practically constant speed, as the load is very light. In fact it is only necessary that the speed be constant during the first and last seconds. The timing device consists of a high grade marine chronometer fitted with a circuit breaking device which operates each second, causing the second chronograph pen to make a mark on the record strip, which is standard f-inch stock ticker tape. Fig. 2 shows at the bottom the type of record made on this tape. After the record of the explosion has been made by means of the ship's hydrophone, the tape continues to pass through the chronograph, each second being marked upon it by the chronometer. The sound from the bomb travels through the water to the hydrophones of the shore station. These are sometimes anchored as much as two miles offshore, depending upon the character of the sea bottom, as it has been found that the system does not operate so well unless the hydrophones are at least 50 feet below the surface. Submarine cable, armored where wave action may cause chafing, connects the hydrophones to the shore station apparatus. The energy from the hydrophones is amplified in a three-stage amplifier, which is very similar to that used aboard ship, and the current actuates an 800-ohm relay which completes a circuit through a 140-meter transmitter, sending out a radio flash. At the same time this relay sets an automatic telegraph key in operation which sends out three additional, equally timed flashes from the transmitter. No two keys have the same timing, so that each station may be identified by its characteristic markings on the chronograph tape. The automatic keys, one of which shown in Fig. 1, are made up with an ordinary musician's metronome as a time element, having the spring removed and weight drive substituted for it. A standard pony relay is mounted directly beneath it with a finger attached to its armature which engages FIG. 2. THE RECORDING CHRONOGRAPH with a similar finger on the pendulum of the metronome when it is in the off position. The impulse from the bomb pulls the armature over, releasing the metronome mechanism, and a suitable system of contacts acting on a notched wheel attached to the time shaft opens and closes circuits in such a manner that the armature is held over during one complete revolution of the time shaft and at the same time sends out three flashes through the radio transmitter. When one revolution has been made the armature is released and the metronome stops. The transmitter is a single ux-210 tube instrument designed to transmit at 140 meters. It is sufficiently powerful to transmit through approximately two hundred miles. The initial and the three identifying flashes from the shore transmitter are picked up on the ship by a standard make of short-wave radio receiver. This receiver contains a detector tube and two stages of audio amplification, and additional amplification is secured by connecting this receiver to the three-stage amplifier previously described. The panel of this amplifier is fitted with a double throw switch, by means of which either the hydrophone or the radio receiver may be connected to it. As soon as the explosion has occurred and has been recorded, this switch is thrown over, disconnecting the hydrophone and connecting the radio receiver ready for the impulse from the shore station to be recorded. The amplified radio signal actuates the second chronograph pen previously referred to, making a mark upon the tape. RESULTS HP HE tape now contains a line ' punctuated at one side with the one second marks recorded by the chronometer. The other side of this line is punctuated with the impulse from the hydrophone and also those from the several shore stations with their characteristic identification marks. The time elapsed between the bomb explosion and the radio reception may be readily determined by counting the number of second marks and by measuring the fractions at each end. Multiplyingthisfigure by the velocity of sound through sea water gives the distances from the ship to the several shore stations. As the geographic positions of these stations are already known, arcs of the proper radii are struck from each and their intersections indicate the ship's position. Amply accurate results for the type of work have been obtained by this means. This system possesses certain difficulties of operation in some localities. Experience 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 offshore, and also where the water is comparatively 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. The system has such attractive possibilities for the location of positions at sea rapidly and economically, that development work will be rapidly carried on in an effort to perfect its use under all conditions. FIG. 3. THE ACOUSTIC RANGE FINDING METHOD The bomb exploded under water by the ship produces sound waves thai are picked up by the hydrophones of the two shore stations, causing the two shore stations to transmit radio signals. These are picked up by the ship, and the elapsed time between explosion and signals gives the data for the calculation of the dis~ tance of the ship from the two shore stations. Thus its position can be located.. Two pens can be seen poised above the tape in the chronograph. The relay operating one of the pens is actuated by the chronometer; the other relay is operated by the signals from the hydrophone and the short-wave receiver on the ship. The type of record made is indicated below the photograph.