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tions hearing a distress call are re- quired to lend every possible assist- ance to the vessel in distress, if in a position to do so. Rescue Communications The receipt of a distress call by a ship or shore radio station sets in mo- tion a comprehensive rescue com- munication system. All communica- tions on 500 kilocycles (600 meters) must be reduced to a minimum neces- sary to properly conduct aid to the scene of distress. The vessel in dis- tress, or another station designated by that ship must control all com- munications on the distress fre- quency. The shore radio direction- finder system is instantly mobilized to obtain bearings of the ship in dis- tress, and to plot her position for the benefit of ships proceeding to the rescue. Rescue ships obtain bearings with their own direction-finders if equipped with such device. Shore radio traffic stations advise other radio stations to remain silent if in- terference on the distress frequency is probable. The owners and the press are notified immediately. Every effort of radio is bent on successful rescue operations. Obviously, to be efficient, strict su- pervision of radio communications is imperative during such a trying pe- riod. Masters of ships must then exercise the greatest vigilance in their communications, and prevent radio operators from transmitting superfluous messages concerning the distress case. Examples in this con- nection will be cited later on to show the necessity of all officers knowing the underlying principles of radio and having in mind at all times a broad knowledge of the world’s radio communication system. Navigation Aids The value of radio under the sec- ond primary use, as an aid to naviga- tion, cannot be emphasized too much. Under this heading, there have been. developed a number of devices or services which have become indis- pensable to the successful navigator. The most important are: (1) Radio direction-finders (radio receivers with loop—directional —antennas) (2) Radiobeacons (fixed radio trans- mitters) (3) Synchronized vices (4) Time signals (radio broadcast- ed ) (5) Fathometers (really sound de- vices ) (6) Weather broadcasts, storm warn- ings, hydrographic broadcasts. radio-sound de- Radio Direction-Finders Radio direction-finders are most valuable in obtaining accurate radio bearings on distant or nearby radio transmitting stations. With this de- vice on board, a radio bearing of a 16 radio transmitting station may be obtained, which should normally be accurate within two to four degrees, the accuracy dependent upon the length of time since calibration and whether conditions have changed in the ship. If the geographical posi- tion of the transmitting station be accurately known, as is normally the case if the transmitting station is on shore, a line of position may be laid down on the chart from the direction- finder. With simultaneous bearings of two or more shore stations affording a suitable cut, crossed lines of position are obtained, establishing a fix, the accuracy of which is of course depen- dent upon the accuracy of the indi- vidual bearings. Two or more bear- ings of the same station (with the known run between) may also be used, just as in the case of visible terrestrial objects. It is considered advisable that the navigating officer of the ship take the radio bearings personally. Through a knowledge of the pos- sibilities, peculiarities and limitations of the radio direction-finder as in- stalled in his ship, the navigating of- ficer may learn by experience the degree of reliance to be placed upon the various bearings obtained. For this reason the radio direction-finder should be installed on or near the bridge of the ship. The value of the radio direction- finder in a fog is obvious, and it is likewise a valuable navigational check even in clear weather. An ex- tensive automatic radiobeacon serv- ice has been established along manv coasts of the world to afford suitable transmission for direction-finding by ships at sea. However, shore radio traffic stations transmitting on suit- able frequencies may be used if their positions are accurately plotted on the chart. The reverse of the process de- scribed above for ship-borne radio direction-finders is also used in the United States and other countries to aid ships in their navigation. A com- prehensive system of radio direction- finder stations has been established along the various coasts. In this system it may be considered that the “radio lighthouse’ is the ship at sea, the radio direction-finder where the bearing is taken being on shore, whereas in the radiobeacon system discussed previously, the radio sta- tion on shore is the ‘radio light- house,”’ the bearing being taken from the ship. In the case of the shore radio di- rection-finder system no direction- finder is required aboard the ship, which merely transmits certain pre- scribed radio signals which enable the shore direction-finder station to obtain a radio bearing, which is then transmitted back to the ship. Groups of direction-finder stations are lo- cated at the more important coastal entrances, thus affording simultane- MARINE REvVIEw—August, 1934 ous cross bearings or fixes to ships in these localities of heavy shipping. Radio bearing from shore direc- tion-finder stations are normally ac- curate within one to two degrees. Their accuracy is affected however by “‘night effect’’ and by geographical features in certain sectors, Bearings of doubtful accuracy are so desig- nated by the shore direction-finder station when they are transmitted to the ship requesting them. The radio direction-finder service of the United States, operated by the navy, gives an average of 260,000 radio bearings per year, approximately nine-tenths of that number being furnished to the merchant marines of various na- tions. A radio direction-finder requires an accurate calibration just as does a magnetic compass. For this reason and because of other readily-learned vagaries of the device, the deck offi- cer should have a good working knowledge of its operation. He must be able to determine when a bearing has been improperly taken, or when errors may have been introduced due to changes of location of metal masses on deck or near the radio direction-finder, such as movable booms, and stays, etc., existing when the radio direction-finder was last calibrated. Broadcast antennas, rigged by the crew, and which are in inductive relationship to the loop of the radio direction-finder, are some- times harmful and can set up errors to cause erroneous radio bearings to be received aboard the vessel. Under the provisions of the safety of life at sea convention (London, 1929), passenger vessels of over 5000 gross tons will be required to have _ in- stalled, radio direction-finders. Cargo vessels or those smaller than 5000 gross tons perhaps cannot be equip- ped for financial reasons, which in- dicates the necessity of maintaining shore radio direction-finders. But regardless of tonnage, the _ ship equipped with a radio direction-finder has a definite advantage over oth- ers, particularly if the deck officers and master become familiar with its operation, possibilities, and limita- tions, being thereby enabled to real- ize the desired results. Many advanced cfficers prefer to take their own radio bearings as pre- viously recommended herein, rather than to rely upon the radio operator who probably possesses no practical knowledge of navigation, At present about 30 per cent of all seagoing United States merchant ships of 1600 tons or over are equipped with radio direction-finders, and this number is increasing each year. Radio Beacons Radio beacons are merely automatic radio transmitters, each sending out a distinctive signal on which radio bearings may be obtained by means of a radio direction-finder installed (Continued on Page 38)