The number of questions asked the writer by officers of all ranks on the subject of radio compasses leads to the belief that a non-technical discussion of them may prove of interest to the service.
To most persons not directly connected with radio work or the navigation of ships, the term radio compass means little. It probably appears to them to be something of purely academic interest. That is because the development of radio compasses is a comparatively recent one, and its influence has not yet been apparent to a great extent. The subject is, however, of direct and growing importance to each of us, and is therefore one which we should know something about.
It affects us in time of peace and in time of war. In time oi peace chiefly because it is a new and potent factor in navigation. Navigation affects commerce and commerce affects all civilization. Think of the money that will be saved in future years by ships being able safely to navigate in fogs! Not only will the saving in the number of ships wrecked or damaged by grounding in fog, be considerable, but the amount of time and anxiety saved by having valuable ships enter port instead of lying off, waiting for the fog to lift, will be enormous. In time of war, the information obtained by the radio compass as to the location or bearing of enemy or of friendly forces, is most valuable and will be more so as the radio compass becomes better known and better developed (which will inevitably be the case).
The following examples of the use of radio compasses may be cited. When the transport George Washington, with the king and queen of the Belgians on board, was approaching New York harbor about the time this article was written, U. S. naval radio compass stations plotted and transmitted to her her positions at short intervals as she approached Nantucket and Long Island, and this procedure was followed until she reached Ambrose Channel, thus providing an additional safeguard to the vessel and the lives of those on board; and furthermore the position of the George Washington at 2.15 p. m., on October 1, as obtained by the radio compass was known to the authorities in Washington at 2.30 p. m. of that date.
Shrewd observers in London during the war are said to have noticed that on the nights when there was to be an air raid, searchlights were not tested. The reason the British authorities knew in advance that the Zeppelins were coming and when to expect them, was that the courses of the Zeppelin squadrons were plotted from their radio signals with considerable accuracy, by the British radio compass stations.
Now just what is a radio compass? A radio compass, briefly stated, is a device for the directive reception of the electromagnetic waves used in radio telegraphy. The basic principle involved in directive reception has been known almost since the beginning of radio telegraphy, being simply that radio waves have most effect on an antenna or loop extended in the direction of the propagation of the waves and least effect when the antenna or loop is at right angles to the direction of propagation of the waves. A number of systems have been evolved for making practical use of this principle. Of these the two most widely used to-day perhaps are the Bellini-Tosi, which is the system usually employed by the British, and the Kolster which is the type used by the United States Navy. I will not go into the detailed description of either, but merely state that in both a pivoted vertical coil, which is part of the receiving system, is rotated by the operator until the position is such that the radiation from the sending station produces a minimum of sound in the head phone. The direction or relative bearing of the transmitting station is then indicated by a pointer on a circular scale.
It may be seen from this brief definition that the radio compass differs from the magnetic compass in that it requires a separate manipulation by a more or less skilled operator for each bearing obtained, whereas the magnetic compass provides a continuous indication automatically. Simple as this fact is, that it has not always been self-evident may be seen from the following incidents which actually happened when radio compasses were first placed aboard United States destroyers. Questioned as to how the radio compass worked, one operator reported that he never could repeat his bearings. Upon investigation it was found that his method was to give the handle a twirl, thinking that the pointer would come to rest on the proper bearing. Another reported that he had been watching the compass for a number of weeks and it had not moved once! The necessity of a certain familiarity on the part of the operator with the principle involved is evident. One cannot expect perfect results with untrained personnel, and it is unfortunate that skilled operators are not always available.
You naturally ask, why, if the principles of directive reception are so simple, the development of radio compasses has been so slow. The great difficulty to be overcome was that while bearings could readily be obtained, they were usually in error, so that no confidence could be placed in them. The source of these errors has been found to be largely not in the compass apparatus (though that too has been greatly improved) but in the incoming waves themselves. These, whenever they encounter an antenna or a loop or a coil or a mass of metal which acts as an antenna, with the proper natural period, will set it to oscillating and a certain amount of energy will be re-radiated; a radio compass located nearby will then indicate not the true bearing of the sending station but the resultant, due to the energy of the transmitting station and that from the re-radiating antenna, which may be anything. Furthermore, the direction of the incoming waves may have changed due to some peculiar qualities or configuration of the intervening surface. To get good bearings therefore, the first step was to standardize conditions as far as possible. For instance, radio shore stations are now carefully located as to geographical position; the distance from any other antenna is carefully prescribed; all telephone, lighting, or other wires within a specified distance are buried; the building in which the compass is located is carefully constructed with metal work eliminated as far as practicable, according to approved specifications; the operating room is surrounded by copper screening; and the compass coil and receiving apparatus is itself of a standard type. When this is done the station will still probably show a certain error or deviation, so that it must be carefully calibrated, a deviation curve plotted, and each bearing taken corrected before being used. The results, however, when this has been done are very good and can be accepted with confidence, as being correct within one or two degrees. There may be of course errors in calibration, and of operation; but the performance of United States Navy shore radio compass stations has been gratifying.
In the case of ship compasses the difficulties are greater, but by following the principle of standardized apparatus and wiring and of careful calibration, together with breaking up as many disturbing metal loops as possible, good results have been obtained, though in general not so good as with shore stations. Best results have been obtained in destroyers, about 125 of which have been calibrated by the Boston Navy Yard. Some of the new destroyers have shown as small as 30 maximum deviation, but the older types show considerably more. The destroyer type compass is much smaller and more compact than the type which was until recently placed on battleships. The latter did not as a rule obtain satisfactory results from their radio compasses, and the destroyer type compasses are now being installed on battleships. When one considers the number of antennas, shrouds, stays, guys and railings, and the masts, cranes and turrets surrounding the compass on a battleship, the problem of obtaining good results may be understood. Still, encouraging progress is being made.
During the war the British and French established numerous radio compass stations along their coasts and many of our destroyer captains first learned their value there.
The United States Navy also developed radio compasses during the war and now has approximately 20 shore compass stations on the eastern coast of the United States and is building stations on the west coast. These are available to all mariners and the service is offered free of charge. A mariner desiring the position of his ship has only to call the nearest control station and request his bearing by means of a conventional signal. Then upon receipt of an acknowledgment to transmit V’s for a few seconds. Bearings are taken by the compass stations and sent to him by the control station. Full details may be obtained from the weekly Hydrographic Office bulletin.
The conclusion to be drawn is that the radio compass is destined to play an important part in maritime life in the future. There are still problems to be solved and difficulties to be overcome before it attains its full measure of usefulness. On the other hand, there is no doubt that it is purely a question of development. Even at the present time the radio compass affords the mariner something that he never had before, and many ship captains have gratefully acknowledged benefit from its use. (A gratifying feature of this service is the good will which will be engendered between the navy and the merchant marine because of it). The device is not yet perfect nor fool proof, and the writer would be the last to counsel navigators to discard all ordinary precautions and rely solely on radio compass bearings for their positions. The perfection for which we all hope will come gradually through the earnest cooperation of all concerned in the elimination of defects of personnel and materiel, and as before hinted in this article the radio compass is a forward looking development. We may yet see it an active competitor of the magnetic compass.