DISCUSSION
A New Method of Coastal Navigation
(See Page 723, Whole No. 219)
Captain J. V. Chase, U. S. Navy.—The author clearly describes this new method of coastal navigation and deduces the trigonometrical formulae necessary for the solution of the problem. The use of these formulae, however, is a process too slow and laborious to make this method attractive. The general solution of the problem by use of tables previously prepared is not practicable as such tables would be too bulky for convenient use. Captain Edmonds of Australia has sought to overcome these difficulties by preparing a set of tables whose use is limited to the case where the first and third bearings make equal angles with the second bearing and the author suggests the advisability of preparing a set of tables whose use would be limited to the special case where the three bearings are taken at equal intervals of time.
It appears to me that recourse to either set of tables is both unnecessary and undesirable as the graphical solution of the general problem is both simple and easy and the accuracy of the results obtained by such a graphical solution is largely a matter of the scale adopted.
The graphical solution is especially easy if the navigator has at hand a universal drafting machine, that useful instrument which is rapidly becoming a part of the standard equipment of the modern chart-board. If such an instrument is not available its place must be taken by a compass rose or protractor, a pair of parallel rulers or a pair of triangles and a graduated scale.
Fig. I shows in full lines the general graphical solution. Fig. 2 shows in full lines the solution for the special case where the initial "fix" lies on the first bearing and the bearings are taken at equal intervals of time.
In Fig I, P represents the landmark, the bearings of which are observed and recorded. A represents the position of the ship at the time t0. PD, PE, and PF represent the bearings of P observed at the times t1, t2, and T3 respectively. AG represents the steady course of the ship steered during the time interval (t3-t0). B is the dead reckoning position of the ship at the time t3. OM equals (t2-t1) to some convenient scale and ON equals (t3-t2) to the same scale. Draw the lines MD and NK parallel to PE. These lines intersect the first and third bearings in the points D and K respectively. Draw through A a line parallel to DK. This line is the course made good. It intersects the third bearing in the point C which is the final position of the ship. Join B and C. BC is to scale the set of the current and wind during the time interval (t3-t0), the direction of the set being from B to C.
In Fig. 2, t0 equals t, and (t2-t) equals (t3-t2). The same lettering is used as in Fig. 1. It will be noted that the solution is simplified because the points A, B and D merge into one point on the first bearing, the points C and K merge into one point on the third bearing and the distance OM equals ON.
It appears to me that the solution shown in Fig. 2 is the one that, in practice, will be the one most generally used. It is very probable that the initial "fix" will lie on the first bearing and I can see no good reason why the remaining bearings should not be taken at equal intervals of time.
It should be noted that the graphical solution can be carried on while the ship is proceeding from the first to the third bearing, little remaining to be done to complete the solution when the third bearing is obtained. This is evident, if we consider the case shown in Fig. 2. As soon as the first bearing and the initial "fix" are plotted, the course AG may be drawn. The time intervals for the second and third bearings are decided upon and the point B is plotted. As soon as the second bearing is plotted the point N is plotted by making ON equal to OA. An indefinite line NK is drawn through N parallel to the second bearing. When the third bearing is finally obtained and plotted all that remains to be done to complete the solution is to draw the lines AC and BC.
A preliminary solution may be obtained if an intermediate bearing between second and third bearings be taken. For example, in Fig. 2 if such a bearing PH be taken after half the time interval between the second and third bearings has elapsed, OR is laid off equal to one half ON and RS is drawn parallel to the second bearing, intersecting this intermediate bearing in the point S. Join AS and prolong this line to meet the line NK in the point C, thus obtaining a predicted final position of the ship.
As has already been stated, the solution shown in Fig. 2 is so simple that it should be the one generally used. In order, however, that preliminary solutions can be readily obtained I would recommend taking five bearings at equal intervals of time. The main solution in this case would depend upon the use of the first, third and fifth bearings. As soon as the first three bearings had been taken a preliminary solution could be made and when the fourth bearing had been taken three other preliminary solutions could be made. All these preliminary solutions should be consistent, provided the bearings be accurately taken. Any inconsistencies not accounted for by the probably inaccuracies of the bearings indicate that the current is variable in strength or direction and, therefore, that the method cannot be relied upon.
In Figs. 1 and 2 is shown in dotted lines the effect produced by inaccuracy of the initial fix. In each figure the ship, actually at A is believed to be at A'. It will be noted that the final positions C are inaccurate. What is more important is that the apparent current effect is wholly erroneous both in strength and direction. If this erroneous current data be used in subsequent navigation serious consequences may result.
If at C, in Fig. 2, the course is changed and another problem solved using the same time intervals as in the first problem, it will be found that the current in the two problems will not be consistent, due to the inaccuracy of the initial fix. It is possible, however, with these two consecutive solutions at hand to determine the true track of the ship. The procedure is as follows:
Plot the dead reckoning track of the ship. Extend backward the second dead reckoning course a distance equal to the run on that course. Through the point so obtained draw a line parallel to the second true course (already determined). This line will intersect the first true course (already laid down) in a point. Through this intersection draw a line parallel to the first bearing of the first problem. This line will intersect the final bearing of the first problem (which is also the first bearing of the second problem) in the true position of the ship on that bearing. The true courses, already determined, draw through this true position will give the true track of the ship, the true initial and final positions and thus render easy the determination of the true direction and true strength of the current.
It is obvious that much of the work just described can be done while the ship is proceeding from the initial to the final position, so that little remains to be done after the final bearing of the second problem has been taken in order to determine the true track of the ship.
This particular application of the method may be found to be very useful in making a running survey of a coast off which may run a current whose strength and direction are unknown. In a running survey the positions of uncharted objects may be accurately fixed by bearings only in case the position of the ship is accurately known at all times. The method just described determines the true track of the ship when only one charted object is available for fixing the ship's position.
Probably much more could be written in favor of using the graphical solution of the problems involved in the use of this method, but it is believed that enough has been said to warrant my taking issue with the statements contained in the quoted sentence with which the author ends his article.
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Trans-Pacific Communication
(See Page 1803, Whole No. 213)
Lieut. Colonel C. A. Seoane, Signal Corps.—In keeping with the sound principle that cable and radio systems should be laid out so as to have radio stations near the corresponding cable stations, which you very well discuss, the thought occurs to me that when the time comes to lay a cable across the northern Pacific it will certainly follow a route along the Aleutian Islands. Cable relay and radio stations would naturally be located there. The St. Paul radio station is admirably located for this and a nearby cable station would probably be at Dutch Harbor.
The next westward relay point might naturally be Attn, 730 miles west of Dutch Harbor, and about 670 from St. Paul. So far, so good, but from here on I do not think that a very clearly defined communication route, either for cable or radio, has ever been laid out.
I have been thinking over the matter and believe that from Attn the next point should be Marcus Island, in 24 N., 154 E., and about 1980 miles from Attu, and approximately 2473 miles from St. Paul.
It may be interesting to know that St. Paul-Sitka Marcus midway form a parallelogram with equally long and short sides.
About 381 miles west of Marcus in 25 N. and 147 E. lies Sebastian Lobos, about 2110 miles from Attu and about 2630 from St. Paul, I have not been able to find that this island is owned or claimed by anybody. Some charts show it under the name of Grampus and Marcus under the name of Weeks.
The reasons why I claim that either of these would make a most desirable relay station in reaching the Orient are these: it would make possible a system by which lines could radiate to Yokohama, 840 miles, Shanghai, 1470 miles, Manila, 1660 and Guam 777—via Grampus, and the distances from Marcus would be, Yokohama, 1050 miles, Shanghai, i860, Manila, 2010, Guam 840.
By connecting up the cable and radio systems with Guam, connection is established with the existing system to the Hawaiian Islands, etc., and would enable communication to be routed over either system, as bulk of traffic or breaks might require.
Let us, for instance, imagine that a great deal of traffic would be going from Manila to the United States and that this would be centered on radio, due to cable breaks. Manila would be able to work alternately with Grampus and Guam, enabling each to relay what it received without holding up Manila (Cavite).
It would make a grid or network covering the Pacific Ocean with a flexibility as outlined, which means that the efficiency of transmission should be at a very high rate indeed and in keeping with our commercial and other requirements in the Pacific Ocean.
It would be very difficult to develop any such possibility as this if the terminus of any new system were to be in a foreign port such as Shanghai or elsewhere. In fact, due to international agreements, that would have to be arranged, it might become impossible to do this, and the result would be that the two systems could not tie up with each other with the ease that the proposed arrangement would permit.
I cannot help reiterating that I consider Grampus a very valuable location indeed and we should lose no time in occupying it, as I think it belongs to the United States.