The science of aerial navigation may conveniently be divided into four parts: (a) piloting, in which the position is ascertained from visible objects on the earth; (b) dead reckoning, in which the position is deduced at any moment from course and speed of the observer from a known point of departure; (c) radio position-finding, in which the position of the observer is obtained by means of radio signals; (d) celo-navigation, in which the position of the observer is found by observations of heavenly bodies, the sun, moon, stars, and planets.
To date very little celo-navigation has been done. In fact, some believe the development of radio will make celo-navigation unnecessary. The situation is more like this: piloting and dead reckoning are most used at present, and radio positionfinding is fast being developed, while celo- navigation has not been extensively used, and the results obtained heretofore by its use have not been encouraging. The time is fast approaching, however, when the practical application of celo-navigation would be a boon to aviation, particularly to naval aviation. The answer to the navigation problem for long-distance flights seems to be the use of radio position-finding in conjunction with celo-navigation. As Commander Victor Herbster states the case: “For efficient operation with the fleet, an aviator must be able to return to a ship by reliance on methods within his plane.” In order to accomplish this mission, he further states that navigation should be used to return to a radius of within twenty-five or fifty miles, which is within range for direction-finding from the plane, then to pick up the ship by means of the radio.
The radio beacon is an entirely different proposition and should prove a great help for the commercial aviator. On the other hand, no matter how valuable to the commercial flier, it would be a mistaken policy for the Navy to put war-time dependency in a method which might be silenced or destroyed for any of various reasons.
Since all of the branches of aerial navigation except celo-navigation are already fairly well developed, this article will be restricted henceforth to the problem of positions by celestial bodies.
If aerial navigation is to come into common use, it must be put on a practical basis, and sea navigation cannot successfully be used in the air unless it is greatly simplified, and some radical changes made. For instance, the ordinary horizon sextant used at sea cannot be used in the air except in restricted cases, for the reason that at night, or over land, or at any but low altitudes over the sea, there is no horizon, and a “bubble” horizon must be substituted. Due to high speeds when in the air, positions at dusk and at dawn, which are standard for sea navigation, are not sufficient, especially when there is a relatively large wind effect.
It would be an excessive and useless load to impose on the navigator to require him to compute the hour angle, and to solve the astronomical triangle by the methods used at sea. Fortunately, we can simplify, to a very great extent, the system used at sea and still be within the limits of accuracy required in the air. In theory we have worked out a very simple method for use in the air, and it now remains to be seen whether or not we present the method to the average aviator in such a practical way that he will put it into common use.
Suppose we ask the question: “How frequently and how accurately must air positions be determined to be of practical value?” As regards speed of solution, will a line of position every ten minutes suffice? If that is too slow, will five minutes do the trick? Somewhere about here we can assume a minimum speed which will suffice for aerial navigation. Similarly, as regards accuracy, an average error of fifteen, ten or some other amount, could be determined. Then if we can obtain without too great amount of mental wear and tear, results within the limits set, there seems to be no logical reason why the method should not be put into common use. I believe this is the way the matter should be considered, and I further believe we can meet the requirements.
Since August, 1927, with the cooperation of the personnel of the Aircraft Squadrons, Battle Fleet, and of the personnel of the Naval Air Station, San Diego, California, the writer has made numerous tests on the ground and in the air in an attempt to arrive at the most satisfactory system for celo-navigation. The actual results have been most encouraging, particularly for night work.
A fix by the observations of two stars is so much easier to obtain than with the sun alone, that we use one system for day work, and an entirely different system for night work. During the day, the best we know is to solve the astronomical triangle by the easiest available method, for each line of position worked. For night work an entirely new system is used which reduces the actual computations necessary to only one subtraction of time to get a fix. The systems for day work and for night work will now be taken up separately.
Day Work
Equipment: See Fig. 1. The first in importance in equipment is a good bubble sextant, as near fool proof as possible. If the sextant goes “hay-wire” in the air, we have poor facilities for making repairs. Furthermore, the vibration and other adverse conditions in a plane impose greater demands on the sextant. We have obtained the best results using the Bureau of Standards Navy sextant. While we have had considerable trouble with lost motion, with the reading and bubble lights, and with other minor features of the instrument, the important features have stood up well under service conditions. No doubt the minor faulty features can be improved.
Next to the sextant, the watch is the most important item of equipment. Thanks to the fact that inexpensive watches may be had which are guaranteed not to vary in rate more than two seconds per day, we can readily provide ourselves with the correct time to the second. If we stop to consider that a watch registers 86,400 seconds in a day, and that the watch is considered defective if it changes more than two of those seconds, regardless of radical changes of temperature and of position, the engineering feat itself wins our admiration. The question of keeping the correct time is greatly simplified by the fact that we can get a daily (or twice daily) radio tick. It is now possible to keep more accurate time on a good wrist watch than was possible a few years ago after a ship was a few days at sea. Mr. Alonzo Jessop, a member of the J. Jessop & Sons firm of San Diego, California, states that a few years ago twelve or fifteen chronometers were usually on hand for rating, while now this number is reduced to one or two. He attributes this change to the fact that navigators use the radio to rate their own chronometers. He further states that the navigator using the radio tick can give the chronometer a more accurate rating than he can, for the reason that by the time the chronometer is delivered on the dock, the rate has changed due to the handling. Mr. Jessop, who is a yachtsman, tells an interesting experience about a voyage from Honolulu to San Diego during which the small yacht rolled so much that the chronometer stopped, but that a watch kept good time through it all. This discussion is aside from aerial navigation but is included for the purpose of making the point that a chronometer is often no more reliable than a good watch, and further that we may expect almost perfect results with a good watch compensated for temperature, and given only reasonably good care. So far as can be detected, a Nardin meantime watch, and a Patek-Philippe sidereal watch have not changed rates due to being carried in a plane as a wrist watch, and given the care ordinarily given a watch.
With the authority of the commander, Aircraft Squadrons, Battle Fleet, two Navy watches, one of which was regulated to mean time, and one to sideral time, were altered by J. Jessop & Sons so that exact seconds could be set on the watch. This makes it possible to take the watch reading for the exact GCT (or GCT minus twelve hours). For use in aerial navigation, the GAT for the equation of time for local mean noon is set on the watch, a direct reading giving the Greenwich hour angle (or its complement). Since sun sights are usually worked after 0800 and before 1600, and further since the equation of time changes not more than 1.3 seconds per hour, by thus using the middle equation of time, a maximum error of only a few seconds can be made, and if this is considered too much, the exact seconds can be set quickly for the equation of time for the instant desired.
By pulling out a ratchet arm, the second dial is moved from one to three seconds in a counterclockwise direction. When released, the ratchet springs back into its housing, and the operation may then be repeated as often as desired. The minute and hour hands are set in the usual way. The ideal condition would be to have a watch which could not only be set to apparent time, but one which could be made to continue keeping apparent time, which would necessitate a varying rate.
The writer has invented an adjustable- rate watch which will keep either apparent or sidereal time in conjunction with mean time, but as the estimated cost of such a watch is $200, and there is at present a small field for its use, it is likely that it will not be manufactured any time soon. The second-setting feature of the altered watches mentioned above is an essential and important feature of the adjustable-rate watch, and is a convenient makeshift. We can thus get nearly all we need by having two second-setting watches, one keeping civil, and one sideral time.
It is surprising that having as accurate time pieces as we do, we have not before this devised some means of reducing the amount of computations in arriving at the proper hour angle. The GAT watch gives directly the value of the GHA. Making this computation in the air every time sights are taken, say every half hour, would impose a great deal of unnecessary work on the navigator, and at the same time increase the chances of error. The slight inaccuracy due to the use of the middle equation of time would be more than compensated for by speeding up the work, and by the reduced chances of error. (See Fig. 2 for sample solution.)
In addition to the watch and the sextant, the equipment includes a chart mounted on a small board with a light drafting machine, dividers, pencil, work book, and a book giving the tables necessary to work the system used in solving the triangle. Extract from the Nautical Almanac is copied in the work book.
The method used in working practically all the sights during this period is that given in the Line of Position Book. The compactness and simplicity of the method seems to appeal to aviators, particularly to the reserve officers who are taking up celo-navigation for the first time. Having complete altitude-correction tables for all bodies for all types of sextants, and time-to-arc conversion table in a handy place speeds up the solution of sights.
Another saving of time is gained by using the form shown in Fig. 2. Since no repetitions are made and since the work is compact and may be easily checked, both loss of time and inaccuracies are avoided. On my first navigation hop, I worked nine sights in ninety minutes with an average error of 5.6 miles. Except for the last sight where five observations were averaged, all the sights were single observations. The conditions for working were good. When the air is bumpy, the average error may be much larger. One day, after a storm, two officers working sights in the Navy San Diego-San Pedro mail plane got seasick due to the bumpiness of the air and to the strain of taking the sights. The average error for twelve sights on that occasion was about eighteen miles.
Night Work
Equipment: See Fig 3. For night work, as for day work, the sextant is the most important item of equipment. The Bureau of Standards sextant is especially suitable for star observations since the star is viewed through a plain glass plate direct. The difficult operation of bringing the reflected image of a star down to the horizon is thus avoided; instead, the “bubble” horizon is conveniently brought up to the star.
The second setting sidereal watch saves more than fifty per cent of the work of getting a fix at night.
The watch reading gives the GST to the second, thus saving the time necessary to make the computations, and the chances of error. Why should the navigator be forced to compute the GST each time a star sight is worked, when a watch which will pass railroad time inspection may be rated to keep the correct GST within two seconds per day? There is a real advantage in avoiding the conversion of watch time to sidereal time due to the fact that a large percentage of all errors made in working star sights are made in this operation.
Method.—It is in the method of solution of star sights that we have been able to make the greatest advance. We pay the price of using particular pairs of stars in order to reduce to a minimum the work of obtaining a fix. The method is based on the suggestion of Mr. Hilding K. Beij, of the Bureau of Standards, in Technical Report No. 198 of the National Advisory Committee for Aeronautics. The basic idea is that the simultaneous altitudes of two stars definitely determines a position on the earth, and that curves plotted against latitude and LST may be drawn to represent the altitudes from which a fix is obtained. Figure 4 will show a sample page of the curves. The latitude scale of the completed curves will be that of the Mercator projection, which makes it possible by drawing in the longitude lines to lay down courses and bearings as on a Mercator chart. Or by using a strip map machine, the longitude lines may be etched on the transparent cover, giving in effect a plotting sheet.
Fig. 4 shows some sample sights worked out by this method. Since the intersection on the curves definitely records both the two altitudes and the latitude to scale, it is not necessary to write down these values. Absolutely the only computations necessary for obtaining a fix after the observations are made, is the subtraction of LST from GST to get the longitude. The watch reading is the exact GST, and the intersection of the altitude curves for the observed altitudes projected vertically to the top or the bottom scale gives the LST. If the sextant has no IC, the sextant reading of the bubble sextant is used directly on the curves, since the bubble correction is incorporated in the construction of the curves. On a test of the method, two altitudes were observed, the time of one noted, and a fix obtained in fifty-five seconds, and as it happened this particular fix was correct in latitude and only one minute of longitude in error. The average error from the ground using the bubble sextant and single observations should be not more than three miles, and in the air this' average error should not be over ten miles. However, the average error in the air depends almost wholly on the conditions of the air.
Since it takes about as long to determine the drift as it does to get a star fix by this new method, why not use a succession of fixes to determine the course and speed made good, and hence the drift and the course to steer? And further, since the methods given in the Line of Position Book and for using the star-altitude curves do not require a profound knowledge of astronomy or of mathematics for a correct solution, why not make our future textbooks on aerial navigation so easy to understand that the thousands of young men who are now taking to the air will welcome rather than shun aerial navigation?