There are four methods of fixing an aircraft’s position and these are described in this chapter
NAVIGATOR OF A WHITLEY BOMBER. In the forward compartment of a Whitley long range bomber, the navigator sits at the chart table on which can be seen a bubble sextant and parallel rulers which he uses to plot his course. At the controls sits the captain, whilst the second pilot pours out a welcome cup of hot coffee from a thermos flask.
NO truer remark was ever made about the art of air navigation than that made by the American authority, Commander Philip Weems, when he said: “There are four possible means of fixing an aircraft’s position,” and then added “But there are times when all four are not enough.”
The methods referred to by the commander were, map reading, dead reckoning, astronomical, celestial and wireless direction finding. Each method is best suited to certain types of problems and when Commander Weems claimed that there were times when all four were not enough he meant that there were circumstances in which each method would fail. Fortunately, all four methods do not usually fail simultaneously, but the moral is clear. The good navigator cannot afford to neglect any one of them.
The maps in use in the R.A.F. are of three scales, each scale being a multiple of the others. First of all there is the 1/1,000,000 map which gives a scale of approximately sixteen miles to the inch on the map. With this scale little detail can be shown of course, but nevertheless a large area can be put on a single map sheet. This map then is used mostly for plotting the track and studying the general lay of the route.
When greater detail is required the 1/500,000 map is used. This gives approximately eight miles to the inch, and allows each feature to be depicted in greater detail. On this map it is usually possible to recognize the towns and villages of the countryside. Only when one comes to a very congested neighbourhood such as greater London or the industrial parts of the midlands or the Ruhr valley is it essential to have still greater detail. For these areas therefore a map of scale 1/250,000 or approximately four miles to the inch is used.
In general, as the flight proceeds the navigator will use maps of larger and larger scale, and the bomb aimer will finally pick out the target from a special large scale target map or a photograph.
Map reading is one of the most difficult subjects to teach, or, to be more exact, it is probably impossible to teach. It comes with experience in the air and hours of observation. Navigation proper is a much more exact science and comparatively easily taught to any one with average ability. There are navigators who will bring their aircraft to within a few miles of their home stations by navigation proper, and then take out a large scale map of their home countryside and proceed to lose themselves entirely.
Dead reckoning navigation is the art of estimating the aircraft’s position by knowledge of the direction and speed in which it has been moving. This sounds simple enough, but in reality can be quite a complicated process, for here, errors are accumulative. If there is an error in the aircraft’s course or if the speed is not quite accurately known then the estimate of the aircraft’s position will differ from the real position and the disparity will increase with the passage of time.
There are many sources of error in dead reckoning navigation. In the first place, the course is set by a magnetic compass, but there is much magnetic material in the aircraft, and it is necessary to “swing” or adjust the compass very carefully on the ground before it can be made to give the correct reading. Even after this, it is not absolutely certain that the course as set in the air is always quite accurate. Bombs are of magnetic material. The machine may have been “swung” on the ground with bombs on, but what when the bombs have been released? The machine was “swung” on the ground with its undercarriage down, but in the air the heavy steel tubes of landing gear will fit inside the wings.
THE MAGNETIC COMPASS. Inverted type aircraft compass. It is fixed in the roof of the aircraft above and in front of the pilot who takes his bearings by means of a mirror. Saving of space is effected by this method.
It has been known for the magnetism of an aircraft to have been substantially changed by flying through an electrical storm with the result the compass had considerable errors. All these are points which have to be continually watched and checked. In the long run, however, the magnetic compass is a superbly reliable and surprisingly accurate instrument.
The next “key” instrument required in dead reckoning navigation is the airspeed indicator. Here again, however, any one who supposes that an air-speed indicator always gives a direct indication of the true air speed would be taking a very optimistic view. First of all, the instrument itself is periodically calibrated on the ground against a master instrument and its errors are noted. Next there is a correction to apply for the position in which the instrument is mounted on the aircraft. This is known as the position error or “P.E.” This is bad enough, but there is worse to come! The instrument is measuring the forward speed of the craft by the pressure of the air stream, so its reading depends on the density of the air itself; the density depends on the temperature and pressure. When flying low these adjustments to the air-speed indicator reading only amount to minor compensations, but when flying high they become very large factors indeed. For example, let us suppose that a bomber goes out on a raid at 20,000 feet where the air temperature is -24°C, and that the navigator sees his air-speed indicator at 165 miles per hour. His combined instrument error and position error correction is usually kept on a card pasted to the instrument board, or inside the cover of his log book. At 165 miles per hour indicated speed, the correction is +9, which means that his corrected indicated speed is 174 miles per hour. Now he takes a computer, that is a special slide rule for correcting air speed. At a pressure height of 20,000 feet and a temperature of -24°C the 174 miles per hour is corrected to a figure of 238 miles per hour. This at last is his true air speed.
Even now, however, the navigator’s troubles are far from over. He has found his true air speed, but that is only his speed through the air. What of the wind? i.e., the speed of the air over the ground. Perhaps he has a 50 miles per hour wind from behind him in which case he is approaching the target at 288 miles per hour. Perhaps the wind is against him, in which case his ground speed is only 188 miles per hoar. Perhaps the wind is on the beam, however. In this particular case he is not approaching the target at all but is merely drifting twelve degrees or so to one side or the other.
NAVIGATOR’S CHART BOARD. Close-up view of chart board with adjustable ruler which can be set to take into account such factors as ground speed, wind speed, air speed and so on, interdependent factors to be reckoned with.
It would be quite impossible in this chapter to explain all that is involved in the term “dead reckoning” navigation, but enough has been said to show that it is full of pitfalls for the unwary. The point to remember is that these pitfalls are apt to increase with the passage of time. For example: if a navigator overestimates his ground speed by ten miles per hour he will plot a position ten miles ahead of his real position for every hour that he flies. After ten hours he would be 100 miles in error. Thus it is very important to obtain other checks on the positions. This may be done either by wireless bearings or position lines from celestial bodies.
Astronomical navigation is the fixing of a craft’s position by observation of the sun, the moon, the planets or the stars. There are many varieties of method, but the underlying principles are similar. In general too, there are three features common to every method. First, the process of “taking a sight,” that is, measuring the “observed altitude” or angle of elevation of the celestial body. This is done with a sextant. Secondly it is necessary to obtain the exact time of the observation by a chronometer or an accurate watch. Thirdly there is the process of reducing the observations of sextant and watch to position lines on the chart. This process, however, involves some small amount of calculation and the use of a naval almanac and some mathematical tables.
It is not possible to obtain the exact position from the observation of any one body. It is only possible to obtain a “position line.” That is, a line somewhere upon the length of which the observer is situated. The position line is at right angles to the direction in which the body is seen. In order to fix the position completely it is necessary to make observations of two celestial bodies and where the position lines of the two cross is of course the exact position.
Air navigation has its roots in sea navigation. The principles are the same and many of the processes are exactly analogous. The marine navigator would however find difficulty in recognizing much of the air practice. The lesser degree of observational accuracy in the air coupled with the urgency of extreme speed has resulted in the development of methods far quicker and simpler than were ever dreamed of by navigators at sea.
ADJUSTING A SPITFIRE’S COMPASS. Magnetic compasses are very sensitive instruments, and as there is much magnetic material in an aircraft, they are liable to be affected and give inaccurate readings. In order to overcome this, aircraft are swung on the ground with full load and the compass adjusted accordingly. The Spitfire above is being swung on a specially constructed turntable.
Despite this it is interesting to find that a marine sextant differing little in essential design from those which have been used at sea for generations is still used very successfully in some circumstances in the air. The flying-boat does most of its flying low over the water on convoy work and anti-submarine patrol. The air over the Atlantic is both cloudy and windy, but when the pilot sees a patch of sunlight shining on the sea ahead of him he may dive down and fly even lower above the surface — no higher than the bridge of a ship. His navigator may then get a “snap” sight of the sun’s altitude above the sea horizon. The resulting position line may be exceedingly valuable to the navigator who may have been working on nothing but “dead reckoning” for the past twelve hours. The particular type of sextant now issued to all flying-boat crews for this purpose is a development of one which was originally made for Alan Gerbault, the French yachtsman who sailed the Firecrest single-handed across the Atlantic Ocean.
Those navigators however who fly high at night, where no natural horizon is available, do not get this advantage. To them the taking of a sight is a more laborious process, for they must use a bubble sextant in which the natural horizon is replaced by a bubble floating in a liquid. On the ground this instrument will measure an altitude to one minute of arc, and would continue to do so in the air if only the aircraft could be persuaded to fly truly straight and truly level. Unfortunately this never happens. In practice, the aircraft, even when it appears to be flying straight, is in reality weaving slightly from side to side. The motion may be imperceptible to the pilot but to the bubble of the navigator’s sextant it causes large and fluctuating errors. This sextant is one of the many devices on which an air crew’s safety depends.
MARK IX AVERAGING SEXTANT. Bubble sextant used for taking bearings on celestial bodies. It incorporates a device for averaging the results of six observations. The bubble replaces the horizon.
The only course open to the navigator then is to take many sights and average them. The taking of an observation with a bubble sextant may be a far more laborious process than the single observation from the bridge of a ship at sea. It often entails standing at an open hatch with frozen fingers for five or ten minutes at a time patiently recording a series of from six to two dozen individual observations. In addition to cold and fatigue, lack of oxygen often renders accurate work even more difficult.
The first bubble sextants issued were designed at the Royal Aircraft Establishment at Farnborough long before the days of R.A.F. expansion, and at a time when very few navigators were trained in the use of the sextant. The demand for the instrument was slight and the sextant was in no way designed for mass production. It was a good rugged instrument and sundry improvements ran up to a MK VIII. In about 1938 it was realized that with the enormously increased range of modern aircraft celestial navigation was going to be more in demand, and with the coming expansion in view the specification for a MK IX sextant, was drawn up so as to be capable of mass production. This sextant incorporated an ingenious mechanism for averaging the result of six observations, and it could be produced in the large quantities which were required. Although there are a number of MK VIII sextants still doing good service, the MK IX averaging sextant is now standard in the R.A.F.
TAKING AN OBSERVATION. Navigator of a Sunderland flying-boat taking an observation by means of a Mk VIII bubble sextant. Depending upon the degree of accuracy, he may have to average out the results of as many as two dozen observations because of the unsteady movement of the aircraft.
Without accurate knowledge of Greenwich Time the navigator would be unable to work by astronomical observations, and it is to the Admiralty that we owe the development of the chronometer, and indeed accurate time-pieces of all kinds. The real chronometer is a wonderfully accurate and very delicate piece of work. Not only does it keep time more accurately than is really necessary for air navigation, but the treatment it receives, the fluctuations of temperature and the vibration are sheer cruelty to such a delicate and expensive instrument.
In practice the endurance of aircraft is not great, and time signals are plentiful. Flying-boats are normally on patrol for twelve or fourteen hours while bombers do not as a rule fly for more than eight or ten. The watches issued to navigators in the R.A.F. are wrist watches of good quality which will keep time accurately enough over flights of such duration. They are fitted with a rotatable bezel engraved with a scale of seconds, so that if the long centre seconds hand is not set absolutely accurately, it may be made to read correctly against this outer scale.
When the sextant observation has been made and the exact time of the observation has been noted, it remains to reduce the observation to a position line on the chart. This sounds a very formidable process and until a few years ago it was so. Now, however, it has been made very easy, for those responsible for developing the methods of celestial navigation to be used in the R.A.F. have realized that only the most simple processes were practicable and workable in the ail. So simple has it been made that it has been found possible to give navigators as little as three weeks’ instruction in celestial navigation, for them to be able to navigate successfully.
This simplicity has been achieved by the production of three things: a “planisphere”, the “air almanac”, and the “air navigation tables.” Little or no arithmetic is required, and it is not even essential to know the principal stars by name. The planisphere is a kind of map on which the positions of the stars can be found. The principal stars used in navigation are numbered, the numbers correspond with those in the air almanac and also with those in the air navigation tables. The numbers in the air navigation tables are “thumb-holed”, so that a pupil may make an observation and turn up the right page in the tables without even knowing the name of the star with which he is working. In practice, of course, the pupil soon becomes interested and learns to recognize and name the dozen or more stars which he requires. In the old days it used to require some twenty minutes’ work with logarithms before use could be made of an observation: now the position line is on the chart in two or three minutes.
The limitations to the use of celestial navigation are firstly due to the fact that there is only one sun by day, which will give a position line but not a “fix”. Secondly, of course, it is altogether useless when the sky is obscured by cloud.
Use of Wireless
As with other aids to navigation there are many variations in the use of wireless. Essentially the process consists of receiving a signal and detecting the direction from which it is coming. This direction can then be plotted as a bearing on the map and the transmitter which sent the signal will lie somewhere on that line. If a second bearing can be obtained cutting the first one, the point where they cross will be the transmitter’s actual position or “fix” as it is called.
Now there are two ways of using this; the directive reception may be on the ground or it may be in the aircraft. First, let us consider the organization on the ground.
Suitably disposed all over the British Isles are many direction finding stations. They are arranged in a number of different groups. There are usually three stations working on the same wave length in each group. If an aircraft is lost or uncertain of its position it calls up on the appropriate wave length and asks for a “fix”. Each of the three “D.F.” stations receives the aircraft’s signals and notes the bearing, that is the direction from which they are coming. Now one of the three stations is a master station and the two others send in the bearings they have received to this one. In the master station all three bearings are plotted and their intersection is the position of the aircraft.
The master station then signals to the aircraft telling it where it is. This sounds rather a roundabout process, but in practice the service can be extraordinarily prompt. An aircraft is often given its “fix” within one minute of asking for it. In some circumstances a navigator may ask for a single bearing rather than a position, which saves a little time.
There are, of course, various drawbacks and limitations as there are to all methods of navigation. In the first place, if the aircraft is close to enemy units and not already discovered by them, it is undesirable for it to break wireless silence and so disclose its position equally to the enemy. Secondly there is the possibility of an enemy ground station answering the aircraft’s call and giving it a bogus position. This happened to the navigator of a night bomber in the first few months of the war. He was returning across the North Sea having been badly damaged. The machine was low over the water and only just holding its height. The navigator wanted a bearing to help him steer for the nearest point on the English coast. No sooner had his wireless operator made the signal than a very loud station replied giving him a perfectly false bearing. Fortunately the bearing given was so wrong as to be quite absurd. It was ignored and the aircraft got safely home. To guard against incidents such as this the wireless signals are now put into code and there is a recognition procedure to ensure that both ground station and aircraft can be certain of each other’s identity. A third limitation of this type of wireless aid is that when there are perhaps a hundred or more bombers in the air, the ground stations get so much overworked that the system is bound to become very much congested.
The other way in which wireless bearings are used is by putting the directive receiver into the aircraft. Here the wireless set has a loop receiving aerial, and the wireless operator in the aircraft measures the direction from which a signal is coming. Here the problem is to find suitable transmitting stations upon which to take bearings. The German broadcast transmitters usually close down during our raids to deny us this assistance. The Germans themselves established a number of beacon transmitters specially for the purpose and moved them about and changed their call signs and wave lengths hoping that we could not use them as well. The broadcasts of the B.B.C. transmitters, of course, have been arranged in such a way that it is impossible for an enemy aircraft to get an accurate bearing from them at any time.
There is one other variety of wireless aid to navigation. That is the beam system which is so universally used on the air lines all over the United States, and which has been brought to a very advanced state of technical development by the Germans. Here a ground station emanates a beam signal in a certain direction. The crew of the aircraft can tell whether or not they are in the beam and so can travel along it. This has the disadvantage, however, that it can only be used on one fixed route and in consequence does not help the navigator who wants to travel in another direction.
A NAVIGATOR PLOTS HIS COURSE. Navigator’s station in a Short Sunderland long range flying-boat. The captain is instructing his crew to stand by ready for the take off. The navigator is already at work plotting his course. Sunderlands fly mostly far out over the sea, so a very high standard of navigation is required comparable, in fact, with that of a transatlantic liner, but with far more variable conditions.
It would be impossible to give the reader a proper conception of navigation in the R.A.F. without mention of the Meteorological Service. Every R.A.F. station has its “Met. Office,” where the forecasting officer and his assistants study the weather maps and prepare their forecasts. The navigator’s training includes a short course of meteorology, and although he is not expected to be an expert on the subject he should know enough of the meteorologist’s work to enable him to get the full value of his advice. This training also gives him some sympathy for the weather man who is attempting the, at times, impossible task of forecasting the weather round these islands. It is to the “Met. Office” then that the navigator goes last thing before he flies. Here he studies what is known of the weather over his route, and he is told what to expect of the wind, the cloud amounts and the visibility. Often the forecast is correct, usually it is a pretty fair indication of what to expect, sometimes it is quite wrong: no fault of anybody, the weather has just “pulled a fast one” again.
Each method of navigation works to its best advantage in some conditions, but has drawbacks or limitations in others. No method is perfect, and there are circumstances in which each may be extremely inaccurate. A navigator is rather like a foreman with various workmen at his command, for he must know which of them will serve him best for any particular part of the job in hand. Often too they will come to him with contradictory pieces of evidence and he must weigh up in his mind which, in the circumstances, is more likely to be correct.
The good navigator must first of all study the theory of his subject in the class room. Next he must accumulate a store of experience in the air to learn just how the theory works out in practice. Then as each perplexing problem presents itself he must use both his knowledge and experience to make what really amounts to an intelligent guess.
In this chapter we have seen what a very important role must be played by the navigator of any aircraft. There have been quite a number of occasions during the Second World War when flying operations against the enemy have had to be carried out under really unsatisfactory weather conditions, and when almost everything has depended on the skill and alert brain of the bomber’s navigator.
NAVIGATION EXERCISES FOR OBSERVERS. Before they become fully operational, navigators are given a final test to make sure that they are fully competent to discharge their very responsible job. The navigator (above, left) seated behind a board on the other side of which is the instructor, is acting in accordance with orders given to him. Electrical instruments record the accuracy with which he carries them out.
