Restless man has never been satisfied to remain within the confines of his . familiar world, either geographically or mentally. Had it been otherwise, progress would have ceased.
In none of his explorations has he been more successful in finding the unfamiliar than in those which took him into the polar regions. Here he found new and strange scenery, as in previous explorations, and, in addition, he found that he had outrun many of the conventional concepts that characterize more familiar regions. Such apparently stable things as sunrise and sunset, morning and afternoon, day and night, parallels and meridians, and even time and direction had lost much of their customary significance. As if this were not alarming enough, the polar navigator found that even his trustworthy magnetic compass became useless, and his gyro compass was little better.
But difficulty serves only to increase man’s determination to conquer. His navigation methods had been devised for lower latitudes, where they served their purpose well. It was logical that familiar methods should accompany him to the far north or south, even though they became increasingly difficult to apply. It seems strange to us now that Admiral Byrd, on his first trip to Antarctica in 1929, took Mercator plotting sheets extending to latitude 89°45'S, only 15 miles from the South Pole! But this was the projection he was accustomed to using.
Special methods and techniques were not available to early polar navigators because there had been no requirement for them. As long as trips into high latitudes were rare, little progress was made in polar navigation, but as operations there became more common, special methods and applications gradually evolved. By the middle of the twentieth century the pattern of polar navigation has become well established. This does not mean that little future progress can be expected. On the contrary, there is perhaps no branch of navigation in which progress is more rapid. But it does mean that polar navigation has emerged from the experimental stage into a practical everyday procedure. Let us review briefly the methods and equipment currently in use.
Charts
So firmly established was the Mercator concept that it was difficult for the polar navigator to discard it in favor of more useful projections. Most marine navigators still use the Mercator chart in high latitudes, believing that its familiar advantages outweigh its disadvantages. Aviators have been less reluctant to discard the Mercator, partly because their flights often take them to higher latitudes, where the Mercator is more objectionable, and partly because they have already become familiar with other projections in lower latitudes and the change is less radical. That the choice of a projection is strongly influenced by the past experience of the navigator is indicated by the fact that navigators of the three groups of Operation Highjump each selected a different projection and all gave the identical reason—plotting was easier!
Projections that have been suggested for polar regions include the following:
Transverse Mercator. In this projection the surface of the earth is developed on the familiar Mercator pattern of latitude and longitude lines rotated 90° from their usual position. That is, the fictitious “equator” of the transverse Mercator is a meridian and the “poles” are 90° away, situated on the geographical equator. In polar regions the meridians are almost straight lines radiating outward from the pole and the parallels are curves closely resembling circles. Fictitious meridians and parallels may be shown to aid in measuring direction (as indicated below) and distance. Near the fictitious equator a straight line is a close approximation of a great circle, being a fictitious rhumb line analogous to a true loxodrome near the geographical equator. Charts on the transverse Mercator projection (sometimes called inverse Mercator are currently published by the U. S. Navy Hydrographic Office for both north and south polar regions.
Modified Lambert Conformal. In this projection the surface of the earth is developed on a cone which intersects the surface at latitudes 74°00'00" and 89°59'58". When the cone is cut along a parallel and spread out flat, a small gap appears along the meridian at which it is cut. This is the familiar Lambert Conformal projection. When all of the parallels are stretched slightly to form complete circles, it becomes the modified Lambert conformal projection. Meridians are straight lines and parallels are circles. A great circle is represented exactly or very nearly by a straight line. This most recent addition to the list of projections useful for polar navigation was developed by the Canadian Department of Mines and Resources. Some experiments with this projection have been made in the United States, but no charts on it have been made generally available.
Polar Gnomonic. The “great circle” chart familiar to marine navigators becomes a special case in polar regions. Points on the surface of the earth are projected from the center of the earth to a plane tangent at the pole. The meridians are straight lines radiating outward from the pole and the parallels are concentric circles. Distortion increases with distance from the pole. Any great circle is represented by a straight line. As on other gnomonic charts, angles are not correctly represented on the polar version and hence the usefulness of this projection is limited largely to planning purposes. The U. S. Navy Hydrographic Office publishes a large polar gnomonic chart, in two parts, of the Arctic.
Polar Stereographic. If the point of projection of a polar gnomonic chart is moved from the center of the earth to the opposite pole, a polar stereographic chart is obtained. It is similar in appearance to the gnomonic, but the expansion of the meridians with increased distance from the pole is less. However, angles are correctly represented, a straight line is a close approximation or exactly a great circle, and small circles on the earth appear as circles on the chart. Polar stereographic charts are published by the USAF Aeronautical Chart and Information Service.
Polar Azimuthal Equidistant. This is a simple projection to construct since meridians are radial lines and parallels are equally spaced, concentric circles. Angles are correctly represented only at the pole and hence the projection is of limited navigational value. However, the projection is useful for showing the area near the geographical poles. A bathymetric chart of Antarctica on this projection is published by the U. S. Navy Hydrographic Office.
Thus, the transverse Mercator, modified Lambert conformal, and polar stereographic projections are all suitable for navigation near the geographical poles. The choice is largely a matter of personal preference, none of them having a clear advantage over the others.
The principal deficiency of polar charts is not a matter of projection, but one of detail. Polar regions have not been accurately surveyed and the information given is not as complete or reliable as elsewhere. Even where shorelines are correctly shown on the chart, they are often difficult to locate by the navigator when a uniform covering of snow blankets both land and sea ice. Ships navigating in high latitudes find it necessary to sound almost continuously, and it is common practice to send a portable echo sounder ahead in a motor launch when entering a strange harbor or unfamiliar waters where shoals might reasonably be expected. Marine navigators sometimes find it safer to plot their positions relative to inaccurately charted land than in their correct geographical locations, for this is one region where a good three-star fix might plot on land and not be incorrect geographically!
Direction
Direction in polar regions poses two separate problems. First, how is it to be expressed, and, second, how is it to be measured?
Thanks largely to Gerardus Mercator, perhaps, we live in a rectangular world. Certainly, in the minds of most Americans all north-south lines are parallel. After all, do north-south streets of a city meet?
Conditions are quite different in polar regions. Here the meridians extend outward from the poles, like spokes of a giant wheel and parallels are circles. Radio waves and light travel approximately along great circles. Two aircraft in sight of each other are each north of the other if the north pole is between them! At the North Pole all directions are south!
Thus, in polar regions direction changes so rapidly that the usual convention is no longer convenient. At the pole itself “direction” can be satisfactorily replaced by longitude, but as the distance from the pole increases, the correction needed to apply this system becomes increasingly larger. A familiar but applicable system is needed in this region.
The need is met by superimposing the usual rectangular system or grid over the spiderweb network of meridians and parallels. On the transverse Mercator projection such a network is provided by the fictitious meridians and parallels. The former are straight lines perpendicular to the fictitious equator, and the latter are straight lines parallel to it but located at increasing intervals in accordance with the usual Mercator principles. Direction can be stated with reference to this grid in the same manner as with the actual graticule near the equator. The pole of this system is on the geographical equator, 90° away, resulting in negligible convergence. On other projections a similar and equally usable result can be obtained by drawing a series of straight lines on the chart parallel to any selected meridian.
Directions relative to this reference are called grid directions and navigation by their use is called grid navigation. It is a system applicable anywhere, but is most useful in polar regions.
Any meridian can be chosen as the origin, and several have been suggested. The one now generally accepted is that through Greenwich, with grid north being the direction of the north pole from Greenwich. Thus, at both poles the direction 000° grid is the same as 000° on the Greenwich meridian. Interconversion of true and grid directions is simple. If G represents grid direction, T true direction, λ longitude, E east, and W west, the following relationships hold in the northern hemisphere:
G= T + λW
G=T - λE
T=G - λW
T = G + λE
The sign (+ or -) in each of these equations is reversed in the southern hemisphere. A modified E-6B computer makes the .conversion mechanically.
But is this system usable in practice? Actually, it is the most convenient and useful system yet suggested. But how does the compass distinguish between grid and true directions? This leads to the second problem, that of measuring direction.
The magnetic compass in its various forms indicates the direction of magnetic lines of force. Thus, it measures magnetic, not true directions (assuming a compass without deviation) and, except along the agonic line (the line of no magnetic variation), a correction must be applied to its reading if true directions are desired. This correction, called variation by navigators, is numerically equal to the angle between the magnetic and geographical meridians. Lines connecting points of equal variation meet at both magnetic and geographical poles, since these are the points at which the magnetic and geographical meridians meet. In the region of the poles the variation is large and may change rapidly with a relatively small change of position.
If the angle between the magnetic and fictitious meridians is used as the correction, it is called grid variation or grivation. It is as easily applied as variation and in exactly the same manner. Lines of equal grid variation meet at the magnetic poles, but ignore the geographical poles. Hence, the area of rapid change is reduced. Also, a craft using grid variation follows an approximate great circle. Thus, the correction for variation is combined with correction for convergence of the meridians, resulting in a single correction, easily applied. Hence, in those parts of the polar regions in which a magnetic compass has sufficient directive force to be usable, an approximate great circle can be followed more easily than a rhumb line!
The marine gyro compass indicates true north directly, usually with a very small error. At high latitudes the error increases and at the pole such a compass would fail as a direction indicator. But such compasses are found only aboard ship, and these seldom operate beyond 80° latitude. Above latitude 70° special corrections must be applied and the gyro error checked frequently.
The marine gyro compass is not used in aircraft because it is too heavy and would not operate satisfactorily at their speeds. The magnetic compass depends for its operation upon the horizontal component of the earth’s magnetic field. Over a large elliptical area (which includes the north pole) this field is too weak for reliable operation. The usefulness of the magnetic compass is also limited by the rapid convergence of lines of equal magnetic variation near the magnetic poles, the lack of reliable magnetic information over much of the polar region, the large diurnal change in variation, and the large change in deviation reported under some conditions. Some of these limitations are being partly removed, but there remains a large area in which the magnetic compass is of little or no use.
In aircraft the directional gyro is used. When this instrument is set in a selected direction, it continues to indicate approximately the same direction (along a great circle) until it is reset. However, the instrument develops a progressively increasing error called wander, the amount and sign of which cannot be reliably predicted. In earlier models this error was often quite large, but in the latest designs it seldom exceeds 4° per hour.
Despite this limitation the directional gyro is a very useful instrument, particularly in polar regions. It is checked about three times per hour by means of the magnetic compass or by the astro compass, an instrument which provides a mechanical solution of the navigational triangle. When this instrument is properly set and the sighting vanes are lined up with a celestial body, it indicates the true heading of the aircraft. By the insertion of an additional scale the instrument can be altered to indicate grid direction.
A careful record of the errors of the directional gyro is kept. This serves not only as a log of past performance, but also as a basis for predictions for the immediate future. However, the keeping of a gyro log is tedious and time consuming. Further, the interpretation of the performance of the instrument is difficult unless checks are made at equal intervals of time. The substitution of a simple graphical solution, called a gyro graph, has reduced the work considerably and provided immediate indication of performance, even when checks are made at variable intervals. The setting of the astro compass preparatory to making a check has been simplified somewhat by the use of a plastic computer to provide a quick determination of local hour angle, an element otherwise determined by means of an almanac.
Thus, a directional gyro checked frequently with an astro compass provides reliable direction guidance over the entire extent of both polar areas. But even after the introduction of this combination one serious limitation remained. The astro compass can be used only when a celestial body is visible. Since modern aircraft can generally fly above an overcast, weather seldom interferes. But during twilight, there is no celestial body visible unless the moon or a brilliant planet is above the horizon.
In most latitudes twilight is of short duration, but in polar regions it lasts for several weeks. When the setting sun makes contact with the horizon at the pole, there are yet about' 32 hours before the top of the sun drops out of sight! During this time the sun makes a trip and a third around the horizon. In this region the motion of the aircraft can be important, causing the sun to rise or set at any azimuth, and at any hour of the day! Twilight exists in a relatively narrow band around the earth. This band is continually moving westward. An aircraft moving eastward can cross it rather quickly, but one moving westward in high latitudes might remain in twilight almost indefinitely.
The best defense against this menace is careful planning to avoid a prolonged stay in the twilight belt. This was difficult until a simple graphical solution was devised and mechanized. Several forms have been tried. The first and perhaps the simplest consists of a map on a polar projection and a transparent template which is placed over the map and oriented by means of the Greenwich hour angle and declination of the sun. When it is so oriented, a shaded band on the template indicates the location of the twilight zone.
It has been indicated that the best way to defeat twilight in polar regions is to avoid it. But this is not always possible—or desirable —especially when military operations are involved. One of the outstanding, and certainly the most interesting, recent contributions to the advancement of polar navigation has been the introduction of the Pfund Sky Compass, which indicates the direction of the sun during twilight, thus removing the last limitation to practical determination of direction in polar regions. This instrument, suggested by the late Dr. A. H. Pfund of Johns Hopkins University, was developed by the National Bureau of Standards for the U. S. Navy.
The operating principle of the “compass” is simple. Part of the light from the sun is scattered as it enters the earth’s atmosphere, resulting in the blue appearance of the sky. The scattered light is said to be plane polarized, meaning that the vibrations are in a plane perpendicular to the line from the observer to the sun. The optical system of the instrument consists of a cellophane star cemented to a sheet of Polaroid similar to that used in sun glasses. When this is kept horizontal and rotated, the star appears alternately brighter and darker than its background. They are equally bright when the optical axis of the Polaroid is in the plane of polarization, or 90° from it. The instrument operates with practical navigational accuracy whether or not the sun is visible, as long as the zenith is clear. It is most accurate when the sun is on the horizon but operates satisfactorily until the sun is 7 or 8 degrees below the horizon, when stars have become visible.
Thus, a practical means of determining direction in the air during virtually all conditions in polar regions has been devised, removing much of the fear of this period. One other aspect of direction remains to be discussed. It is not sufficient that heading alone be accurately determined: drift angle must also be available if the direction of motion of an aircraft is to be known. When the surface is visible, both drift angle and ground speed are observed with a gyroscopic-stabilized drift meter if breaks in the ice or distinctive ground features are visible. Radar is used for the same purpose when an undercast obscures the surface. Both of these methods provide essentially instantaneous values. An average value of drift angle is often obtained over sea ice by means of the Bellamy method, a meteorological method based upon the fact that the cross-component of the wind is proportional to the atmospheric pressure gradient at constant altitude. This method often provides results that are more reliable than instantaneous values for dead reckoning.
The comparable problem at sea—the determination of currents—has not been adequately solved.
Celestial Navigation
Since celestial bodies are often the only means of determining both position and direction, celestial navigation is of greater relative importance in polar regions than elsewhere. Celestial observations are usually solved by H.O. 249 in the air and by H.O. 214 aboard ship. Weems’ star altitude curves and trigonometric methods such as H.O. 208 and H.O. 211 are available.
H.O. 249 is particularly useful in polar regions, where consecutive meridians are not far apart. Since the local hour angle of Aries replaces meridian angle of the body as one of the entering arguments, only one sight of a series need be solved if the observations are made exactly four minutes (1°) apart.
Aviators must be careful not to omit Coriolis correction in high latitudes.
Since refraction near the horizon is large and somewhat variable, navigators generally distrust low altitude observations. However, in polar regions these are often the only ones' available and high-latitude navigators do not hesitate to use them when needed. In fact, when the observer is several thousand feet above the surface of the earth, observations are sometimes made when the sun is considerably below the celestial horizon. A limitation of previous celestial navigation tables has been their failure to list values for low or negative altitudes. The “declination” tables provided in the 1952 edition of H.O. 249 will correct this deficiency.
A unique method of solving observations, fully available only in polar regions, is that of using the pole as the assumed position. At this point the celestial horizon and celestial equator coincide. Celestial bodies circle the sky, changing altitude only as they change declination. Declination and computed altitude are numerically the same, and the Greenwich hour angle replaces azimuth. Hence, computed altitude and “azimuth” can be determined by inspection from an almanac. Lines of position are plotted, advanced, or retired as in any latitude. This is a special case of meridian altitude. By means of the “Ellsworth Table” the method can be used when the craft is a considerable distance from the meridian over which the body is situated at the instant of observation. The table indicates the offset of the circle of position from the line perpendicular to the meridian under different conditions, assuming it is plotted on a polar stereographic chart.
Celestial observations aboard ship are frequently not available during the summer, when the ice is sufficiently broken to permit surface navigation; and when celestial bodies are visible, ice on the surface of the water often makes it difficult to estimate the position of the horizon. When a ship is in heavy pack ice, it often constitutes so stable a platform that more reliable results are obtained with an aircraft bubble sextant than with a marine sextant!
At the poles the day and year coincide, for the sun is above the horizon for six months and below for the following six months. Numerous celestial bodies are available for observation during the long polar night, but when the sun is visible, it is often the only body available. The moon is above the horizon about half the time, but is in a favorable position for only a few days each month, for when it is near the sun or almost opposite to it, the sun and moon lines are nearly parallel. When the sun is the only body available, three observations are customarily made with an interval of four minutes between observations. A most probable (estimated) position is established, using either the middle or average line of position thus determined. An hour later the process is repeated. A line of position or fix cannot be obtained by celestial observation when the sun is just below the horizon and no celestial bodies are visible.
Greenwich civil time is usually used for navigation in polar regions, since all time zones meet at the poles. Whatever a clock reads it is correct at some meridian, even if the clock is stopped!
Electronic Navigation
Electronic navigation is little used in polar regions for several reasons:
Electronic aids to navigation are almost non-existent in high latitudes. After preliminary experimentation, there was great hope that low frequency loran would provide the position fixing information so much needed in the arctic, but the first permanent installations proved so disappointing that they have been abandoned.
The vagaries of propagation are imperfectly understood anywhere, but some of the limitations reach such extreme amounts in polar regions as to render electronic aids unreliable, as in the case of low frequency loran.
The twilight period is the most difficult for electronic aids, and in the polar regions this may last for several weeks. During this period reliable electronic aids would be most helpful.
The radio direction finder is useful, but is seldom within range of transmitters.
Radar is very helpful but is limited in application by the almost complete absence of identifiable features. Reliable interpretation of the PPI requires training and experience in an area where land and water areas sometimes appear interchanged.
Practice
The techniques of polar navigation are becoming well established as the problem is being continuously studied by those engaged in regular or frequent operations in these regions. Most significant, perhaps, are the contributions made by the United States Air Weather Service, which operates a flight from Alaska to the north pole and back every second day; the United States Navy, which frequently sends combined sea and air task forces into both polar regions; and the Royal Canadian Air Force, which continually operates in high latitudes.
When regular aerial operations in polar regions began in 1946, it was customary to carry three navigators. It has now been possible to reduce this number to two, and a single navigator has demonstrated his ability to perform all the duties, but he did not have many spare moments! One navigator customarily makes all celestial observations and calculations for lines of position and heading checks. The other does all the plotting, performs the dead reckoning, keeps the log, and prepares half-hourly position reports. The navigators are assisted by a radar operator and at intervals by a weather observer.
The duties of the navigators have been simplified considerably by a number of contributions, each of which has been of little importance in itself, but which collectively have aided materially in reducing his work. The twilight computer and gyro graph have been mentioned, as has the modification of the astro compass to provide grid direction readings direct. The grid system itself is a distinct simplification of earlier systems of indicating direction. Celestial methods such as H.O. 249 simplify their work in establishing celestial lines of position. Improved instruments—-such as directional gyros, gyroscopic drift meters, and radar—have all contributed to the simplification of the navigators’ work. But perhaps the greatest contribution has been the confidence which they have gained from experience.
We have come a long way since the first intrepid explorers entered the polar regions, but the navigators are still the busiest members of the crew of a large aircraft on a polar mission. We still have far to go before the simplicity of their tasks approaches that of the pilot. This can be accomplished by the further development of their tools. When their celestial observations and astronomical heading checks are made automatically, when their dead reckoning is continuous and automatic, when their log is kept mechanically, the polar navigators will find that their principal duty is to interpret the results of their instruments—as the pilot does today.
But there will never be a substitute for the experience accumulated by navigators who have and are facing their polar assignments with confidence and determination to succeed even though their tools be imperfect. The polar navigators at the middle of the twentieth century use methods pioneered by those who preceded them and are in turn pioneering methods which will add to the simplicity and reliability of the polar navigator of the future.