AERIAL NAVIGATION - POSITION FINDING.
utility of dead reckoning navigation is limited by the need of at least occasional observation of the ground, and, unless it is possible definitely to recognize some landmark from time to time, the method is subject to accumulative error. In fog, or when an airman is flying above cloud, the ground is not visible and an al ternative means of navigation must be provided. One such means is direction finding by wireless (see WIRELESS TELEGRAPHY) ; while another, navigation by astronomical observation, has been used at sea since the i6th century.
All methods of direction find ing by wireless have one underlying principle. If a rectangular coil, capable of symmetrical rotation about a vertical axis, be so placed that the plane of the coil is at right angles to the direction from which the wireless waves are transmitted, no current is set up in the coil ; in any other position an oscillating current flows having a maximum value when the coil is set "edge on," i.e., when the two vertical sides and the sending station are in the same vertical plane. If, therefore, the coil be rotated until the current is observed to be a maximum or a minimum, the bearing of the sending station can be determined. In practice, telephonic detec tion is employed and the position of the minimum or zero signal is preferred to that of maximum strength owing to the much greater sensitivity obtainable. For ground reception the Bellini Tosi system is usually employed ; this consists of two small fixed coils, mutually at right angles, each being connected to an aerial loop, and a rotatable search coil symmetrically arranged with respect to the two fixed coils.
This is the only method at present regularly employed on commercial air routes. The aeroplane carries transmitting and receiving sets, and calls for its position. Two or more ground stations observe the bearing of the aircraft, and one of them plots the position on a chart and communicates the information to the aeroplane. This method is fairly satis factory for day use, but reception by a ground station at night is subject to inaccuracies known as "night-effects." Moreover, it is for many reasons desirable that the position should be deter mined by the aircraft itself and not by a ground station. For this purpose it is sufficient if the ground station transmits signals and the aircraft carries a receiving set, together with means for detect ing the direction of the wireless waves. The following are various modifications of this system ; each has been the subject of experi ment but none is yet in regular use.
This method has undergone successful air tests and can be operated by the pilot. Two sets of coils mutually at right angles are fitted in the aeroplane wings and connected to a receiver. One set, the main coils, is directed fore and aft, and the other, the auxiliary coils, is athwartships. When the aircraft is headed toward the sending station the auxiliary coil receives no current but the main coil receives maximum energy. Under these conditions reversal of the auxiliary coil has no effect upon the signals received by virtue of the current set up in the main coil. If now the aircraft be turned so that its fore and aft axis is no longer toward the station, a current is set up in the auxiliary coil, and alternate reversals of the auxiliary coil will, under these con ditions, result in unequal strengths of signals; the aircraft must then be turned until equality of signal strength is obtained. The disadvantage of the method is that it is convenient only when the sending station is the objective or lies on the desired track of the aircraft ; moreover, the time taken to fly to the sending station When navigating by wing coils will not, in general, be the shortest possible time, though this is not a very serious objection. The method cannot conveniently be employed to determine position ; "homing" by wing coils merely ensures that the aircraft will eventually reach the sending station.
An alternative method is to use two coils mounted on frames suitable for fitting in the fuselage of the air craft ; these coils are mutually at right angles and may be rotated about a vertical axis. With this arrangement the bearing of a sending station may be obtained, without change of course, by the method described for wing coils. Two or more such bearings on different sending stations, of known positions, would enable the position of the aircraft to be determined. This method neces sitates special apparatus and requires the attention of a skilled observer.
In this method the ground station apparatus consists of a fairly large loop which rotates at a speed of about one revolution per minute. No energy is radiated in a direction at right angles to the plane of the coil, and by "tun ing in" to the station and observing the time between two consecu tive points of silence, the exact speed of rotation can be estimated. Further, if the station emits a distinctive signal when the loop is directed, say, due north, the bearing of the sending station can be calculated by observing the time interval between the distinctive signal and the next zero signal. In practice it would be necessary to use two distinctive signals, preferably 90 degrees apart.
The radio range beacon has proved highly efficient as an aid to navigation in the United States. This system involves use of transmitters or range beacon stations vir tually throughout the entire country, the transmitters broadcast ing a guiding signal and weather reports 24 hours a day.
A radio beacon in normal operation transmits its signals within a circle having a radius varying from 25 to I oo miles, depend ing upon its power. The two Morse telegraph characteristics N (dash-dot) and A (dot-dash) are broadcast. The N is audible in two quadrants of the circle—the one in which the true north line lies, and the one opposite—and the A is heard on the other two.
The on-course signal is heard along the lines dividing the quadrants where the pilot hears both A and N at equal strength. Along these courses the A and N merge into a continuous mono tone, thus forming what are technically called equi-signal zones, but more commonly termed the "radio beams" of the radio range beacons, and the orientation is so adjusted that these "beams" coincide with routes of the airways which the beacon is installed to serve. This beacon is interrupted at periodic intervals, usually 12 seconds, by the automatic transmission of an identifying characteristic, informing the pilot as to the range beacon to which he is listening. Further interruption of the beacon occurs at much wider intervals (usually twice each hour) to allow the broadcast by radio telephone of weather reports from observing stations located throughout the area served by the range beacon together with selected adjacent sectors.
Reception of the broadcast is audible by means of earphones worn by the pilot so long as he is flying the "beam," although the system has so progressed that registration of the strength of the signal, indicating on or off course flying, is now possible by use of a visual indicator.
The deter mination of the position of an aeroplane by astronomical observa tion is attended by many difficulties and is hardly likely to be employed on a developed air route along which it is convenient to erect wireless beacons. For long distance pioneer flights over un developed country and for trans-Atlantic flights, astronomical ob servation remains the only method of checking the D.R. position. The principles employed are essentially those used for maritime navigation (see NAVIGATION), but both the accuracy needed and the accuracy attainable are considerably less on aeroplanes. Deter mination of the altitude of a heavenly body necessitates either that the horizon shall be visible, or that the true horizontal or vertical be otherwise known. The horizon is often indefinite from an aircraft, and this fact has led to the development at the Royal Aircraft Establishment, Farnborough, of an instrument known as the Bubble Sextant, and the Aircraft Octant, developed by the Pioneer Instrument Company in conjunction with the Bureau of Aeronautics, U. S. Navy. A description of the Bubble Sextant will serve to illustrate the basic principles of these two devices.
image of a bubble, con tained in a chamber having a spherical glass cover, is observed by reflection from a pentagonal prism and subsequent trans mission through a collimating lens and an index glass. The optical distance of the bubble from the lens and the radius of curvature of the glass cover are both chosen to be equal to the focal length of the lens. This arrangement provides that if an image of the sun, obtained by reflection from the index mirror, is brought into coincidence with the bubble image and the instrument is rocked in the hand, the bubble remains in focus, and bubble and sun ap pear to move together. When measuring the altitude of a star it is more convenient to observe the bubble image by reflection from the under-side of the index mirror and the star by direct trans mission through the mirror. The bubble is, of course, subject to displacement if the aircraft experiences an acceleration which is not truly vertical, and, in practice, an average of ten readings is taken.
