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Aviation History
1949
1949 - 1283.PDF
JTTLY I4TH, I949 FLIGHT Astronomical Navigation Eight Papers by Specialists in the Art ENTITLED Astronomical Navigationin the Air—a Discussion on Methodsto Produce Speed and Accuracy, an important symposium was held recently inLondon by the Institute of Navigation, with the Astronomer Royal, Sir HaroldSpencer Jones, F.R.S., in the chair. Sum- maries of the papers read appear below: —Requirements for Astronomical Naviga- tion (J. B. Parker, read by D. O. Fraser).—The requirements for aids to air navigation were examined, and the suggestion madethat they fell into two distinct categories: (a) general aids for controlling passagesafely and efficiently; (b) more accurate and completely universal aids for specializedproblems, such as homing and the main- tenance of traffic patterns. It was argued that astronomical naviga-tion cannot fulfil the requirements of a type (b) aid and should, therefore, be con-sidered solely in terms of the requirements for type (a); a consideration of these re-quirements suggested that a 95 per cent fix- ing error of 10 nautical miles is sufficient. The paper advocated a simplication of the Air Almanacand/or reduction tables at the expense of accuracy, provided that the errors, when combined with those of the observer andhis equipment, fell within the tolerable 95 per cent error. Use of Astro in Fast Aircraft (A. H. Jessell).—This paperdiscussed in general terms the practical requirements of high- speed flight. The principal weakness in astro was consideredto be the time needed to take and plot a fix; unless this time could be much reduced it was unlikely that much use would bemade of astro in fast, high-flying aircraft. Astro, like any other navigation aid, could provide informa-tion either to the navigator or to the pilot, and possible de- velopments were considered under these two headings. Inthe first case, some sort of sextant was required, and features of such instruments were discussed by the speaker in the lightof the requirement for rapid fixing in pressure-cabin aircraft. The next requirement was for more rapid methods of reductionand plotting. A certain amount of accuracy of reduction methods could be sacrificed to this end, without reducing theoverall accuracy of astro fixing. The most direct method, perhaps, was to use star curves, on a reduced scale. In the case of astro as an aid to the pilot, the most likelydevelopment was some form of automatic astro compass. Altitude-Azimuth Almanac Suitability of an Altitude-azimuth Almanac (D. H. Sadler).—In this paper a number of possible methods of combining the almanac and reduction tables were examined. The conclusionreached was that none of them offered any substantial advan- tage over the present methods and was worth developing indetail. The only exception was a permanent table for the sun; this table was considered to be of some merit in itselfand worthy of further investigation, though it still left un- answered the problem of the moon and planets, and it isprobable that their inclusion would make it desirable to have a special method for the sun. Methods of Plotting and Reduction (P. F. Everitt).—Thispaper described a position-line protractor designed for use with long intercepts. The protractor has been constructed onthe horizontal stereographic projection to a scale of 1:3,000,000 and provides altitude-circle slots, through which a pencil canbe run, uniformly spaced at right angles to a central line of azimuth. The protractor can be used on a horizontal stereo-graphic projection or, without appreciable error, on the Lam- bert Conformal; it cannot be used with the Mercator projection. One suggested advantage of the protractor is that the"mesh " of altitude-azimuth tables such as A.N.T., H.O. 214, and H.O. 249. can be opened from one degree to three,thus achieving a ninefold reduction in bulk. It was also sug- gested that the pre-computation of altitudes and azimuthscould in most cases be done by an operations staff on the ground, the navigator making his observations within fiveminutes of the given time and correcting for run and difference of time. An Altitude-azimuth Sextant (R. Genty—read by Prof. A. T. R T7 The Hughes position-line protractor which was the subject of Mr. P. F. Everitt's paper. Bastien).—A method was described by which the sphericalastronomical triangle is solved using the altitude, azimuth and declination of a star. Altitude and azimuth are measuredsimultaneously with a sextant measuring azimuth from a reference star (Polaris or 0 Hydrae Male). Using this method,a single twin-star observation allows a fix to be obtained in terms of its geographical co-ordinates by means of special tablesand certain additions and subtractions. Air Navigation by Zenith Photography (R. d. E. Atkinsonand E. Brett Hilder).—This paper showed that provided a camera can be suitably stabilized in an aircraft, zenith photo-graphy may be a rapid and sufficiently accurate method of astro navigation, with the advantage that a single observationgives a fix instead of a position-line. A method of stabilizing the camera was outlined, in whichtwo equal gyroscopes rotate at the same speed in opposite directions, and are connected by a linkage so that the mid-point between them is stationary when they precess by equaj and opposite amounts. The resulting photograph is fitted toa star map by a procedure which ignores distances and relies only on directions. Astronomical precession is.applied to thefix, not to the individual stars; corrections both for precession and for map distortion are made by simple graphical methods. Instrumental Reduction (A. J. Bastien).—The speaker de-scribed the principles of an instrument which would record continuously and instantaneously the latitude and longitudeof an aircraft in flight. The Vertical Datum (Francis Chichester).—Some suggestionsas to the accuracy required from astronomical navigation in the future were put forward in this paper. In particular itwas argued that a relatively high order of accuracy would be required for the determination of wind effect over short periodsfor the purpose of plotting pressure gradients in the vicinity of the aircraft. It was concluded that the principal failings of astronomicalnavigation at the moment were due to the vertical datum in use. THE DISCUSSION The papers were summed up by Capt. R. C. Alabaster, ofBritish South American Airways, and Mr. W. A. W. Fox, of the Ministry of Supply, gave a valuable summary of thedifferent methods proposed to define the problems of ttie ver- tical by gyroscope. In discussion, some further useful con-tributions were made, notably by W/C. E. W. Anderson, of Shawbury, who suggested that the pilot of a modern fastaircraft wanted to know not so much where he was as what he ought to do about it; he felt that plotting methods werenow rather beside the point, and suggested the tabulation of altitudes and azimuths of the best celestial bodies availablefor observation in specific areas of the world. The papers read at the symposium are to be published inextenso in the October issue of the Journal of the Institute of Navigation. The offices of the Institute are at the RoyalGeographical Societv, 1. Kensington Gore, London. S.W.7.
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