The earlier papers in this series described the principles, and some of the techniques, which determine the approach to separation standards made at the Royal Aircraft Establishment. In this approach to the problems of estimating safe separation standards and specifying the quality of navigation needed, emphasis is laid on the observations of flying errors which occur in operational conditions. Dependence on theoretical assumptions is limited to the extent necessary to make usable ‘upper limit’ estimates of collision risk. Where critical assumptions are unavoidable, their nature and effects are shown.

In America and Great Britain people are on the whole well acquainted with the picturesque circumstances surrounding the discovery of the ‘line of position’ by the Merchant Navy captain, Thomas Sumner. On the other hand, less has been written about the personality of Admiral Marcq de St. Hilaire, and about his work which led him to propose the comprehensive and exact method, now universal, which bears his name. My intention, therefore, is not to go back over the theory or the practice, nor even the value of this system of position finding by comparing points. It is just a question of making this brilliant naval officer better known, his upbringing, character, and the historic circumstances of the perfect achievement of which he is the immortal author.

Before considering in detail the instrumentation involved, it is necessary to establish first of all what is meant by microwave radiometry and mention briefly its possible applications. A radiometer is a device for measuring heat radiation and the title ‘Microwave Radiometry’ is a little surprising in that it implies heat radiation at microwave frequencies. In fact a consideration of the underlying physics shows that heat radiation and electrical thermal noise are indeed the same phenomenon, although their properties were investigated independently by physicists and electrical engineers. This concept is particularly valuable in microwave radiometry, where the electronic techniques are appropriate to ultrasensitive noise measurements, but the system applications are most easily visualized in terms of heat, or, more particularly, temperature.

The following papers were presented at a meeting of the Institute held in London on 12 March 1965. Mr. W. P. Davies (Polhemus Associates) first outlines the height measurement requirements for a supersonic transport aircraft. The other speakers review the available techniques: Mr. R. Whalley (Ferranti) dealing with inertial techniques, Mr. B. Cape (S. Smith & Sons) with pressure measurement techniques and Mr. W. L. Garfield (Standard Telephones and Cables) with radio altimetry. Mr. G. E. Beck (The Marconi Company) describes a recently developed radio altimeter.

1. Introduction. In outlining requirements for height measurement in supersonic transport aircraft it should first be said that not all the requirements are special to supersonics. Some of the requirements are the same as already exist, or will exist in the 1970's, for subsonic aircraft. The main problem in supersonic aircraft will be to maintain the same accuracy over increased measurement ranges of height and height rate and in an environment of higher speeds and temperatures. Where-ever possible then, the requirements are of the same standard expected for subsonic aircraft of the 1970's. More stringent requirements are stated only where they are considered necessary; there are enough real problems in the advance to civil supersonic operations without creating artificial ones.

This paper examines the requirement for a new type of display for the pilot navigator of the S.S.T. (supersonic transport). The author holds that inductive reasoning, which is the basis of manual navigation techniques, should not be dispensed with and the benefits of automatic systems will not be achieved until a similar awareness of the processes going on about the navigator can be reproduced. A well-designed display system might have this potential.

The ideas behind this paper were first described at the Annual Meeting of the American Institute of Navigation in Long Beach, California, in June 1965 and further developed at a meeting of the Deutsche Gesellschaft für Ortung und Navigation held at Munich in August of 1965.

This is the third article of a series of three describing the methods developed at the Royal Aircraft Establishment for costing the effect of air traffic control deviations in long range systems. The first two discussed the general principles of costing and their application to the current and future systems for subsonic aircraft over the North Atlantic. The present paper considers systems for the S.S.T.

The characteristics and mode of operation of the supersonic transport are untested, and it will operate at an altitude for which information is relatively scanty. This means that the results obtained for the cost penalties in a supersonic traffic system must be regarded as tentative. On the other hand, because we are considering traffic systems for aircraft at such an early stage of development, the situation can be analysed in a more fundamental way, with particular emphasis on the effect of the system on the aircraft design.

Inertial navigation techniques have been applied as a means of improving the height information in military aircraft for some time. It is almost certain that the supersonic transport will use inertial techniques for plan navigation and it is interesting to see how far it is worth extending this to include height, and also height rate, information.

It is convenient to start by looking at the fundamental principles of inertial navigation as applied to height measurement; Fig. 1 shows the essential features. There will be an accelerometer measuring vertical acceleration; this must be maintained approximately vertical and can be very conveniently mounted on the main inertial platform. The output of the accelerometer contains a number of unwanted terms for which corrections have to be made. The expressions are well known and are explained in the Appendix.

There does not appear to be a standard British notation for the parts of the spherical triangle formed by the pole (P), the zenith (Z) and the observed body (S or X) that is such a familiar feature of astronomical navigation. The main value of such a standard (or generally accepted) notation would be academic and theoretical rather than practical, since most navigators are concerned only with the one system that they use. However, it can be most confusing when comparing different methods to find that (for example) both h and H are used for both hour angle and altitude.

From an examination of the outline requirements given by Mr. Davies, it is apparent that pressure measuring techniques can be used in all stages of a flight although it must be acknowledged that some of the tolerances he has called for cannot be met. The advantage of the pressure technique is that it uses the Earth's atmosphere which is experienced by all airborne vehicles and so provides a common datum for height or altitude measurement. It should be noted that wrong impressions can be got from the present pattern of accepting altitude or height in feet as a fixed physical measurement. A pressure sensor measures the pressure and, as the millibar is a widely accepted unit of pressure in aeronautics, it would really be more sensible to express aircraft altitude and separation values in millibars.

The input—output behaviour of a human operator in a control system is characterized by his ability to predict the course of a target based on past performance. With a stochastic signal or a more complicated system, however, prediction is necessarily imperfect and the operator's response lags behind the input. Mechanical prediction might therefore improve the human behaviour in a control system. The principles of the prediction display are described and two methods for mechanically calculated prediction are discussed: the model method and the extrapolation method.

This paper gives an account of a study carried out by the Institut Français de Navigation for the Administration de la Marine Marchande. The object of the study was to investigate the possibilities of introducing automation into the navigation of merchant ships. In addition to examining projects already underway in France and other countries, the Institut Français de Navigation sought the cooperation of numerous companies working in the electronics and computer fields and notably the Compagnie Générale d'Automatisme (C.G.A.), Compagnie Européenne d'Automatisme Électronique (C.A.E.) and Compagnie Générale de T.S.F. (C.S.F.).

Only the main features of this study are described here. Professor Hugon is Secretary-General of the French Institute.

Summary

Examples have been given of certain words which seem to be used correctly by navigators where there is any chance of ambiguity. It is suggested that, if these and other terms can be formally denned and agreed by the Institutes of Navigation, bad language will gradually die out and confusion in navigation terminology will be reduced to a minimum. A similar exercise might also be undertaken with regard to abbreviations.

It may be that the British Institute of Navigation working with other English-speaking Institutes of Navigation could make a start by defining certain words in the hope that they will gradually come to be used with a specific meaning. The alternative is a continuation of the present chaos whereby an official body such as the Airlines Electronics Engineering Committee can decide that ‘course’ and ‘track’ have the same meaning and therefore that a direction is a line.

This note is published with the kind permission of Mr. K. Fearnside, Technical Director of Smiths Aviation Division, but is not intended to represent Company views. Mr. M. W. Richey's advice and help is gratefully acknowledged.

The following discussion took place at an open meeting of the Technical Committee held in London on 8 December 1965. Captain Cotter, Extra-Master, is Senior Lecturer in Navigation at the Welsh College of Advanced Technology. Captain Wepster is Marine Manager of the Holland-America Line. Commander Leathes' contribution was prepared by the navigation section of H.M.S. Dryad and Wing Commander Rayson's contribution was presented on behalf of the Director of Navigation at the Ministry of Defence. Mr. Page is Chief Navigator of British United Airways.

Weather Radars. The presence of heavy rain and hailstones in cumulo-nimbus clouds is a natural phenomenon which is of vital importance to people who fly in aircraft. With a suitable radar fitted in the nose of an aircraft, this precipitation can be detected and the information presented to the pilot so that he can steer the aircraft to avoid the dangerous turbulent conditions associated with cumulo-nimbus clouds. This is, of course, the basis of weather radars which have been used for many years, and are now regarded as more or less essential for civil aircraft.

In the terminal area and for the approach and landing phase, radio provides the most appropriate method of height measurement. That is to say that the altitude reference frame is switched from barometric to terrain clearance measurement. It is at first a little surprising to see from Mr. Davies' requirements for this phase that the accuracies are in line with those currently in use in subsonic jets. But consideration shows that this is because the approach speed and general handling characteristics of the S.S.T. must approximate to those of subsonic jets in the approach and landing phase. Mr. Davies has also stated that automatic landing is essential for the S.S.T., so that the picture of requirements for radio altimetry in the approach and landing phase is fairly clear.

In a recent note, D. A. Lloyd has obtained a formula for the frequency distribution of time dependent errors in terms of the exponential integral

This note clarifies Lloyd's derivations by referring to a theoretically identical, though conceptually different, time-independent, physical model, relates this distribution to one described by Anderson and concludes with a short appreciation of the role of the negative exponential distribution in navigational studies.

It is, I believe, generally agreed that if the aspect of a ship could be seen on the radar screen from a range of 4 or 5 miles down, the information provided by the radar picture would be sufficient to prevent collisions from occurring. The aspect cannot be directly observed on an ordinary 3-cm. marine radar, but I think such a set could be modified in the following way to enable the aspect to be shown.

Progressive changes in the relationship of aircraft navigation and Air Traffic Control have produced a disturbing discrepancy between terrain clearance theory and practice. In theory, terrain clearance is assured by the aircraft adhering to certain specific and well-known traditional techniques; but in practice these are now often allowed to go by default, due to pressure of the A.T.C. environment. The purpose of this paper is to illuminate the exact nature of the problem and to suggest that clarification is overdue.

This contribution is somewhat aside from the flow of the discussion so far. Mr. Garfield has dealt with the general features of radio altimetry and I am going to describe only one version of a radio altimeter in a little more detail. What I have to say is also a progress report in continuation of an earlier note published in the Journal; at that time the possibility of obtaining altitude as additional information from an airborne doppler navigator was discussed, and it was concluded that an accuracy of ± 5 per cent ± 2 ft. over the range of 10–500 ft. was possible. It was emphasized that the work was in an investigational stage, and no specific design was described. Today, over five years later, I can report on the flight trials of an engineered equipment using these principles. The long gap is not a true measure of the total time from design study to a unit in production, but the hazards of either commercial or military supported development programmes can extend the time in a manner which is all too familiar to many people.

The problem of determining longitude without reference to a timepiece is one which even today commands the interest of a number of mathematicians, astronomers and navigators. Having recently undertaken a crossing of the Atlantic in a small boat, I was particularly interested in Mr. Chichester's article and Mr. Sadler's comments in the January edition of this Journal.

The use of the visible horizon as a reference point for measuring the lunar distance from a celestial body is described in some of the older books on Nautical Astronomy. One method consists of observing the altitudes of the Moon and second body on opposite sides of the meridian and thence computing their respective hour angles. The sum of their hour angles is then equivalent to the difference in right ascensions of the two bodies. With this information, the lunar distance can readily be found and compared with tabulated or calculated lunar distances for given instants of G.M.T. to obtain the G.M.T. of observation. Mr. Chichester's variation of the method is certainly ingenious, and the need to compare lunar distances is thereby dispensed with. I cannot, however, see any reason why intervals as large as one hour between plots should be necessary: the overall size of the plot could well be reduced to one-sixth of the size, without loss of accuracy, by using times say 5 min. on each side of the ‘guessed at’ G.M.T.