Any discussion on the navigational problems of hovercraft must obviously be closely associated with the control characteristics of the craft. Although these machines have been widely publicized as capable of operating over almost any terrain which is reasonably flat, it will be assumed that the present discussion is concerned primarily with operation over water.

The word hovercraft may be regarded as a generic term in the same category as aircraft, ship, motor vehicle &c., and practical examples are likely to take a variety of forms depending upon their particular application. Thus, the control characteristics and the way in which they affect the navigational problem are likely to vary, in degree at least, amongst the different types. Again, it must be realized that the hovercraft as a transport system is only at the beginning of a long development phase, and any remarks of a general nature which are made at this time must be read in that context. Even at this early stage, however, two distinct applications of the hovercraft principle, as applied to over-water transport machines, have emerged and it is important to appreciate the difference between them.

1. Introduction. By definition a hovercraft or air-cushion vehicle is a vehicle capable of being operated so that its weight is partly or wholly supported by a self-generated cushion of air that is dependent for its effectiveness on the proximity of the surface over which the vehicle operates. Thus from the safety aspect of navigation a hovercraft must be treated as a surface vehicle or craft; for any collision risks involved are proportional to the amount of conventional surface vehicles or craft and static obstructions, in the area of operation. The clearance of minor obstacles less than the hoverheight is practicable; but on present experimental craft with an undersurface clearance of three feet, these clearances are limited to small buoys, waterlogged obstructions and rock outcrops of limited height.

This paper has been written with the SRN-2 and VA-3 type of craft in mind; they are basically over water craft but with a capability of operating over land for the purpose of embarking or disembarking passengers and freight or of crossing mud flats or sandbanks. Immersed sidewall craft have not been considered since their problems are similar to those of high-speed launches. Overland air-cushion vehicles will, as Mr. Lamb has pointed out, most likely operate over prepared routes and therefore navigation should not present any problems.

The main advantage offered by air-cushion craft is speed. The second advantage is its amphibious capability. Because of this an air-cushion craft can operate routes that are out of the question for any other kind of vehicle except perhaps a helicopter. An example of such a route is the proposed Rhyl–Hoylake ferry, which at low water is almost an overland route.

The hovercraft will be required to move over land and water and, dependent on siting and the state of the tide, will also move over mixed conditions—land, mud, water, rocks, &c. It is a flexible vehicle and it will be capable of working in many parts of the world, and thought should be given right from the start of development to the use of aids to navigation which are self-contained.

The various weather conditions it will be expected to operate in and to maintain scheduled movement will vary from daylight—fine weather and clear visibility—to night and bad weather.

1. Navigational equipment. The radio navigational equipment likely to be carried in a hovercraft will depend on the type of craft, the route or area of operation, and the economics of operation.

At present the operational requirements for navigational equipment are not fully crystallized because no hovercraft is yet in commercial operation and the operational requirements may well differ from trial requirements. However, two distinct types of hovercraft could become operational soon, the immersed sidewall hovercraft and the amphibious air curtain hovercraft.

The preceding papers have shown that the present generation of hovercraft being used mainly on overwater routes should be considered as ships. The problems of operating these very fast craft in the same media as conventional vessels is receiving a lot of thought at the present time and perhaps a few more words on this subject would not be amiss.

In framing operational rules for hovercraft it has been proposed that they obey the Regulations for Preventing Collision at Sea in so far as their size and construction will permit. This is in fact the point of view which has been put forward by the Hovercraft Operational Requirements Panel, representing Hovercraft Developments Ltd. and the industry, in their provisional British Air-Cushion Vehicle Operational Rules.

If this approach is accepted, three questions immediately spring to the mind of the mariner;

(1) How manœuvrable are these machines that skid around on a cushion of air? The present papers have shown very clearly how well the turning circles and stopping distances of hovercraft compare with conventional ships. Control systems have improved tremendously since the first generation of machines and they are now far more manœuvrable than ships proceeding at a fraction of their speed. Experience in driving these craft has shown that this problem causes, little trouble, and as a result of various hovercraft demonstrations their manœuvrability at speed now seems to be generally accepted as satisfactory.

A number of air traffic control authorities are now introducing electronic digital computers into air traffic control systems in order to speed operations and reduce the burden on controllers. These machines will initially be used for the routine clerical and data storage operations, but their full capabilities can only be used if they are given a much greater share of the control task and allowed to make logical decisions and intervene in the handling of the traffic. In such cases the computer will be performing a task that cannot be handed over to a human operator at a moment's notice in the event of a machine failure, and extreme reliability becomes probably the most important single attribute of a computer used in such a system.

Although computers of adequate speed and capacity are being produced for commercial and scientific uses, with reliabilities sufficient for these purposes, the frequency of faults and the time required for routine maintenance are both far too large to permit their full-time use in an air traffic control system. The reliabilities of typical components and systems are discussed in the following section.

1. The reliability of typical components and systems. The reports of a number of computer users suggest that ‘availabilities’ of 97–99 percent are readily attained, so that only 1–3 percent of the available working time is lost due to unscheduled maintenance.

The work of the air traffic controller is necessarily closely interwoven with the instrumentation of the control system and it is difficult without actual operational studies to establish which tasks ought to be automatized and which left to the man. This paper describes tests carried out with existing instrumentation to convert a conventional radar display into a television-type display. The indication from the tests is that a 1000-line system would be needed. The paper was presented at the annual meeting of the German Navigation Society in Dusseldorf, 5–7 June 1962, and is printed with the kind permission of that body.

A great deal has been written, especially in the last few years, on the subject of collision at sea. The increase, however, in the speed of ships and in the density of traffic so magnifies the complexity of the problem that even closer and more earnest study is still called for. Three fields of inquiry demand our attention: technical advances, procedure, and the human factor. Of these, the widest and most promising might seem to be the first. Its importance is undeniable, and I am by no means the only one to have pointed out that with the new difficulties it raises it supplies also the appropriate solution. Even wider, however, and more complex is the field of human study; this covers both the others, for in this the final objective is to enable men to master what they have won for themselves, and this they cannot do—here we meet the familiar problems of philosophy, ethics and religion—unless they first learn to master themselves.

Compared with these two fields, the technical and the human, that of procedure must at first seem quite insignificant. Can one seriously imagine that there is in fact some new method of avoiding collisions, so abstruse that no one has yet been able to discover it? In my small book on the use of radar I wrote: ‘The Regulations envisage two ways of avoiding collision:

1. A movement carried out in concert with the other vessel.

2. A reduction of speed, if necessary to zero.

Airborne weather radar equipment, carried for detection of meteorological phenomena, is now well established as a standard equipment for most transport aircraft, and is even being specified for the larger executive types. It may be useful, therefore, to review the basic factors which influence the value of the information obtained and the progress made with equipment designs during the last decade.

Turbulence on its own cannot be detected by radar. The radar defines areas in which some form of precipitation is occurring, i.e. heavy rain, hail, ice crystals or snow. Of these, the areas in which large water droplets fall as heavy rain on the ground or exist in suspension in the cloud are of most interest, since they are inevitably associated with convective cloud formations. All other information relating to the possibility of turbulence, ice formations, hail, &c., must be deduced by the observer from his knowledge of the mechanism of such meteorological phenomena. However, it is clear that the radar must display information on large droplet type clouds only. The presentation of small water droplet clouds not associated with convective processes would serve only to confuse the picture.

The theory of reflections from meteorological phenomena is dealt with very thoroughly in the literature and here only the main conclusions will be summarized to show how the characteristics of a radar set affect the information produced.

At present the rate of failure of a weather radar set is of the order of 1 failure in 50 sector flights. Probably the most reliable unit of the equipment is the scanner, which is fortunate because it is the most difficult unit to locate in an aircraft by virtue of its size and the need for uninterrupted forward scanning. With its present size the installation of two scanners is not feasible. In a piston-engined aircraft a radar failure when confronted by prolonged turbulent cloud conditions means no loss of aircraft range due to the adoption of optimum turbulence airspeed. The serviceability rate afforded by a single equipment is, therefore, acceptable since speed is not critical to performance. In a jet the inability to pick a way around turbulent cells if the radar fails, necessitates, by the adoption of optimum turbulence airspeed and altitude, a considerable shortening of range. It may be that in a supersonic aircraft, an unserviceable weather radar will make the flight abortive, since the ability to become supersonic will depend on smooth flying conditions.

The possibility of completely automatizing the navigation of merchant ships has been the subject of a recent study and prototype anti-collision radar equipment has already been engineered and evaluated. Indications are that, particularly from an economic point of view, there is little likelihood of such equipment going to sea in the foreseeable future, if it is accepted that a principle aim of any such system should be to reduce manning requirements below three watch-keeping deck officers and a captain. The main reason is that complex navigational problems exist at the beginning and end of the voyage and the presence of a trained watch-keeping deck officer offers the most economic solution to these problems. Detailed analysis of these navigational problems shows that the simple application of automatization techniques to existing equipment offers improved, standards of navigational efficiency and thus increased safety and higher productivity. A new radar plotting system, the Decca 969/ARP, is designed to assist a single watch-keeping deck officer to navigate more efficiently by automatizing certain difficult navigational tasks now performed manually.

That the economic need for automatic or semi-automatic navigational equipment and the design of such equipment is a relatively complex matter is illustrated by the recent very full report issued by the U.S. Maritime Administration. It was clear by 1958 that new navigational radar equipment would be required; a design study was therefore commenced.

Many airborne doppler radars in use at present work with several beams in fixed directions and a pair of beams switched in sequence. In this paper, which was presented at the convention held in Dusseldorf in May 1961 (Journal, 14, 480), a new type of doppler radar is described in which a single beam is rotated around the vertical axis with a uniform angular velocity, keeping the depression angle constant. With this type of radar, combined with certain circuits in the transmitter-receiver, the frequency tracker and the data stabilizer, it is possible to measure the velocity of positive as well as negative values (including zero); it also allows easier functioning of data-stabilization. Further, a small and lightweight radar system may be constructed in this manner. This type of doppler radar is considered to be of particular use for small, low-speed aircraft.

When trials of civil aircraft doppler navigation systems are carried out over land the pattern of results generally show a small along-track error component and a large across-track error component, whereas when the basic doppler radar is assessed, the errors over land are about ±0·2 per cent of distance gone and ±0°.2 in drift (standard deviation). It is not possible to quote system errors for all permutations of doppler equipments and compass systems but it is suggested that a good working figure for the across-track system error component would be about ±3 per cent (standard deviation), i.e. a compass error of ±1°.7. Advantage can be taken of the inherent accuracy of the doppler radar equipment to determine the heading error from a knowledge of track made good and the mean doppler drift along that track.

The standard twin-channel civil doppler navigation computer combines aircraft heading with doppler drift and doppler distance gone to form a ‘computer track’ in coordinates of distance along and across a preset track and distance. The computer is, in effect, meaning the doppler drift between selected points and adding this value to an arbitrary datum (aircraft compass) to form the computer track. If we can determine the true track over the ground we can assume that the error between computer track and actual track is caused entirely by error in the arbitrary datum, i.e. heading error.

The system proposed by Manley (15, 172), to produce true motion on relative displays seems to suffer from two serious disadvantages which were not referred to. The first is that there would be a serious degradation in the resolution of the stored picture, since every time an original echo is rewritten into one of the Tenicon storage tubes the echo size would be increased both in range and bearing by the size of the beam spot. No doubt the full effect of this increase could be reduced by the introduction of pulse shaping circuits between the read-out and write-in points, but nevertheless the original small echo would be considerably increased in size if a number of read-out write-in operations were carried out as would be the case if the information was stored for a sufficiently long time to show true track. As an example, without applying any special pulse shaping techniques a small echo, two spot diameters long and one spot diameter wide, would be increased in size by 65 spot diameters if stored for 30 seconds (i.e. 10 scans of the aerial) and would be increased by 1890 spot diameters if stored for 3 minutes.

The second disadvantage is probably the more serious. Since any signal entering the system remains there until the stored information is cancelled, the effect of interference would be to flood the screen completely in a short space of time.

Having some time ago been requested to design a sundial for the Hydrographic Institute of Genoa (Fig. 1) I went through various books and found that the methods indicated for drawing sundials were all based on graphical constructions. Although quite interesting these methods have the inconvenience of all graphical solutions, that they require constructions which, besides being complicated, turn out to be rather inaccurate, especially when lines cross at small angles, as inevitably happens outside a limited central area of the drawing. To avoid this I worked out a numerical system, related to the familiar methods of astro-navigation, which, by means of simple formulae, gives the coordinates of the points necessary to design a sundial. As the bibliography I examined is, of course, limited, I am not in a position to state whether this method of calculation is new or not; I describe it here hoping it may interest people who have sometimes wondered how sundials are made.

Recently I was asked to assist the engineers in my company (a Canadian electronics firm) devise a method for using radiation from the Sun to check the alignment of heavy ground radar sets to within very fine tolerances. The degree of accuracy would have to be better than five minutes of arc in azimuth for search antennas, and three minutes of arc in elevation angle for height finding antennas. Solar radiation in the microwave region of the radio frequency spectrum would be used as a reference. The antenna reflector would be rotated in azimuth and elevation until maximum solar radiation was reflected into the waveguide horn and thence to the radar receiver. The moment peak detector current was obtained, the antenna would be locked in position and the exact G.M.T. noted. Normal astro-navigation calculation methods and tables would then be used to calculate the Sun's true azimuth and elevation angle, which would be compared with that measured by the antenna to determine its accuracy.

Since none of the engineers had previous navigation experience, they had run into difficulties when they tried to devise a method for calculating the Sun's position to within the requisite accuracy limits.

The first difficulty was the fact that most antenna calibrations would be performed by field technicians having no knowledge of astro-navigation tables and calculation methods. They would be at extremely remote radar stations across Canada and thus would not have an opportunity to ask questions if our instructions proved ambiguous.

The imaginary question I propose to answer in this address is—how does it come about that a person with such a very ordinary sort of intellect, and certainly with no particular flair for mathematics, should be the author, or nominal author, of such an imposing publication as a volume of nautical tables?

To build up the background to the story briefly let me explain that in my training ship days I received the impression that a ship's position was found by mathematical calculations. True, we knew that sextants and chronometers were necessary instruments in the scheme of things; but they only seemed to supply certain ingredients to the calculations that ultimately produced one of the components, latitude or longitude. It was when I was consigned to a stretch of service on the North Pacific in 1927 that, for something to do, I decided to try and improve my limited understanding of astronomical navigation. In the volume of Raper's Tables which I had always used there was an appendix by the Naval Instructor William Hall which explained what was then called by the Navy ‘The New Navigation’, and gave four-figure cosine-versine tables for the work. It was in reading, and re-reading, and thinking about Hall's marvellously lucid explanation of the St. Hilaire process that the light suddenly burst upon me.

Over a decade ago there was some controversy about the selection of an international standard airfield approach lighting system. At a very late stage, the R.A.E. produced a visual simulator of great simplicity on which any administrator could fly approaches using any of the rival systems. His performance could be checked but, what is far more important, he obtained personal and realistic visual experience. If this simulator had been available in 1945 it might have saved many millions spent on the flight evaluations of the rival systems and years of international conferences.

In the present controversy on collision avoidance regulations in the air and at sea, time and wealth could be saved by the creation of a visual and dynamic simulator in which all persons attending symposiums on collision problems could obtain synthetic visual experience of the present day avoidance regulations and of the new conventions proposed by E. S. Calvert and S. H. Hollingdale.

Curiously enough, such a simulator would not be very costly. It would consist of a very large hangar and a number of small electric two-seater cars somewhat similar to those known as ‘Dodgems’ often seen at Funfairs.

It would be used quite effectively for simulating air traffic as a high percentage of all near-misses occur in the horizontal plane but it would be desirable to gimbal the cars so that they can bank when turning.

Simulation of nocturnal traffic requires only black-out and navigation lights.