When the problem of collision in the air is discussed, it is usual to start by pointing out the enormous closing speed of two modern aircraft meeting head-on, and to conclude from this that avoidance on the ‘see and be seen’ principle has ceased to be possible. The fact is, however, that the great majority of mid-air collisions (about 85 per cent) occur within five miles of an airport and the typical case is not the head-on one, but the case in which the two aircraft crab into one another from a direction which may be anywhere around the whole enclosing sphere. Since the field of view of the aircrew covers only about 20 per cent of the enclosing sphere, the aircrew of colliding aircraft seldom see each other. It would seem, therefore, that the ‘see and be seen’ principle never did afford much protection, even when speeds were low. In other words, the fact that the number of mid-air collisions in Europe has hitherto been small is not primarily due to seeing and evading, although this sometimes happens, but to the fact that the airspace is very large compared to the volume of all the aircraft in it at any given time. However, as traffic densities go up, the risk rapidly increases, and in congested airspace, such as that around New York, the problem of avoiding collision has already become acute. In the period 1948–57, there were 159 mid-air collisions in the United States, and many of these made headlines in the world press. One can imagine the public outcry if two large transports were to collide over a housing estate; but unless something effective is done, something like this will presumably happen eventually. At very high altitudes the ‘see and be seen’ principle certainly fails, by day, because the speed will be high, and in addition, the range at which a pilot can see an object the size of an aircraft may be less than 1½ miles due to what is sometimes called ‘high-altitude myopia’.

The analysis of low-altitude observations of celestial bodies specially made for the purpose, indicates that no significant amendment is required to the refraction corrections published in the (Abridged) Nautical Almanac.

In recent years there has been an increasing interest in low-altitude astronomical sights, particularly in regard to the accuracy of observation and of the corrections to be applied for atmospheric refraction. As a result of the analysis of many thousand observations made under different conditions, Professor C. H. Smiley had deduced tabular refraction corrections which differed significantly from those in current use and also from those deduced from modern theories. Suspecting some systematic error in Smiley's results, possibly associated with the phenomenon of irradiation, Dr. G. M. Clemence of the U.S. Naval Observatory undertook an independent investigation using observations made at his request by ships of the U.S. Merchant Navy. Although the number of observations was rather small, leading to a fairly large uncertainty in the deduced refractions, there was little possibility of any source of systematic error; the observed values agreed with the theoretical values well within their observational uncertainty, and differed significantly from those of Smiley. A systematic difference between the observed altitudes of the upper and lower limbs of the Sun was ascribed to irradiation (the spreading of a bright image on the retina of the eye, making the diameter of a bright object appear larger than it is) and, taken in conjunction with a similar phenomenon depressing the visible horizon, was allowed for by subtracting 1.2 from the observed altitudes of the Sun's upper limb and leaving those of the lower limb unchanged. This correction was later incorporated in the altitude correction tables of the (Abridged) Nautical Almanac.

This paper briefly reviews some of the research findings with respect to factors which affect the efficiency of radar operation. One of the most important factors in determining whether or not a radar target will be visually detected is the brightness of the radar scope. A simple method is described for setting optimum scope brightness. Visual search habits are discussed and the implications of such habits for the design of radar displays are pointed out. Reference is made to some of the design factors which determine the accuracy with which target range and bearing are reported. It is shown that human monitors perform less efficiently as a watch progresses and factors which impair or aid their vigilance are described. Finally, a brief account is given of illusions on radar displays.

Shortly after merchant marine radar came into use as a navigational aid, it was felt that some consideration should be given to the development of electronic aids for vessels too small to use radar, such as tugs and fishing vessels. It was obvious that the use of the microwave frequencies of radar would best suit the development of a small directionfinding receiver. The final system developed by the National Research Council used a compact 3-cm. pulse transmitter on a shore site, such as a lighthouse, or harbour breakwater. The antenna was designed to radiate energy across the water from all angles of approach.

The receiving equipment was built into a case on the back of a small radar antenna consisting of a six-inch slotted waveguide at the focal plane of a parabolic cylinder. The receiver used a radar crystal detector and transistorized audio amplifier for reception on phones or a speaker. The receiver could be hand held or mounted on a pelorus head, and rotated around the horizon to take a bearing on the shore transmitter. The signal from the shore is audible over an arc of only about six degrees, and can be maximized with an accuracy of ± 1 degree.

Dr. Ahrens's paper, which was presented in a more extended version at the meeting in Paris, describes a number of ways in which automatic processes may be applied to, or operated from, echo-sounders.

Knowledge of the depth of water is of secondary navigational value to the surface ship since (as compared with the submarine), she has no changes of course to make in the vertical direction. The present state of development of surface navigation systems makes the use of the depth as a method of navigation superfluous in all cases in which sufficient water is available, where there is no danger of grounding. It can, however, when the topography of the sea-bottom is known from accurate charts, be employed for checking and confirming the data determined by other forms of navigation.

A computer known as the Radio Track Guide, of which both air and ground trials have been held, is described. As a system it can provide accurate automatic guidance for all anticipated operational conditions in a manner which is of value for both navigational and movement-control purposes. Mr. Britnell's paper was presented at the Paris convention on automatic navigation.

The criterion for assessing the efficiency of a navigational aid must include a reference to the ease and precision with which it enables a vehicle to maintain a specified track. In aircraft navigation the most accurate tracks are, of necessity, those made in the vicinity of the runway during the landing and take-off phases. At such times the errors must be limited to a few feet and such small values are a result of the pilot using an ideal reference frame for lateral guidance purposes. This is centred upon the runway and within it he is able to detect, with a high degree of accuracy, the two quantities necessary for precise track keeping. These two quantities are the displacement and rate of displacement from the required track.

Dr. Ramsayer's paper, presented at the tripartite meeting on automatic navigation held in Paris last April, describes a dead-reckoning computer on which position is presented instantaneously on a map.

The so called Automatische Koppelkarte has been developed by the Institute of Air Navigation of the Technical University of Stuttgart, and is a new type of dead reckoning computer differing from the usual navigation computers in that position is immediately indicated on a map; there are many possibilities for combining the device with other navigational aids.

Fig. 1 shows a schematic diagram of the computer. It consists of the computer, the map display and the course and track indicator. The station selector and the coordinate transformer shown in Fig. 1 are additional units, only used when monitoring by tacan is desired, as will be described. The computer is fed by a gyro-magnetic compass and a true airspeed transmitter. The wind is set at the map display, and ‘grivation’ at the course and track indicator. From these data both the flight path and the track are computed. The flight path is resolved in two rectangular components and is displayed by a light pointer on a map of conformal conical projection. The standard parallels of the map are selected in such a way that the distortion in length is practically negligible for the area of interest.

The route direction-finder is a device giving an accurate directional reference. It uses two distant transmitters, A and B, radiating two continuous waves, the frequencies of which are different but very proximate. The paper was presented at the Paris convention on automatic navigation.

Classical methods of navigation by dead reckoning have been considerably improved in recent years by the use of autonomous systems such as doppler and inertia which enable true speed to be measured. When using these systems, the principal cause of errors is the unreliability of the directional reference, which may be the meridian or any arc of a great circle, since it is with respect to this that the speed vector is obtained. The directional reference is given by the magnetic compass which often has an error of up to several degrees or is given by gyroscopic devices which can have considerable drift after several hours' operation. Errors in directional reference are especially marked in polar regions where magnetic or gyro compasses cannot be used.

In this paper, methods of defining heading are enumerated and the limitations of magnetic and gyroscopic devices are analysed in detail. Particular attention is paid to the development of the rotated race principle which is likely to be responsible for a large improvement in the basic azimuth gyro. The gyro-magnetic combination is scrutinized and the errors inherent in this compass system are classified. The possibility and consequent effect of diminishing certain categories of error are presented. The current position is summarized and an attempt is made to suggest possible future trends. The authors are, respectively, with Smiths Aircraft Instruments and the Sperry Gyroscope Company.

A description is given of a floated rate-integrating gyroscope and of the way in which it may be used to provide a high-accuracy aircraft heading reference. The sources of error of the system are examined and the overall heading accuracy is estimated as being comparable with the distance accuracy of a doppler system. This paper was presented, on 15 January, at the same Ordinary Meeting of the Institute as the preceding paper by Green and Glenny.

The advent of the high-accuracy single-axis floated gyroscopes which have been developed so rapidly over recent years, primarily for inertial navigation use, has raised the question of whether the accuracy of these instruments can be made use of in other applications. Can systems which have been employing the older established types of gyroscope benefit from the greater stability of the new type? If a direct change of instrument with no modification of the method of use is considered, the answer will in many cases be no. For example, an improvement in the accuracy of the north-seeking marine gyro compass may not be found rewarding with existing marine navigation techniques, as other factors such as the accuracy with which a ship can be steered would become limiting, defeating the advantage of improved compass accuracy. The introduction of the doppler system has created a different situation in the field of aircraft navigation. Doppler navigation equipment enables an aircraft's drift angle to be measured accurately. That is, the aircraft's track can be defined with an accuracy dependent on the accuracy with which its heading can be measured. In this new situation the overall navigation accuracy is limited by the accuracy of aircraft compasses. These have in general been magnetic compasses, given improved dynamic performance in later years by the incorporation of a gyroscope or use of an associated directional gyro but still dependent on the magnetic element for long-term accuracy. Such a direction reference, though simple and reliable, is at no time precise by modern standards, and as the region of the magnetic pole is approached it becomes practically of very little value. For this reason in recent years the technique of using a free gyro alone as a long-term memory, to store direction information, has been developed, particularly by the airlines operating transpolar services. It is a system such as this that can reap most benefit from use of the new gyroscopes.

The International Regulations for Preventing Collisions at Sea are of the first importance to the navigator; as Marsden puts it, they are ‘the paramount Rules of Navigation’ and it follows that any trouble taken to perfect them is well worth while. They must be comprehensive and clear, simple enough for the sailor yet precise enough for the lawyer. Considering this, the wonder is perhaps that they are as good as they are. Since they are to be discussed at the International Safety of Life at Sea and Load Line Convention this year, the subject of their revision is a topical one and worth the attention of every mariner.

The story of the Regulations is not without interest. They did not, of course, spring fully fledged into being and they are little more than one hundred years old—a very small fraction of the time that man has been navigating the seas.

It is almost two years since Wing Commander Dickie's survey (11, 259) emphasized the importance of improving methods of control so as to allow more movements in a given volume of airspace. Since then, much effort in many countries has been devoted to the problem. The magnitude of these difficulties is not surprising when it is recalled that, in addition to the professional difficulties of flying, navigation and control, various sections of industry are involved, and international agreement must be obtained. At every turn there are political and commercial problems which cannot be ignored by those responsible for initiating action to solve a very difficult practical problem. This note is written in an attempt to examine once again the various possibilities for improving airspace utilization, with the object of discovering some pattern which will lead to progress.

It has been stated that at all times both the controller and the pilot must know the absolute ground position of the aircraft, but it is by no means clear that this really is a requirement. It could well be sufficient to maintain the position of the aircraft relative to the traffic pattern, for this would satisfy the pilot's aim which is to travel in accordance with his flight plan, and also the controller's aim which is to ensure safety while making best use of the available airspace.