Separation standards are used for assuring the safe and effective operation of aircraft in a traffic control environment and as aircraft operations increase in number the need to reconsider separation standards is strongly voiced both by operators and controllers. A panel, the Review of the General Concept of Separation Panel (RGCSP), has been organized by ICAO and the possibility of reducing conventional separation standards has been discussed. Mathematical models for estimating collision risk due to loss of separation between aircraft require data on navigation accuracy. In general, three-dimensional radars are used for height finding but these are complicated and expensive.

The altitude of the cruising aircraft is required within about 100 ft. In the simplified radar here described a rotating antenna produces a fan beam for scanning aircraft flying overhead. The data obtained from several scans are processed by a mini-computer and the height of the aircraft estimated by a least-square method and sequential approximation. Between the items of information obtained for each scan (the distance, the elevation angle and the data acquisition time) a theoretical relationship is established as an observation equation, one of the coefficients of which includes the height information. The coefficient is determined by a curve-fitting method. The paper first describes the method of analysis for height estimation and then an evaluation of the accuracy of the method by a computer simulation.

Automatic collision avoidance systems are being installed on board more and more ships and this necessitates verifying their characteristics, as compared with postulated requirements. The requirement for such anti-collision systems may be specified as follows:

(i) Radar echoes should be tracked automatically, regardless of weather conditions, the number of echoes, or the relative distance between tracked targets.

(ii) The system should be operable by a single observer, assuming that the information given is used efficiently.

(iii) The information should be displayed in the clearest and the most comprehensible form.

(iv) The system should facilitate alterations of course and speed.

(v) Information relating to collision risk should be provided in ample time if the system is to operate in dense traffic.

(vi) The system should include alarm devices which operate in pre-determined circumstances.

In this paper, which was presented to an Ordinary Meeting of the Institute held in London on 22 November 1978, Captain Woodfield, who was for nineteen years master of Royal Research Ships in support of British Antarctic Survey operations, discusses some of the problems of navigating in this ill-charted and ice-ridden area. The presentation was made jointly with Lieutenant Commander P. Le Pla, formerly Navigating Officer of the Ice Patrol Ship HMS Protector whose paper on Antarctic navigation was published in Vol. 31 of the Journal. The President was in the chair.

Every day several hundred subsonic aircraft travel between the European and North American continents. Because of noise limitations at major airports, aircraft turn-around times and the time difference between the continents, the direction of the main traffic flow changes through the day. In the afternoon (GMT) the flow is predominantly westbound, while in the early hours of the morning the flow is predominantly eastbound. Oceanic air traffic control for the North Atlantic region has evolved into what is now known as the Organized Track System; traffic is divided into streams, each stream flying at a specified set of flight levels on one of several adjacent tracks. Figure 1 shows an example of the subsonic track system during the westbound peak. To secure operating economy the layout of the track system on any day is matched to the pattern of the upper winds.

The adoption by the International Civil Aviation Organization of the MNPS requirement in the North Atlantic Region was based on two premises:

(i) The need to maintain an acceptable level of safety in an ATC environment in which a major systems change was taking place—the withdrawal of the Loran A Navigational System on 31 December 1977.

(ii) The evidence of an option to take economic advantage of the provision of advanced navigation systems now being used by the majority of traffic operating in the Region. This advantage would be a reduction in the lateral separation minima.

In this paper, which was presented at a symposium on electronic aids to air navigation held in Paris in November 1977 (and subsequently published in the Journal of the French Institute of Navigation), the authors first analyse in detail the mental processes of an aircraft pilot and the ergonomic factors that should underlie the design of any flight data display. They then describe a cathode-ray tube display designed for use in civil transport aircraft and the modifications they found desirable in the light of simulator trials.

Inertial navigation systems have for many years been mechanised as a cluster of gyros and accelerometers mounted in a servo-controlled gimbal system which isolates the cluster from the motion of the carrying vehicle. Until the spectacular advance in computer technology over the last decade no other mechanization was really feasible, but serious consideration is now being given to strapped-down systems. This paper compares the two mechanizations, outlines some of the advantages of strapped-down systems, and considers some of the contenders for rate-sensing in such systems.

The Department of Trade has recently issued a Merchant Shipping Notice (No. 845) entitled ‘Dangers in the Use of VHF Radios in Collision Avoidance’. Though they recognize that ‘the practice of using VHF radio as a collision avoidance aid may be resorted to on occasion, especially in pilotage waters’, the tenor of the Notice seems clearly designed to inhibit the use of this valuable aid to navigation. Their attention seems mainly absorbed by the undoubted dangers attending its haphazard use, which appears to be the salient feature of most of the ‘official’ attention directed towards the use of this device.

The following paper was presented at an Ordinary Meeting of the Institute held in London on 21 March 1979 with Mr S. S. D. Jones in the Chair.

This paper is not so much a forecast of future navigation systems as a personal view of the philosophy of the increased use of micro-processors. Digital computers were first introduced into airborne systems in the late 1950s to meet the demands of new sensors such as inertial navigators and star trackers for which analog computation methods proved inadequate and inaccurate, but their cost restricted their application to specialized high accuracy systems.

The current situation with aircraft inertial systems is that virtually all are based on gimballed platforms so that this technology is very well understood, including the cost of maintenance. Strapped-down IN promises the already well demonstrated advantages of gimballed equipment but at a much lower cost of ownership; but although the technical feasibility of strapped-down systems has been demonstrated, the realization of the economic advantages is still in process.

Recent advances in the speed and capability of miniature computers have made possible a strapped-down mechanization for airborne inertial navigation systems. The implementation of this mechanization is critically dependent on the availability of suitable accelerometers and gyroscopes able to measure, and yet withstand, the effects of vehicle motion. The laser gyroscope is now considered a major contender for angular measurement in that it offers the possibility of a cheap, reliable instrument with the wide dynamic range required for strapped-down implementation.

The anonymous author of a paper published in the November 1857 issue of The Nautical Magazine, introduced a novel philosophy in relation to the location of lighthouses. At the same time he discussed the proposition for a coastal traffic separation scheme for the English Channel — notorious then, as it still is, for its high incidence of collisions and groundings. The proposal was made just two years after Walter R. Jones of New York made his suggestion to Lieut. M. F. Maury of the United States Navy for an ocean traffic separation scheme which materialized in the form of Maury's ‘steam lanes’ for the North Atlantic.

The practice of safety can be treated as the operation of a system with which it is intended to control the level of some parameter ‘risk’. The following is an attempt at a description of the behaviour of a population of individuals engaged in a risky occupation in terms of such a system. In the first section the casualty figures for commercial airline operations are analysed to show that there is a prima facie case for a belief that in this case risk level is regulated by those immediately engaged in the industry. The second section contains a description of a proposed ‘input/output’ model of an individual regulating his own risk, and preliminary results are given for a simulated population of aircraft operators having these characteristics. Lastly, the credibility of the model in terms of known human behaviour is examined, some tentative conclusions are drawn and an indication is given of its proposed use and development.

During the months of October and November 1835 a series of magnetic observations was made, under orders of the Lords Commissioners of the Admiralty, on board the iron steamship Garryowen by Commander Edward J. Johnson R.N. The results of Johnson's observations paved the way to Airy's solution to the crucial problem of compass management on board an iron ship.

Johnson related in a paper in which he described his experiments, that he proceeded to Ireland taking with him the necessary instruments for ascertaining the deviation of the magnetic needle produced by the local attraction of an iron steam-vessel, together with instruments for measuring dip and magnetic intensity. Every facility was afforded him by the owners of the Garryowen, the City of Dublin Steam Packet Company, and by the builders of the vessel, Messrs. Laird of Liverpool.

The Garryowen, built in 1833, was a paddle steamer of length 130 ft, breadth 21 ft 6 in and depth 11 ft. She was equipped with two engines each rated at 85 h.p. and her tonnage was 281 tons burden. She was fitted with four water-tight transverse bulkheads of wrought-iron ¼ in thick. Her hull and boilers were of malleable iron and the total weight of iron in her structure amounted to 180 tons.

It is now more than thirty years since Commander W. E. May, R.N. (formerly of the Admiralty Compass Observatory), drafted this hitherto unpublished paper, recording his opinions based on a study of documents made available to him in 1947 by Messrs Kelvin, Bottomley and Baird. The documents referred to in the paper are:

The Thomson v. Moore case as presented to the House of Lords in the Thomson v. Moore case (Patent Design and Trade Mark Cases, Vol. VII, No. 36.)

The case of Thomson v. Hughes (Patent Design and Trade Mark Cases, Vol. VII, Nos. 9 and 22.)

Report of Proof of case of Kelvin v. Whyte Thomson &. Co.

Bound volume of patent specifications referred to in the last named.

It is a curious habit of editors and publishers to invite well-known persons to write articles on subjects outside their normal orbit. Thus in 1874 Sir William Thomson was invited to write for Good Words an article on the mariner's compass. He took up the task and soon realized that he did not know enough of the subject to complete the article. He then began to study the compass and the final part of the article was published in 1879. Such is one of Lord Kelvin's explanations of how he came to interest himself. In 1885, in an affidavit for the Moore case, he said that he took up the study of the compass in 1871, whilst elsewhere he said that it was the necessity of writing for the Royal Society an obituary notice on Archibald Smith, who died in 1872, which first turned his attention to compasses.

A recent study by digital computer simulation modelling of traffic interactions in the Dover Strait by Batkin made use of a so-called ‘decision-distance’ criterion for locating the point at which a crossing manoeuvre was to begin. This decision-distance was measured along the course of the vessel involved. The manoeuvring vessel was regarded as making an abrupt change of course, by an angle β to starboard on reaching the decision-distance, it having been ascertained beforehand that, in the absence of a manoeuvre, the vessel having the right of way would infringe the other's domain. Although it seems unlikely that such a simple procedure is used by mariners in practice, it seemed useful to examine the implications of the method in terms of the angles of manoeuvre involved, and to make a comparison with an alternative procedure involving the range between ships.

Dr Corlett's paper on hydrodynamic interaction (Journal, 32, No. 2, May 1979) is of great interest, coming as it does from a practising naval architect with considerable experience of dealing with complex issues arising from casualty investigations. His paper gives an insight into the causes of interaction and the practical illustrations given in the paper add weight to his arguments.

Model experiments to study interaction between ships in shallow water have been undertaken at the National Maritime Institute for some years now and many of the results obtained agree with conclusions reached by Dr Corlett. For example experiments carried out with models close to sloping surface-piercing and flooded banks have shown that at moderate to high speeds the side forces and turning moments induced by the banks vary not as the square of the speed, but as the speed raised to a power greater than two. It is of interest to note that the rudder force used to counteract these bank effects varies roughly as the square of the speed, so that when close to a bank at too high a speed, use of the rudder is less likely to maintain control than would be the case at a more moderate speed.

The author correctly emphasizes the advantages of a ‘kick ahead’ with a single screw ship, but it is well to bear in mind that this device is not applicable to some ships, notably those with twin-screws and a single centreline rudder.

In a previous article about Escape Time the writer mentioned automatic acquisition; this is taken by the writer to mean acceptance by the computer/processor of a radar echo and the subsequent initiation of the tracking process if the echo satisfies the requirement of one or more paints every 6 scans, and is one of the 40 (in Digiplot) closest to own ship. But he took the advantages of it to be so obvious that no special stress was laid on the desirability of its inclusion as standard practice. However, it is clear from the replies to the IMCO questionnaire on Radar Plotting, notably that of the International Chamber of Shipping, that although a very high percentage of informed maritime opinion (97) is in favour of automatic (computerized) radar plotting, a somewhat smaller though still a preponderant (65) percentage favours automatic acquisition. A similar view is taken in the United States' reply. Although to the writer this seems to be illogical or without good reason, it may be of interest to examine some possible subjects of controversy.

The distribution pattern of navigational errors is inherently an important subject bearing as it does on matters of navigational safety. The work of Hiraiwa on the basic distribution of Omega observations is thus a most welcome addition to the literature. There is, however, a major limitation in the data base used by Hiraiwa which could lead to serious errors in analysis of safety based on his findings.

This paper first presents some of the results of my recent work on error distributions in navigation which may be worth summarizing for navigation practitioners. Some further explanations and insights in the second part may be useful for future studies on the subject.

Discussion on this subject was initiated by E. W. Anderson who questioned the adequacy of the gaussian distribution as the probability model for navigational position errors. He observed that empirical position error distributions are usually long-tailed (i.e. with raised skirts) compared to the gaussian and have a strong tendency to form an exponential function of error magnitude, especially in the far tail-region. Attempts to explain this phenomenon have been made by Abbott, Crossley, Lloyd, Parker, Lord, O. D. Anderson, etc., based on various aspects of navigation performance. To provide a general structure for characterizing the error distributions, Anderson and Ellis made a significant step forward in suggesting ‘Student's t’ distributions as error models. As pointed out by Parker, however, this suggestion has to be taken with reservation and requires further verification based upon empirical data. My own recent work has indeed taken the direction advocated by Parker and has attempted to fill a theoretical deficiency in Anderson and Ellis's work, to provide examples of empirical model fitting and to discuss the merits of Student's t distributions relative to the double exponential model. The main findings are summarized below.