The problem of underkeel clearance is not new; since time immemorial navigators have been concerned to know the minimum depth of water in which they can sail with a ship of given draught. Until quite recently, underkeel clearance requirements were determined almost entirely empirically, and in many cases the rule of thumb values used can be shown to be greater than the requirement of navigational safety would dictate.

In the years since the war the pattern of the oil industry has changed and very large ships are now commonly used to carry crude oil cargoes to a large number of ports around the world. It is obvious that use of these ships, with their deep draughts, has meant that a number of expensive dredging projects have been put in hand to provide adequate access to the ports served. Continuing escalation in size means this will probably continue.

These notes on collision avoidance by an experienced airline pilot are presented in the form of practical advice to young pilots. The paper deals first of all with blind avoidance and then with visual avoidance.

Collisions are rare and about one quarter of colliding aircraft have landed safely. Collisions are not always fatal to everyone, but they are serious accidents. Pilots need knowledge, skill, good habits and/or a little luck to avoid them. Collisions have occurred for years although in many cases the investigators have proved that the approach of the aircraft and the layout of the windows around the pilots were such that some of the pilots could have seen the other aircraft. Yet they still collided, so that obviously visual avoidance is not easy. In other cases especially in the circuit the aircraft approached and collided without much possibility of seeing each other; so remember there may be someone under your nose or above your tail who can't see you either. We all know how the high closing speed of fast aircraft is too high for human visual acuity to sort out the situation and prevent collision visually. But a number of collisions have occurred at low closing speeds and a number have occurred between light aircraft (e.g. two Convair 240's over U.S.A. in August 1953, and two light aircraft over Reading a few years ago).

1. INTRODUCTION. The distribution of jet streams over the North Atlantic, Europe and the Mediterranean for the two years 1957–8 was described in an earlier paper (Crossley, 1961); the present paper extends the study to the area which is here referred to somewhat loosely as the Middle East —it extends between latitudes 40° N. and 10;° N., and from longitude 30° E. to 90° E. in the north but to only 65° E. in the south (Fig. 1). The aircraft trunk routes intersecting this area include those from the Eastern Mediterranean across Arabia and the Persian Gulf to West Pakistan and north-west India.

The jet streams which affect the Atlantic-European area in the upper troposphere are mostly of the ‘polar’ type with a core in the neighbourhood of 300 mb. so that they can be conveniently located as a rule on the contour charts for 300 mb.; in the Mediterranean region and in the corresponding latitudes of the North Atlantic the westerly subtropical jet stream is also concerned and charts for both 300 and 200 mb.

The limitations to the accuracy of an inertial navigator are first of all the inherent accuracy of the inertial components—gyros and accelerometers; secondly the accuracy with which these are controlled, i.e. by power supplies, and electronic circuitry; thirdly the accuracy of the associated navigation computer. All these three can be checked during manufacture and by laboratory testing of components and systems. However, there remains the effect of the aircraft environment on accuracy. It is possible to simulate some of the environment, for example vibration and temperature, in the laboratory, but it is not possible to simulate realistic accelerations, velocities and displacements, and so flight trials form an essential part of the evaluation of an inertial system. Flight trials may be preceded by trials in a land vehicle on grounds of economy, but such trials are of limited value because of the limited velocities and displacements available. In this paper we shall attempt to summarize our experience at R.A.E. and A. & A.E.E. of laboratory, van and flight testing of inertial navigators which has been acquired in joint trials over the past five or six years. During the next twelve months we shall be associated in trials of three further inertial navigators, the Elliott E5, the Sperry SGN 10 and the Ferranti FMP-B, and we shall also describe in this paper some of the techniques which will be used for the first time in these trials. This is not intended to be an exhaustive treatment of system testing, but will be mainly confined to the systems and tests of which we have practical experience.

The Central Inertial Guidance Test Facility (C.I.G.T.F.) at Holloman Air Force base, New Mexico, has the responsibility for testing all aircraft inertial navigational systems, aided and unaided, for Department of Defence applications. From 1963 to the present, ten systems have been evaluated. The experience derived from past test programmes has always been valuable to the C.I.G.T.F. engineers in the design of increasingly sophisticated programmes.

This paper retraces some of the experience of the C.I.G.T.F. in its inertial navigation system testing. Test philosophy, objectives and approach are discussed briefly. Flightpath design, radar coverage, range instrumentation, aircraft instrumentation, data reduction and analysis of system performance are then considered. In addition, some mention is made of the test programmes presently being designed, and the technical problems of INS evaluation that the C.I.G.T.F. is now attempting to resolve. Because of the breadth of subject material, ideas are presented mostly at the concept level. Technical details are contained in the references.

The C.I.G.T.F. test method is to fly the test specimen within the coverage of the FPS-16 radars or other instrumentation of the White Sands missile range. An airborne tape recorder records where the system ‘thinks’ it is, along with a range reference time signal (irig-b). Likewise, the FPS-16 radar site records where the radar ‘knows’ the aircraft is, referenced to the same time base. Post-flight data reduction compares the outputs of system and reference at each sample/time, and forms an error plot of the differences in position in each channel. This is the basic error data used to describe system performance.

This paper covers the performance of the Sperry SGN-10 Inertial Navigation System as demonstrated by the operation of dual systems installed in a standard operational configuration aboard four jet aircraft of Pan American World Airways incidental to a preoperational engineering evaluation programme conducted for the Federal Aviation Administration during the latter part of 1966. The primary object of this evaluation programme was to determine the system's capability of meeting the following requirements of the F.A.A.'s Advisory Circular covering the use of inertial navigation systems in U.S.- registered civil aircraft, namely:

(1) Maintain a position accuracy within 20 n.m. in the across-track dimension and 25 n.m. in the along-track dimension for 95 per cent of the time on flights up to and including ten hours duration.

(2) Automatically accomplish initial platform alignment in a normal manner in latitudes up to and including the highest normally used in civil aviation.

(3) Perform all its designed navigational functions in a normal manner at all latitudes, inclusive of polar and equatorial overflights.

The word ‘Automation’ when applied to the shipping industry seems to invoke in the individual mind such matters as direct control of propulsion units, planned bridge lay-outs, cargo plan computers, engine data loggers and such other items of ships' equipment. Automation is thus limited in effect and confined entirely to the ship internally. Clearly this ensures that the ship as a unit is as economically efficient as modern techniques allow and is a reflection of the up-to-date attitude of mind of many shipowners.

On the other hand there is a very large field open to automation which concerns sea transport as a whole in respect of the actual conduct of ships at sea both individually and in relation to their effect on each other. It is in this field of sea transport operations that the author sees many longterm possibilities and this paper tries to give some lead into these, which so far seem to have received little, if any, widespread thought or discussion.

It is now two years since a working group set up under the direction of the French Institute of Navigation at the instance of the Secretariat d'Etat á la Marine Marchande delivered a report which was the fruit of a thirteen-months study of the various possibilities of bridge-automation for commercial shipping.

The terms of reference of this study embraced a ship all of whose essential functions were under the control of a central computer, the system, briefly including:

A dead reckoning loop (ship computer dialogue);

A heading speed conversion loop (officer computer dialogue);

An observation calculation loop (officer computer dialogue);

An anti-collision control loop (radar computer dialogue).

Naturally since this was a formal study, the most complete and complex case was examined, and the approximate cost of the data-handling equipment for such a ship was estimated to be of the order of two million francs.

Over the past two decades there has been a steady increase in the amount of electronic equipment carried by transport aircraft. This reflects the expenditure of a large amount of research and development effort, particularly in the automatic flight control field. Yet despite these developments complete automation has not been applied to air transport in the sense that it has been applied to some industrial processes. The attempts made to introduce partial automation have not always had complete success either operationally or economically.

The reasons for this somewhat piece-meal growth of automation in civil transport aircraft are varied. The most important seems to be that any new aircraft represents an evolutionary step forward. Much of the equipment must either be carried forward from a previous generation or based on it, for a mixture of operational and engineering reasons. Inter-fleet standardization in the interests of maintenance and the need to achieve safety by building on past experience are contributory factors.

A great deal has already been written about the navigational problems presented by the appearance in the near future of supersonic civil aircraft; in this paper I shall try to give a quick description of the methods that will be available in the supersonic Concorde in 1970, with an account of the chief considerations that governed their choice. While there is no doubt that there is a specific navigational problem proper to supersonic aircraft, it does not present itself in as critical a form as some writers believe. When the present-day jets came into service there was not, strictly speaking, any real gap to be bridged. The conventional methods and means used in prop-driven aircraft were taken over, with whatever further adaptations were required for particular cases—such, for example, as polar navigation. In spite of the increase in speed, there was still sufficient time for the navigator to check position and heading at regular intervals.

However, the 300 per cent increase in speeds now envisaged (1300 knots = mach 2.2 for the Concorde) introduces a more formidable discontinuity in relation to the standards of today. At a speed of 22 miles a minute it becomes impossible for the navigator to work as rapidly as he must if he is to deal with the information provided by his sensors and carry out the necessary corrections.

Automation is generally embarked upon with the idea of improving the economics of some process, largely by reducing the manpower, but it may be used also as means of solving computations which would otherwise present an almost impossible task for ordinary human effort, or involve so many people that this also would entail problems of organization. It may be difficult to demonstrate that automation of aircraft navigation can cheapen the job by saving in manpower, but it can be seen that the same manpower can be provided with solutions to navigation problems which will improve the aircraft navigational capability, but which would otherwise have demanded more crew. Processing the data obtained from a navigation satellite might be so complicated as to be beyond the scope of what can be expected of human effort in an aircraft, and even if this is possible the time taken to compute the fix may make the information so out of date as to be of little practical value. It is quite certain that by introducing automation into aircraft navigation, the flight crew can be relieved of some tasks so that, particularly in respect of the captain, more time can be made available for the management of the aircraft. We are, in fact, dealing with the question of workload and, therefore, we must look at the size of the job and the time available to do it.

With the advent of increasing numbers of high-speed, high-altitude aircraft on the airways of the world, airlines, aircraft manufacturers, government agencies responsible for navigational standards and air traffic control, as well as military agencies are actively engaged in studies and action directed towards significant changes in longrange air navigation practices and procedures.

These changes have been dictated by the need to utilize to best advantage the ‘limited’ air space available on long-range air routes to each individual aircraft, because of the fantastic growth rate in numbers of aircraft operating the trunk routes of the world.

In order to limit the amount of airspace required around each individual aircraft separation standards are enforced by the various agencies responsible for control of air traffic along these major trunk air routes. These separation standards are based on longitudinal, lateral and vertical limits between adjacent aircraft. In order to ensure that operating companies and flight crews are aware of these requirements, appropriate legislation is introduced by either international or national agencies and promulgated in readily assimilable form in regulations, orders, or flight or operation manuals.