It is expected that the number of military and civil aircraft operating supersonically all over the world may remain the same during the next decade. The problem of sonic boom, which has been by-passed for military aircraft through the assignment of specially allocated ranges for supersonic operation, is still with us and is likely to influence the operation of supersonic transport (SST) aircraft, the Concorde and the Tupolev 144. Concorde is still short of reaching the break-even point, despite the large traffic gain registered on the routes to North and South America. The problem is to get overland routes; the difficulties encountered by the UK in the discussions with India and Malaysia for the establishment of a London–Bahrain–Singapore route are well known.

Before presenting the effects of the sonic boom on the ecology it may be well to summarize the nature and the main features of this phenomenon.

(i) The N-wave signature. A complex body like an aircraft, moving in supersonic flight, generates a pattern of local shock waves which tend to coalesce in two main shock cones, the front (or bow) and tail, as the distance from the aircraft increases. A shock wave is a discontinuity surface for the thermodynamic characteristics of the fluid, the most important being the static pressure jump responsible for the N-shaped pressure signature characteristic of the sonic boom. Other parameters are of minor importance to the environment. A shock signature is denned by the following parameters:

For short range air navigation the most general method of determining the distance between two points, and the course to be steered from one to the other, is by means of a grid system of coordinates based upon some map projection. Problems must arise, however, in routing aircraft between Traffic Control centres in adjacent regions which employ different grid reference systems, even though the airway markers are identified by their latitude and longitude. A difficulty which may result in steering-angle errors arises when these markers, which are generally VOR stations, are referenced to the local magnetic field rather than the geographic pole.

The recent death of Philip Van Horn Weems, at the advanced age of 90, has removed from the field of navigation one of its most outstanding and colourful personalities. His contributions, made over a period of more than half a century, have ranged over almost every aspect of navigation: practising navigator, instrument inventor, pioneer of the GHA form of Nautical Almanac and of the first Air Almanac, introducer of the Star Altitude Curves, brilliant expositor (as his books so clearly demonstrate), successful business man who trained many thousand navigators through his training schools and correspondence courses – he devoted his whole life to navigation with boundless energy and contagious enthusiasm.

This is not the place to give the factual details of his life, or to put on record his many achievements, both inside and outside his professional career. Many of his most lasting and significant contributions to the practice of navigation, although so familiar to myself, can only be appreciated by a detailed study relative to the contemporary ‘state of the art’; Weems was, above all else, an innovator of ideas many of which were developed by others and which deserve a fuller and more technical treatment.

A brief summary of his career follows. Weems, left an orphan at an early age, had a hard life on the family farm in Tennessee, with little formal education.

On 18 May 1979 the Nautical Institute held a one-day meeting at Dover to discuss some of the more burning problems of traffic congestion in the Dover Strait. The meeting was attended by shipmasters, pilots, representatives of the Dover Strait Information Service, the Department of Trade, etc. The Institute was formally represented at the meeting by the Director and Captain A. N. Cockcroft. The papers had been circulated beforehand. The following contribution from Commandant Oudet was read out by Captain Cockcroft.

Animal navigation is magic. Nobody really knows how they do it. We can only suggest the senses they must be using, but how their brains convert this information into action is a mystery. In their heads are computers infinitely smaller, lighter and more flexible than the fantastic silicon chips that clumsy humans employ. Let us first consider what animals have to measure and, to make sure we cover the whole field, we shall look first at what giant tankers, nuclear submarines, Concordes and Apollo space-craft require. [Wing Commander Anderson's article was presented at the Young Navigators’ Meeting in London on 18 December 1979.]

A tedious problem which confronts many a yachtsman is determining the deviation of his magnetic compass. He can hardly justify the expensive services of a professional compass adjuster, so this necessary but difficult operation must be carried out by himself. Should he decide to use bearings of shore objects when swinging ship he is faced with the difficulty of observing these bearings on his compass. A small-boat compass is hardly likely to come equipped with azimuth mirror and sight vanes. The alternative equipment for taking the bearings is a pelorus, but the expense of this instrument could hardly be justified for such rare and intermittent use.

The favourite way out of the difficulty appears to be to use a hand bearing compass. This method suffers from two disadvantages in particular:

(i) The hand bearing compass should be situated in precisely the same position as the main compass if it is to reproduce the latter's readings precisely. Not only is this impossible to achieve in practice – the nearest they can be brought together is side by side – but when brought close together their mutual magnetic reaction can produce errors of an unknown amount.

(ii) The use of bearings to shore objects naturally entails being offshore. Stationary small boats are notoriously unstable observation platforms and even a light chop would make it impossible to read the main compass with the accuracy necessary for a good deviation card.

In a recent paper the author reported a distribution theory which rationalizes some fairly well established phenomena in navigational errors. Adequacy of the implied model and other existing models was investigated in the light of empirical evidence. A summary of the findings has been available in a previous issue of this Journal.

This paper provides further insights into several theoretical models, establishes formal procedures for the estimation of parameters crucial to the evaluation and design of navigational systems and investigates the potentials of a broader system of distribution models. In addition, some simplified results aiming at practical use for navigators and air traffic control authorities are provided. A real-world application of the suggested methods and theoretical results is also demonstrated.

From about 7 years’ research work on the phenomenon of Ship Squat I have produced curves (Fig. 1) that will give predictions of maximum squat for merchant-ship types having block co-efficients, CB, ranging from 0.500 to 0.900. They are for vessels operating in ‘confined channel’ conditions and ‘open water’ conditions, i.e. those without breadth restrictions.

The major purpose of an air traffic control system is to ensure the separation of two or more aircraft flying in the same airspace, with an efficiency that can be expressed in terms of capacity and cost. As air traffic grows in numbers it becomes necessary to reduce the workload of the controllers by relieving them of many monitoring tasks, and eventually some decision-making tasks, through computerized automation. In this context many developments tend to build up an efficient conflict-alert subsystem.

The problem of conflict-alert in the air needs strategic tools, to make collision unlikely or even impossible, and tactical tools to detect impending collisions. The latter detect potentially hazardous aircraft encounters and alert the controller in time to warn the pilots (if necessary) and should obviously provide this capability with a minimal number of false alarms and no increase in workload.

I note with interest Dr Barrass' comment in the January issue of the Forum (33, 139). He deals with three subjects: (i) the ultimate trim of tankers in shallow water; (ii) the question of increasing rpm in interaction situation; and (iii) the final turning diameter in shallow water.

Dr Barrass' curves (p. 140, Fig. 1) are useful approximations to squat and to ship characteristics but cannot be treated as definitive.

Fig. 2 is a particular interpretation of practice and not necessarily universally accepted, particularly in respect of the LCB position. In some recent VLCC work benefits have been obtained by moving the LCB aft in association with a buttock-flow stern. Furthermore, the dependence of block coefficient upon Froude number is not realistic, especially in the finer vessels; it is the prismatic coefficient that is dependent upon Froude number. It cannot be taken, therefore, that absolute curves for these factors exist as inferred from Fig. 2.

Turning to Fig. 1, the curves showing sinkage in confined channels are for the blockage ratio bracketing 0·1–0·3. This is a large range of blockage and as a result the curves can significantly underestimate and overestimate sinkage, depending upon where in this bracket a particular example falls. Similarly, the solid curves show sinkage in open water conditions over depth/draft ratios 1·1 – 1·2. This is quite a large range and a unique curve cannot represent these two values.

This paper, which was presented at an Ordinary Meeting of the Institute in London on 21 November 1979, with S. Ratcliffe in the Chair, traces the evolution of airborne doppler navigation systems over the past two decades and discusses the treatment of errors introduced when flying over water. Both authors are with Decca Radar Ltd.

The object of this paper is to describe briefly the major developments which have taken place in doppler navigation systems during the past 15 years or so, to indicate the current state of the art and to examine in some detail the behaviour of doppler systems when flying over water. The year 1960 is taken as a starting point since by that time commercial doppler systems were established. A typical general purpose system of that generation (Fig. 1) consisted of the following units:

1. An antenna, probably mechanically stabilized.

2. A transmitter/receiver using either pulse or FMCW modulation.

3. A tracking system.

4. An analog computer.

5. A display of ground-speed and drift-angle.

6. A display of present position.

When airline fuel is calculated before flight, ‘contingency fuel’ has been included either separately or in combination in various forms over the years. The primary calculation has always been the fuel required in expected conditions to fly to the destination and thence to a nominated alternate. The next is the provision, in the words of the Air Navigation Order, of a ‘safe margin of fuel’ for contingencies. For aircraft operated under an Air Operator's Certificate, CAP 360 specifies ‘an allowance for contingencies such as errors in forecast winds and temperatures, navigational errors and ATC restrictions on altitude and route’. Also, ‘45 minutes holding reserve’ is stated to be a normally overriding requirement. The wording of CAP 360 is, perhaps intentionally, not precise but is broadly in line with Annex 6 of the ICAO Chicago Convention which requires:

(i) a reserve ‘to provide for contingencies’ plus, for turbojet aircraft,

(ii) 30 minutes' low level holding, plus

(iii) an amount ‘to provide for the increased consumption on the occurrence of any of the potential contingencies specified…’