Since the large-scale development of offshore oil installations started in the 1970s the requirements for navigating men and machines around structures and on the sea bed have increased dramatically.

The earliest forms of navigation were by tracking with fish-finding sonar, either directly or with the aid of an acoustic transponder. The sonar would usually be fitted to a surface craft, whose position was found by using one of the commercial navigation systems such as HIFIX. The range and bearing of the object being tracked was then applied to this position. Most of this work was done by hand and the position up-date rate was of the order of minutes. The error of position was the sum of the surface system and tracking system errors and was of the order of at least 10 m.

A deeper understanding and love for the creatures with whom we share this planet must develop as we read the works of animal specialists. These great men and women will surely forgive a navigator who suggests alternative methods by which creatures may find their ways, methods which do not seem to conflict with the remarkable facts they have brought to light but which happen to conform to modern navigational principles.

Secondary surveillance radar (SSR) is nearly as old as radar itself. It has evolved from a need in the last war for an Identification system between Friend and Foe (IFF)

Primary radar has a scanning aerial beam which transmits a burst of energy, and the reflected signal from an aircraft is detected. In the simplest form of IFF the aircraft carries a receiver/transmitter (transponder), which detects the primary radar beam and transmits a pulse (on the same frequency) a few microseconds later. Thus two ‘blips’ appear on the radar display, one at the shorter range due to reflected energy from the aircraft followed by a stronger reply from the transponder. The system requires all aircraft to carry a transponder which upon receipt of an ‘up link’ signal or message transmits a ‘down link’ signal or message. SSR has evolved (and is still evolving) from this early IFF technique into the present-day system which serves both military IFF needs and the requirements for Air Traffic Control. This paper concentrates on the civil use of SSR where it is the prime sensor for the control of aircraft.

The stringent maintenance requirements that apply to North Sea structures make the cost of inspection a significant factor in North Sea operations. A failure on a large production platform necessitating its shut down would result in a loss of revenue of between half and three-quarters of a million pounds per day. Current practice is such that the whole inspection process is very weather-dependent, and only a portion of a large structure can be examined during the ‘weather window’ (roughly speaking, the period between April and October). The area to be inspected has to be planned in advance, acoustic nets have t o be laid down and calibrated, a range of sophisticated equipment has to be deployed; divers also require relatively long lead times prior to the operation and considerable time for decompressing after the operation. The use of divers will, it is hoped, diminish. It is now general practice to use Remotely Controlled Vehicles (RCV) and television for sub-sea inspection.

In a notable series of articles, Hsu advances theoretical models which are used to graduate 7582 observations of aircraft lateral deviations. The goodness of fit of these models, as judged by the χ2 test, is satisfactory. Hsu's main theoretical model is the Double Double Exponential distribution (DDE), a three parameter model whose probability density function is given by

Other model types are also considered, such as the family of exponential power distributions whose probability density is cited by Hsu insection 9. This leads to a four-parameter model, and the fit is (not surprisingly) better even than that of the DDE.

Secondary surveillance radar is a combined radar and communication system, which provides aircraft position (range and azimuth) on the radar side and identity and height on the communication side. It is a main working tool in Air Traffic Control today and the information it produces is an ingredient in providing separation between aircraft in flight. This paper is concerned only with the reliability of the identity (Mode C) and the height (Mode A) data, and not with the radar position data as such. ‘Reliability’ is taken to mean ‘can the values reported by the SSR be relied upon?’ Consequently we are not concerned with cases where an interrogation elicits no response, but only with responses which are present but not correct. Furthermore, a ‘Mode C discrepancy’ is defined as a read-out that disagrees with the altimeter reading observed by the pilot, so that we are not concerned with altimeter errors, but only with the faithfulness (or otherwise) with which the SSR reproduces the altimeter reading. In cases where the Mode C read-out is obviously wrong, but no pilot's report is available, it is not always possible to distinguish between an altimeter error and a Mode C discrepancy.

In an earlier contribution to this Journal the Author described some striking characteristics of the casualty statistics for the airline operations of the western world and proposed a model for the generation of a perceived risk by an individual living in a dangerous environment. It was hoped that a simulation including a population of such risk observers in the feedback loop of a control system might help with the understanding of what was happening in the real world of operations. Since then he has published an account of the development of the model, including some results from the simulation.

For many years there has been keen interest, especially in the avionics industry, in the possibility of devising some airborne device which would warn aircrew of an impending mid-air collision. At sea, the use of radar as a collision-warning device is now common, and the carriage of a computer-based collision warning device on tankers in US waters is expected to become mandatory in the near future.

The Earth's magnetic field has long provided us with a directional reference of almost worldwide usable coverage. This paper examines the use of the magnetic field for ground referenced motion and position measurements

Where E is the vector representation of an electrical field, v vehicle velocity and B a magnetic field, the electromagnetic law of induction, E = v × B, indicates one possibility for measuring ground speed; the magnetic and electrical fields experienced by vehicle mounted sensors being used to solve the equation for v. This method however only gives the component of v perpendicular to the magnetic field. There are also certain difficulties associated with the measurement of B, which should be only the magnetic field of the Earth at the location of the vehicle, and E, which should be only the electrical field resulting from vehicle motion relative to the magnetic field of the Earth. The main problem appears to be the inseparability of motion dependent and non dependent electrical fields, a problem analogous to that of gravitation-acceleration inseparability for inertial navigation systems. The relative magnitudes of the vehicle-motiondependent E-field, of the order of 10−5 (volt/metre)/(metre/second), and the ever-present and very variable non-motion-dependent E-field between a highly conductive atmospheric layer at an altitude of about 50 km and the surface of the Earth, of some 102 volt/metre, are particularly unfavourable. Another potential basis for a ground-speed measurement system is the heterogeneous character of the intensity of the Earth's magnetic field.

This paper, which was presented at a meeting of the Institute in London on 2o May 1981, with Captain R. Maybourn in the Chair, describes some studies of the conflicts arising between a multiplicity of aircraft in straight-line flight through a volume of airspace. Topics include the effect of coercing traffic to fly fixed routes, the direction from which a threat can be expected and the choice of alerting criterion for a groundbased advisory service or an airborne collision-warning system. There are some analogies, perhaps, with marine traffic problems.

There exists, worldwide, a complex scheme for the segregation and control of various classes of air traffic. To fit into this scheme aircraft must make detours, horizontally and/or vertically, the cost of which in Europe alone must be at least £50M per annum. The cost of the rare failures in traffic management is higher still. To authors trained in electronic engineering it is therefore rather surprising that the published theory of air traffic management and control is still in a rather rudimentary form.

The Meteorological Office makes full operational use of pictorial weather satellite data in preparation of routine analyses of the atmospheric fields of wind, temperature, pressure and humidity. There are two types of satellite, polar orbiting and geostationary: the polar orbiter scans the globe from a height of about 830 km and as the globe rotates beneath the satellite each area is scanned twice in 24 hours; the geostationary satellite rotates with the globe and takes frequent pictures of the same area from a height of about 35000 km.

The pictorial data are in the form of visual and infra-red images and when used together can provide considerable information about the height, type and density of the clouds. Vertical temperature profiles within the atmosphere are also retrieved from polar orbiting satellites using linear regression techniques, but there are problems yet to be solved in areas of heavy cloud and precipitation. The paper was presented at Oceanology International 1982, held at Brighton.

Since programmable electronic calculators were first employed in marine navigation, a variety of formulae has been used for calculating astronomical refraction. As the choice is wide, some formulae have been selected from commonly used reference sources and their accuracy and suitability examined. No attempt has been made to assess the validity of the selected formulae to represent astronomical refraction in practical circumstances. Accuracy comparisons have been made using the refraction algorithm proposed by Garfinkel – the standard adopted by the British and American Nautical Almanac Offices. New formulae are given that are simple and accurate, even over a wide range of temperature and pressure, and which for all practical purposes may be considered equivalent to the tables of refraction given in the Nautical Almanac.

On the whole global surface of land and sea it falls to the lot of some few places to emerge through time to be of immense importance. Lizard Point is just one such place. Who were the first mariners to realize that it was the most southerly point of the English mainland or to use it for landfall and for caping, we shall probably never know. What is clear, however, is that, by the time Claudius Ptolemy was compiling his Guide to Geography, C. AD 160, it was sufficiently well known to the Roman world for the name by which it was then called, Damnonium or Ocrinum Promontorium, to be included, with its geographical coordinates, in his famous gazetteer. It is of academic interest only for us to query whether those coordinate values were right or wrong, whether they were a good approximation or not: what is important for us, with the benefit of very long hindsight, is to marvel at the sheer brilliance of the conception of a spherical coordinate system, and at the industry and far-sightedness of one or more men who set about compiling lists of known places with assigned coordinates. What, in so doing, they handed down to posterity is of inestimable value.

Radar operating in the dekametric or high frequency band permits the measurement of sea-surface parameters at very much longer ranges than is possible with other techniques: 0–200 km is practical using a coast-located radar and oversea propagation, while 1000–3000 km is achievable using ionospheric reflection from a radar which may be located inland. The paper, which was presented at Oceanology International 1982 at Brighton, discusses the extraction and mapping of wave height, wave period, non-directional spectrum and surface wind parameters from Doppler radar echoes. The techniques are illustrated by the results of the UK ground-wave and sky-wave measurements and computer simulation.

A swath sounding system is under development which is capable of being towed from ships of opportunity. The principle on which it is based is an acoustic interferometer to which a vernier has been added to resolve the ambiguity of phase ramp identification. The ‘fish’ is equipped with both rotational and translational attitude sensors, though no attempt is made to stabilize it. This is in the interest of size and hence of both instrumental and deployment economy.

The paper, which was presented at Oceanology International 1982 at Brighton, proposes a novel method to permit replication of the swath depth measurements, and also to provide an independent means of ground scale correction, which may obviate the need to deploy a positioning network for deep-water high-resolution swath surveys.

In general terms the principal problem in astronomical navigation is the solving of a spherical triangle - the PZX-triangle. The fundamental formula of spherical trigonometry for finding an angle given the three sides of a spherical triangle is the cosine formula. By transposition this formula can be used for finding a side given the opposite angle and the other two sides. Because the cosine formula is not suitable for use with logarithms numerous formulae have been derived from it with the aim of simplifying logarithmic computation. The term ‘direct method’ applies to a method the basis of which is generally the cosine formula or any of its derivatives although some direct methods are based on Napier's Rules for right-angled spherical triangles.

The number of ships involved in collisions in Japanese waters, excluding collisions with fixed or floating objects, increased rapidly in the 1960s, as can be seen in the statistics of the Marine Accidents Inquiry Commission: 1578 in 1955, 3604 in 1960, 6043 in 1965 and 7642 in 1970, while the increase in average tonnage resulted in numerous serious accidents. To cope with this situation, the Maritime Traffic Safety Law (MTSL) was enacted in 1973, defining eleven traffic routes in congested waters. The Tokyo Bay Traffic Advisory Centre was established in 1975 and started operation in 1977. The trend in the number of traffic accidents requiring assistance shows the effect of traffic management in reducing the number of accidents.

This paper, which was originally presented at the Fourth International Symposium on Vessel Traffic Services held in Bremen from 28 to 30 April 1981, here summarizes the results of an investigation into collisions world-wide, with particular emphasis on the circumstances of collisions associated with different types of encounter for conditions of both clear and restricted visibility.

Information relating to the circumstances of collisions is available from various sources but analysis of casualty information has usually been restricted to ships under one flag and been mainly applicable to accidents in restricted waters.

The voyage from the Bermudas, or Somers Isles, to Lizard became in the seventeenth century, following Richard Norwood's use of it in The Seaman's Practice (1637), the standard example to illustrate the principles of great-circle sailing. The title chosen here suggests an association. Norwood, as a matter of interest, carried out the first survey of the islands, and his measurement of an arc of the meridian, from York to London, led to the re-knotting of the log-line to preserve a sea-mile comprising 60 to a degree.

(This paper was presented at a meeting to celebrate the tenth anniversary of the Solent Branch held in Warsash on 3 December 1981.)