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.

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.

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.

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.

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 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.)

In the Journal for May 1980, Adlard Coles, author of Heavy Weather Sailingand an Institute Gold Medallist, gave an account of his own investigation into the weather situation which decimated the ocean racing fleet during the Fastnet Race of 1979. Neither the formal report of the Fastnet Race inquiry nor the official meteorological analysis seemed entirely to explain what had happened. Alan Watts, a meteorologist particularly concerned with small craft navigation, from his own analysis of meteorological factors, including the barometric readings of many of the competitors, here suggests an explanation for the particular violence of the seas which was responsible for most of the damage.

Over the past ten years at least the Law of the Sea has been a subject of continuing debate. This would have been inevitable even if there had been no UN Law of the Sea Conference to focus people's minds; the increasing importance of offshore hydrocarbon deposits, the potential exploitation of seabed mineral nodules and the competition to exploit fish stocks would have ensured that. Add to these factors the demands of the large number of newly formed coastal states, which did not see their interest well served by the law that had largely developed under the influence of the western maritime nations, and something clearly had to be done.

A major research project to investigate the causal relationships of collisions and groundings involving Norwegian registered vessels has recently been completed. This article reviews mainly the Final Report, by K. H. Drager, but earlier project reports have also been taken into account.

The project was launched in 1976 by Det Norkse Veritas in co-operation with Norwegian maritime authorities, shipowners and research organizations. It was inspired by what was considered to be a high rate of collisions and groundings, attributed mainly to human error. The aim of the project was to investigate the causes of the accidents and to develop measures to reduce the incidence of collisions and groundings.

Before a helicopter takes off from an airport on the Norwegian coast to fly to an offshore installation it is cleared in the usual way and is followed by the air traffic control; even after having left controlled airspace the helicopter gets flight-following and alerting services as long as contact can be kept by vhf radio.

When the helicopter approaches its destination it has to enter the platform area by flying over a certain reference point (e.g. an outer platform, Fig. 1), which is to be passed at an altitude of 500 feet and which uses a special frequency for its non-directional beacon (NDB) transmissions; it can thus be easily recognized. Before the reference point is reached, radio contact between the helicopter pilot and the platform area will have been established so that necessary information can be transferred. The pilot then asks all platforms to turn off their NDB's and his destination platform to turn on its own (all NDB's unless otherwise mentioned use the same frequency).

Rule 13 of the Collision Regulations reads as follows:

Overtaking:

(a) Notwithstanding anything contained in the Rules of this Section any vessel overtaking any other shall keep out of the way of the vessel being overtaken.

(b) A vessel shall be deemed to be overtaking when coming up with another vessel from a direction more than 22·5 degrees abaft her beam, that is, in such a position with reference to the vessel she is overtaking, that at night she would be able to see only the sternlight of that vessel but neither of her sidelights.

(c) When a vessel is in any doubt as to whether she is overtaking another, she shall assume that this is the case and act accordingly.

(d) Any subsequent alteration of the bearing between the two vessels shall not make the overtaking vessel a crossing vessel within the meaning of these Rules or relieve her of the duty of keeping clear of the overtaken vessel until she is finally past and clear.

Every navigator is familiar with the sight of two intersecting course lines on a nautical chart. And every navigator knows that no ship can sail through the intersection. To get from one course line to the other the ship has to follow some other line for some time. For small ships this is not important, but for large ships the transitory line and the time sailed on it have gradually become more interesting for several reasons.

Navigation lights for ships were first regulated in the United Kingdom by a statute introduced in 1848 and revised four years later, since when there have been few changes. Originally the regulations referred to oil-burning lamps fitted to vessels rarely in excess of 400 tons and rarely sailing much above 10 knots. The sheer increase in average size and in speed of typical modern ships, particularly marked over the last four decades, raises the question as to how effective the regulation lights actually are in practice. This question is made more pertinent in view of the other great change in ships, namely their general arrangement: the superstructure may be forward, midships or astern, masts and derricks vary in number, complexity and size. Although the regulations for the positioning of lights are reasonably comprehensive they inevitably leave considerable freedom to the designer because of the variety of superstructure configurations.

An experiment is described in which colour-coded information concerning the aspect of target ships was provided in addition to a relative-motion radar display. This was compared to the normal relative-motion display alone, while a third condition used the normal true-motion display alone. The performance of twenty-one experienced seafarers on six exercises was assessed by five expert judges as an index of display effectiveness. The method of evaluation was found useful and practical, while user reports were shown to be inappropriate. No overall difference between the three displays was found; the reasons for this are discussed and the conclusion is drawn that training is required to gain the benefit of colour in a display.

In a previous paper a model was outlined for an encounter between two vessels. This paper shows how the model has been developed to include the simulation of the behaviour of more than two ships, the entrance to harbours, and narrow channels.

In a previous paper by the authors the concept of a domain was examined. Goodwin's definition of a domain was used, namely, ‘the effective area around a ship which a navigator would like to keep clear with respect to other ships and stationary objects’. The concept of distinct sectors for sidelights and sternlight was modified mathematically so that an area equal to the total of the segments was contained within a circle. By off-centering the position of the ship within this circle, the weighting of the differing areas for the various sectors was retained (Fig. I). A second circle with the ship off-centre was introduced called an arena or ‘sphere of influence’. When a ship is inside the arena a navigator becomes aware of the other ship and decides what action, if any, is needed to keep his own domain unviolated. This resulted in a model which obeyed the Collision Regulations.

Having been concerned in the mid-fifties with developing the general equations of most economic flight and applying them to the profitability of an airline, I was surprised to discover when I was a ship financier in the early seventies that the much simpler equivalent marine problem had never been addressed by our clients, nor did the question arise apparently in the negotiation of their time charters with oil companies which are not conspicuously unsophisticated in some matters. I was therefore most interested to read Captain P. M. Alderton's paper on ‘The optimum speed of ships’, this Journal, 34, p. 341. If it were not for his charming reference to Napier's paper on the same theme dated 1865 one would be tempted to think that this was a case where the air leads the sea by decades.

The proposal by A. J. Wright (this Journal, vol. 34, no. 1, p. 142) that banded contour charts should be introduced, appears attractive at first sight. It appears to provide immediately recognizable indication of safe areas for the navigation of those ships whose draught is within certain limits. However, I feel that the system as proposed suffers from some major weaknesses

Allowance for height of tide. Tidal ranges of several metres are by no means uncommon. Failure to make proper allowance for this could result in a wrong assessment by the user of where navigationally safe water lies. Such tidal ranges frequently occur in areas of high-density traffic. Here, the pressures on the mariner are such that the temptation to ‘eye-ball the navigation’ is greatest – yet it is in such areas that positional accuracy based on sound chartwork procedures is vital.