A study of many collisions at sea in which the misuse of radar may have been a contributory cause leads to the conclusion that much difficulty might be overcome if a less ambiguous method of displaying radar information were provided. To this end an entirely new type of radar presentation has been embodied in the latest equipment produced by Decca Radar Limited. Type TM 46 displays the radar picture so that the true movements of all vessels, including own ship, can be assessed by inspection of the display. This continuous presentation on the radar screen of the true course and speed of all targets is an entirely new feature in radar equipments. Unlike former systems, where own ship appears continuously at the centre of the display and the movement of other echoes is relative, ‘true-motion’ presentation shows own ship moving across the display at the correct course and speed, and the true movement of other echoes. Stationary objects such as buoys or anchored ships do not move on the PPI and can be readily identified.

The main problem in polar navigation is one of direction, and the problem has two facets. First we have to measure direction and then we have to describe it. Generally, the seaman has a gyro-compass by which he can measure the heading of his ship. As you know, the gyro-compass is simply a gyro which keeps pointing towards the pole. If we have a gyro close to the north pole and pointing north, it will continue to point in the same direction however the ship turns beneath it. So also will it continue to point in the same direction if the Earth turns beneath it. In twelve hours it will be pointing south instead of north.

People often think that early seafarers must have clung to the coast, but there is plenty of evidence that from the earliest times seamen have not only ventured out of sight of land but have conducted regular passages across the open sea. No doubt, however, landmarks did provide the earliest aid to navigation and in the eastern Mediterranean where the science was born, ships would not have been out of sight of land for more than a day or two.

The invention of the balloon was almost a casual affair, unrelated to other contemporary inventions and unheralded by prior developments. The hot-air balloon could have been invented at any time since the mastery of thin textile manufacture, which can be dated well before the Christian era. The hydrogen balloon could not have come about until after Cavendish had isolated the gas in 1766. As it finally turned out, both types of balloon were invented in the year 1783.

Captain Scott went to the pole over the Ross Barrier, up the Beardmore Glacier (Fig. 1) which is about 100 miles long, and then on to the south pole, coming out by the same route. The great Norwegian explorer Amundsen went in from the other side over the shelf, up one of the glaciers and on to the pole. Admiral Byrd flew in and out again in 1929. Apart from these expeditions very little has been done in Antarctica and nearly all of it, apart from the fringe, is untouched.

If a navigator wishes to determine the closest passing distance of his ship to any other ship by means of radar, it can be done quite simply by plotting the ranges and bearings of the other vessel's echo. The line joining the plotted positions will indicate the relative course of the other vessel and by extending the relative-course line the closest passing distance can be measured. With the existing method of measuring bearing by means of a mechanical cursor with its attendant possible in-accuracies, it is necessary to plot a number of positions and draw the relative-course line as a mean through the plotted positions. The time taken to establish the nearest approach is therefore comparatively long. If an electronic bearing cursor is used the time to determine the nearest approach can be reduced since the accuracy of the measured bearings will be higher, and fewer measurements required. It may, however, be inconvenient to move from the radar screen to the plotting table and if plotting is done on the face of the PPI or a reflection plotter, the accuracy may be poor, particularly if the nearest approach is determined by extending a relative-course line formed by two plotted positions close together, using a blunt wax pencil.

As a result of the war, radar instruments developed in one particular way, namely as relative-motion instruments, which is only one of a number of possibilities. The relative-motion display is above all a position-finding aid; it places own ship in the centre of the screen in exactly the same way as the human eye has its position in the centre of the circle formed by the horizon.

Although early meteorological records were naturally confined to the weather experienced at the surface of the Earth, as soon as meteorology began to be a science it was realized that what went on above the surface was important and that the physics of the air needed to be studied in three dimensions. An obvious illustration is the use made of cloud observations—the nature and structure of clouds, the method of their formation and their movement. Something could be learnt on those questions by observations from the ground, aided by such a simple instrument as the nephoscope. Most people, and certainly all concerned with navigation, know that the movement of clouds may be very different from the wind at the surface. It was also realized that since the atmosphere is a heat engine a knowledge of the temperature of the upper air was required. Anyone who has climbed a mountain knows that the temperature falls, and this was confirmed for the free atmosphere by observations with kites. It was clearly of interest to know whether the fall of temperature was maintained until the absolute zero was reached, and if not, why not. It is in the measurement of temperature and wind in the upper air that balloons have found their chief use.

Over the better developed land areas of the world, where radio and communication facilities enable aircraft positions to be known on the ground, the system of Airways flying has been introduced. In areas of high traffic density it is of the greatest value, as it allows the traffic control authorities to separate aircraft at moderate to close intervals. The system requires good and continuous radio track guidance and a regular reporting procedure at specific radio fixes. (Airways are normally looked upon as fully controlled areas and until some more reliable long-range aid which can present continuous fixing is developed, Oceanic Airways would be more properly referred to as Oceanic ADR's.)

The operation of helicopters in all weathers is an essential development if full advantage is to be taken of this class of vertical take-off aircraft. Helicopters must operate in weather minima at least as low as the current fixed-wing limits of 300 yards visibility and 300 ft. cloudbase, and should approach the limits approached by surface transport which continues to operate down to visibilities of the order of 10 to 20 yards. Present experience suggests that it should be possible to conduct regular operations down to 200 yards visibility and 200 ft. cloudbase provided the necessary aids are available. As experience is gained, these limits might be reduced to 150 yards and 150 ft. However, as the pilot needs time to change over to visual guidance before landing, it does not seem likely that these limits could be much reduced before the development and acceptance by civil operators of either completely automatic landing or some radical new landing aid.

For a number of purposes it is convenient to have a platform floating at a constant level high up in the atmosphere. This can be contrived by the use of a special arrangement of balloons for heights up to about 100,000 ft. There are at least three ways in which this can be achieved. In the first an extensible balloon is filled with sufficient gas to give it buoyancy for a definite rate of ascent. As it ascends and the atmospheric density decreases, the gas and the balloon expand and if the ascent goes on long enough the balloon bursts. If when the balloon reaches a certain dimension (or alternatively when the exterior pressure falls to a certain figure), a valve opens and allows sufficient gas to escape to stop the ascent, the balloon will float in equilibrium. A second method is to use a non-extensible balloon only partially filled with gas and with an open mouth; as the balloon ascends, the gas expands to fill the whole balloon and overspills thereby reducing the rate of ascent until equilibrium is reached. A third method involves the use of a number of extensible balloons tied together. When they have risen in the air and the balloons have stretched, some will burst before others and thereby a rough equilibrium is attained and the remaining balloons will float at nearly a constant level. A variant of this is to detach some of the balloons from the assembly at a predetermined pressure level, so that the remainder are just able to support the instruments and equipment.

The first equation displayed in Appendix A to Mr. J. E. D. Williams's paper (Vol. X, p. 44) should have read:

The weather at an airport affects the ability of an aircraft to land in so far as it controls the pilot's vision of the runway as he approaches it. This aspect of weather is at present described by (a) Runway Visual Range (RVR) as measured along the runway by Air Traffic Control, (b) Visibility as measured from and by the Meteorological Office, and (c) the amount and height above the airfield of the lowest cloud. Weather minima are specified in terms of RVR or visibility and of cloudbase by each Company for each type of aircraft using each available type of approach aid on each runway at each airport, such that an aircraft should not try to land in worse conditions. A landing forecast known as a TAF is issued regularly for each airport and specifies among other things visibility and cloudbase, giving either single values for each, or subsidiary ‘possible’ values indicating temporal variations or values with a small probability of occurrence. Examples are: ‘Visibility 10 miles TEMPO (temporarily) showers Visibility 2 miles’. ‘Fog Visibility 500 yards GRADU (gradually becoming between) 0400–0600Z mist Visibility 1 mile’. The TAF for the destination is seen by the aircraft captain before departure and during flight: if it is unfavourable he may decide to increase the fuel reserve reducing the payload or to delay departure or to divert. A wrong TAF may have costly and occasionally dangerous results.

The value of plotting radar observations at sea has often been emphasized and some knowledge of the errors to be expected will be of use to the navigator. Small random errorsare always present in radar observations. Under certain conditions, particularly when ships are approaching end-on, even a correct interpretation of the observations can lead to a misinterpretation of the situation. The Steering Rules are operative only when vessels are in sight of one another, since if it cannot be assumed that there is a mutual understanding of the situation there can be no mutual rules of conduct.

The object of this paper is to compare modern interception theory with the better known classical concepts. We shall examine the curve of pursuit, the classical straight-line interception and the modern theory of proportional navigation.

The suggestion in this note is the result of reading D. E. Adams's paper on inertial navigation (this Journal, 9, 249) and Captain Maya's paper on dead reckoning (this Journal, 10, 84).