The problem of navigation, reduced to its very simplest terms, is how to get from X to Y. But this statement appears at first sight to require the proviso ‘given the means of transport available’, whether camel, ship, plane or rocket. Each sets up its special limitations to free movement, but in effect merely determines that XY must be broken up into segments, X Y1, Y1 Y2 and so on. Further reflection would probably add two additional considerations, the navigator must get safely and expeditiously from X to Y. But this again can only break down the route into fresh segments whereby dangers and difficulties are avoided. It is still for the navigator just the problem of getting from X, where he is, to a designated Y, where he wishes to be.

The rotation of the Earth provides the ultimate standard of time. As the fundamental unit of time we can use either the mean solar day or the sidereal day; these two units are related in a definite manner, so that when one is determined, the other can be inferred. The purpose of any timepiece is to subdivide the day into shorter intervals, and so give the time at any instant. No timepiece will give exact time; the error of the timepiece at some definite instant and the rate of change of that error, or, briefly, the rate, must be determined in order to extrapolate for the correct time at some subsequent instant. The accuracy of the extrapolation will depend upon the uniformity of the rate of the timepiece. Radio time signals sent out from an observatory, which is responsible for the determination and distribution of time, provide the most convenient means for deriving the error and rate of a timepiece. For normal navigational purposes an accuracy of about 0·05 seconds is adequate. But for the purpose of frequency control a very much higher precision is needed—but a precision in time interval rather than in absolute time. Some of the radio-aids to navigation depend upon the accuracy in standardization of frequency, so that high accuracy in time interval has become, indirectly, a navigational requirement.

A Brief introduction on the purpose of the paper constitutes Part 1.

Part 2 of the paper traces' the historical development of navigational radar in the United Kingdom from the beginning of the late war to the present day. The use that was made of the very early radars is mentioned and the improvement brought about by the introduction of the PPI and centimetre radar is described. An account is given of how radar assisted the Normandy landings, of the techniques employed and the lessons that were learned. It is shown how a change to shorter wavelengths improved the performance, and how this made possible a large experiment in the Scheldt Estuary designed to help convoys to reach Antwerp in fog; the navigational difficulties of this experiment are described and it is shown how an elaborate drill had to be prepared for navigating these difficult channels to allow the troubles to be overcome. The thought that was devoted to the navigational needs of the Merchant Service in peacetime is dealt with, together with the elaborate trials that were carried out in the Thames Estuary to check the plans for peacetime development. It is shown how the new equipments which were developed brought about a large improvement in performance and ease of operation. In particular, the use of new displays allowing the PPI to be superimposed on the chart is dealt with and the simplification of navigation and pilotage which this allowed; the success with which ships were navigated and piloted on radar information alone is also dealt with. Some notes are included on the work of the two first International Meetings on Radio Aids to Marine Navigation. A short account is given of some experiences with radar in Norwegian Fjords.

Part 3 describes the present position where commercial navigational radars are in production and fitted to a number of British and foreign vessels. This part compares the performance of some British and United States equipments and also compares the requirements of the British and United States Government performance specifications.

Part 4 reviews some of the problems outstanding today and makes some suggestions on possible future developments. In particular, suggestions are offered on improved PPI'S, on new systems of chart comparison, on an alternative form of display for ship-handling and, looking rather further ahead, on a new form of close range and pilotage radar.

Almost ten years ago (14 March 1938), in this same lecture hall, the Astronomer Royal, our newly elected President, opened a discussion on the recently published Air Almanac. That meeting, held under the auspices of the Royal Geographical Society, can justly be said to mark an epoch in the application of astronomical methods to air navigation in this country; but it was remarkable in other ways too. First, it provided that opportunity for criticism, by users and by others interested, of the provision to be made for astronomical navigation in the air, which it is hoped the Institute will provide for all aids to navigation. Secondly, the preparation of the Astronomical Navigation Tables was first announced; this may not appear very remarkable in itself, but it was the first time, since the days of Nevil Maskelyne, fifth Astronomer Royal, that the Nautical Almanac Office had prepared any major tables for the reduction of navigational observations!

During the course of the Second World War, consideration was given to the means for operating post-war air routes. Concurrently, discussions were taking place as to the type and performance of the aircraft likely to be available. It was, however, quite obvious that, as we had had no previous experience at the higher altitudes at which these operations would be carried out, account would have to be taken of the winds which would be experienced, and steps were taken therefore to procure these data whereby to plan the routes.

The Mercator chart has been used for navigational purposes for nearly four hundred years. Before Kramer devised his chart in 1569 navigators generally used a chart based on the simple cylindrical projection. The Cylindrical Orthomorphic projection devised by Kramer (or Mercator, as he is commonly called) revolutionized navigation and from that time has been the standard navigational chart for marine use. It was to be expected that, when the time came, air navigators would copy and perhaps modify the technique of the seaman. This was done, and today air navigators continue to use the Mercator chart. In many ways, however, it is not an ideal chart; it is now time to depart from this tradition and to use charts more suitable for air navigation. That this is possible has been shown by experimental flying in the Royal Air Force and by other countries, notably the United States and Canada.

As A preliminary to the discussion of the pre-determined Position Line, it will be advantageous to consider the system of working a Position Line on the Intercept method. Although this is well-known to navigators, a certain analogy exists between the two and it may be demonstrated quite simply.

During the cruise of H.M.S. Van Kinsbergen, of the Royal Netherlands Navy, with 92 midshipmen on board (of whom 66 were Executive Branch), as may be expected, a great many sights were taken. Very often the ship's position was obtained by bearings shortly before the star observations were made; in this way the DR position was known and the accuracy of the sights could be determined.