The four papers which follow were read to a meeting of the Institute held at the Royal Geographical Society on 19 May. They provide an account of the four systems of radio position fixing that are most widely used at sea, and give both a description of the experience gained and a general survey of what has been achieved since the international meetings on radio aids to marine navigation were held in 1946 and 1947. Certain requirements for marine radio position fixing systems were formulated at these meetings and a measure of agreement was reached on the types of navigational aid which should be developed. This review of the four principal systems, and of aspects of some of them which may not be widely appreciated, may perhaps give an opportunity for assessing how much progress has been made over the last few years and of the problems that remain to be solved.

Before the war the only radio navigational aids in current use by shipping were direction finding systems, operating in the medium frequency band of the radio spectrum.

During the war, not only were many new types of position fixing systems evolved, but radar techniques were also developed, thus making available a wide range of new facilities of potential use both to ships of many different categories and to port and harbour authorities for the supervision of marine traffic.

Direction finding is the oldest, simplest, and probably most reliable radio aid yet offered to the mariner. In some ways it may be likened to a magnetic compass and, if its range is very much smaller, it has the inestimable advantage over a compass of giving bearings on a very large number of stations instead of one only. It is the only navigational aid, other than radar, in which the performance is controlled by the shipborne installation and is not subject to technical failures which may occur on land-based transmitters.

The determination of a ship's position by means of radio observations from fixed shore transmitters has now been practised for a considerable period of time, but up to the outbreak of the last war the accuracy obtained from the conventional direction finding systems then in use was only sufficient to provide a general estimate of position suitable for off-shore navigation in waters where the ship's position need be known only to within a mile or two. It was not until, under the pressure of wartime requirements, the modern range of radio aids to marine navigation were produced that means were provided by which the mariner was enabled to determine his position with an accuracy comparable with that of the best visual fixing but with the great advantage that these observations could be made in all conditions of visibility and weather. At the end of the last war there were three systems of outstanding merit available: Gee, Loran and Decca. All three systems, as is well known, employ a combination of master and slave stations, the transmissions of which provide radio position lines capable of accurate calculation and plotting on navigational charts.

The present-day trawler carries more aids than any other vessel of comparable length and tonnage and, more often than not, more than the majority of big liners and cargo ships. She carries a powerful wireless transmitter giving an output in the neighbourhood of 100 watts in the modern sets; a main receiver with a standby, or emergency one, for listening in to ships of the same Company for information regarding the state of fishing in their areas; and one or two direction finders, one of which (known as a ‘fish snatcher’) is tuned to the trawler wave band to take snap bearings of rival ships who may be on a better living and foolish enough to say so. So far as Consol is concerned the direction finder is used to ascertain the sector in which the vessel is, should the accuracy of the dead reckoning position be in doubt.

It might reasonably be considered that any discussion of interplanetary navigation at the present moment is slightly premature. So of course it is, from the practical point of view, since no well-informed person seriously imagines that space-travel will be possible for at least twenty or thirty years, despite the colossal efforts which are now being devoted (unfortunately for quite other purposes) to the solution of its engineering problems. Nevertheless the subject is one of peculiar fascination—which is a completely sufficient excuse for discussing it—and the navigation of guided missiles into astronomical space, which will precede the manned exploration of the planets, has of course already begun and will continue on an ever-increasing scale during the next decades.

It is clearly not possible, in the space of one paper, even to touch on all aspects of interplanetary navigation, and some whole fields must be ruled out from the start. This paper will not be concerned with any discussion of the space-flight as such; there is no point in considering navigational problems at all unless one can assume (at least for the sake of argument) that there is to be some practicable way of lifting a ship out of the Earth's gravitational field, and then of accelerating it out of the Earth's orbit; but even granting this major premise a considerable amount of selection is still essential.

The practice of precomputing astronomical sights in the air, so reducing the delay between taking the observations and plotting the position lines on the chart, appears to be gaining in popularity. The observer selects a future instant in time, and calculates the altitude and azimuth of a suitable body or bodies, for an assumed position near the D.R. position at that time. Then, if the observation is taken at the time chosen, the intercept is obtained directly by comparing the sextant reading with the precomputed altitude, corrections having previously been made for systematic errors (e.g. dome refraction, personal error, &c.)

Recent papers on the future of astronomical navigation in the air indicate that the present trend of development is directed towards surmounting two principal obstacles: the time lag between the shots and the final fix, and inaccuracies due to accelerations of the bubble. Even in aircraft at speeds between 200–300 m.p.h. these two factors tend to discourage the use of astro-navigation even where, as is particularly the case in lower latitudes, radio aids are not available.

The need for the rapid reduction of astro-sights has been constantly stressed for use in high speed aircraft outside radio coverage. The method described here is a combination of precomputation of altitudes and graphical representation of intercept from a given position. By this method a fix can be plotted at the instant of time for which it has been precomputed.

The paper reviews some of the major problems which have been encountered in the development of airborne direction finding equipment, and describes, in rather more detail, the latest electrical and mechanical improvements which have been evolved to provide the modern, fast moving aircraft with a more rapid or instantaneous position indication.

Various methods of utilizing the information provided by the direction finder are discussed and questions of calibration and ultimate accuracy considered.

The introduction into both the Royal Navy and the Royal Air Force of the air navigational equipment that is at present envisaged emphasizes the need for a reliable and accurate compass. Further, since no compass can be expected to give completely accurate information in all conditions of use, the errors that are likely to arise and their magnitudes must be known. Before an error can be determined, it is first necessary to define what may be regarded as an error, and then to determine its magnitude as accurately as possible. Gone are the days when an expression such as ‘plus or minus two degrees’ would be regarded as satisfactory. The determination of compass accuracy to a tenth of a degree is now an accepted requirement.

The use of Consol at long ranges in ships is hampered by the lack of facilities for accurate plotting. Of necessity ocean charts (scale 1:1,000,000 to 1:12,000,000) are used; and on these it is not practicable to plot running fixes, lay off position lines from celestial observations or to compare a succession of cross bearings with any degree of accuracy.

When Consol bearings can be plotted on a large scale chart and used in conjunction with other position lines, such as Marcq. St. Hilaire, lines of soundings, m.f./d.f., &c, they are of assistance in defining an observed position; this is especially so in conditions of poor visibility, such as prevail in the approaches to the British Isles and in the North Atlantic.

The publication of the paper Astro-fix by Computation by Oliver C. Collins in the April number of the Journal serves as a reminder that the observation for the determination of the latitude from two altitudes, usually known as the double altitude problem, is one of some celebrity in the history of navigation.

The following notes on the subject may be of some interest although they are far from complete, having been compiled from such books as happened to be readily available, and can only be considered as an introduction to the subject.