When harbour surveillance radars were first fitted in the United Kingdom in 1948 it became apparent that under conditions of high traffic density difficulty might arise in identifying the ship being assisted into harbour with a particular echo on the radar screen. The group at the Admiralty Signal and Radar Establishment (A.S.R.E.) working on navigational aids for the Ministry of Transport were set the task of examining the problem and suggesting a solution, and this paper will describe the results of this work.
THE system proposed by Manley (15, 172), to produce true motion on relative displays seems to suffer from two serious disadvantages which were not referred to. The first is that there would be a serious degradation in the resolution of the stored picture, since every time an original echo is rewritten into one of the Tenicon storage tubes the echo size would be increased both in range and bearing by the size of the beam spot. No doubt the full effect of this increase could be reduced by the introduction of pulse shaping circuits between the read-out and write-in points, but nevertheless the original small echo would be considerably increased in size if a number of read-out write-in operations were carried out as would be the case if the information was stored for a sufficiently long time to show true track. As an example, without applying any special pulse shaping techniques a small echo, two spot diameters long and one spot diameter wide, would be increased in size by 6^ spot diameters if stored for 30 seconds (i.e. 10 scans of the aerial) and would be increased by 1890 spot diameters if stored for 3 minutes.The second disadvantage is probably the more serious. Since any signal entering the system remains there until the stored information is cancelled, the effect of interference would be to flood the screen completely in a short space of time. In the case of receiver noise and sea clutter the effect could be reduced by the introduction of signal clipping and swept gain but at the expense of receiver sensitivity. However, it would be extremely difficult to reduce the effect of interference from another ship's radar on the same frequency, and interference of this type would be received from ships within at least an 8 miles radius irrespective of the relative position of the two radar aerials. In congested waters such as the Dover Strait there is a high probability of receiving this type of interference from a number of ships, but with the proposed system the stored picture would be completely saturated by the interference from only one ship in a very short time. Mr. B. W. Manley comments:If an echo of size A o is stored on the Tenicon target, and its size is increased by an amount (p x a) at each transference, where a is the spot size and p is a numerical factor, then after n such transferences the size A n of the stored signal is given by:Milwright seems to have arrived at a form:which would indicate an alarming and unobserved rate of growth. What small amount of evidence we have at present on the use of the Tenicon in a 'ping-pong' mode, indicates that p is less than unity. After three minutes scanning, therefore,
The need for investigations into the detection of ice by radar became apparent when merchant ships started reporting that ice formations were inconsistent radar targets, and that ships relying upon radar to navigate through ice areas could, in some circumstances, have their safety endangered.Certain investigations, notably those carried out by the U.S. Coast-guard Service and by the Swedish Defence Research Institute, resulted in the publication of a great deal of useful descriptive information but it was not accompanied by measurements of the actual echoing power of ice targets. With the object of carrying the matter further, a special enquiry was made during the 1950 and 1951 North Atlantic ice seasons, when a number of British ships plying North Atlantic routes completed questionnaires in which the sizes and shapes of ice formations were noted together with their radar detection ranges. This enquiry was instituted by the Operational Research Group in the Marine (Navigational Aids) Division of the British Ministry of Transport, which later analysed the data collected and issued a report. There were, however, certain limitations in this investigation. For example, the performance of the radars was not known, because of their inherent differences and the fact that none of the ships carried instruments, such as echo-boxes, for checking performance; further, propagation conditions at the time of making the observations were not established.
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.
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