True Motion in Relative Displays

Abstract

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,