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Aviation History
1945
1945 - 1215.PDF
JUNE 2IST, 1945 FLIGHT 663 Radar Partial Release of Some Systems Used by R.A.R in Defeating the Enemy by Defence, and Fighter and Bombing Attacks C^em?^^?^ °f ( ^o-AmenCim censorship the exploring station measures the difference in frequencySSon with th? m niU"f • y l0W/?evel.in com- between the transmitted and reflected waves in order toPof aTtivt W SZ^^.Z^^J?™. letermine the SPeed of .the object,. _ Owing to the small whilst the subject is still shrouded in official secrecy L, this country. However, as the disclosure was made avail- able to all the world, we, too, publish the information while awaiting the official release. Radar is a generic term covering all forms of radio-electronic identi- fication and orientation systems. Of these there are a considerable number, many of which are still undisclosable although the basic principle of operation is common to all, and the description of how one device or system operates can be taken as a basis for all others. Briefly, radar can be said to be the application of basic radio principles to the general problems of deter- mining distance in three dimensions of an object which will reflect radio frequency energy relative to an exploring station. The fundamen- tal principles involved are the con- stant known speed of radio wave propagation and the reflective or echo principle which is a major characteristic of the ultra-high fre- quencies. Two Systems Without going too deeply into the individual systems, there are two major methods of radar detec- tion which have been employed, either separately or in conjunction with each other. First is the system based on Doppler effect whereby exploring radio frequency energy, continuously transmitted, impinges on the target object whence part of the energy is reflected and its apparent frequency changed. The receiving equipment of Fig. 1. Diagram of basic radar system showingdirection " sighting " and transmission-reception time for range. TRANSMITTED PULSE CATHODE RAY TUBE SCREEN fast moving target objects are involved. The second system is based on pulse modulation of the exploring wave emanations, and the determination of the reflected impulses against a time scale. Radio frequency energy is modulated for pulsation intervals of from i to 50 microseconds (p sec.) and the pulse ends before the re- flected energy returns to the re- ceiver, so that an indicating time scale set against the '' blips'' shown on the cathode-ray tube screen will determine the distance of the object. Time Datum A fundamental quality in radar is the ability to convert time in- terval into distance due to the fact that radio energy radiated into space travels at a constant velocity, and that on striking an object is re-radiated in part with no loss of time. The velocity is that of light, which is 186,000 miles/sec, or 327 yards/microsecond. Referring to Fig. 1, if a 1 ,u.sec. pulse is trans- mitted, the time taken for the pul- sation to reach the objective 32,700 yards away would be 100 ju.sec, and as the reflection will require the same time to get back to the re- ceiver the total time from trans- mission to reception will be 200 /(.sec. Since the indicator registers total elapsed time it is usual to calibrate distances (transmitter to object) as 164 yds./,u.sec between the initial and reflected pulsations. In Fig. 2 is shown a diagrammatic illustration of the cathode-ray screen, across which the exploring wave and reflective pulsations sweep, and upon which the time and distance scales are graduated electronically. Initial and any reflected signals will appear on the screen as vertical "spikes." We will assume that the linear sweep of the wave pulse is 1 centimetre per 100 /(.sec, and that a pulse of 1 M.sec. duration is being transmitted. This pulse will be 0.01 cm. wide on the screen, and during this time the exploring wave will have travelled 327 yards from the transmitter. Assuming that the target object is 32,700 TIME /1SEC 100 •• •••" 200 I 1 1 11 > 1 I 300 400 1 I I 11 1 11 i I I I | I I M S00 I- i 1 •+• K 16400 -t-0 10,000 20^00 30,000 4Q000 50,000 60,000 DISTANCE YARDS Fig. 2. Initial and reflection "spikes " shown on the C.R. screen can be read against a time-distance scale for determining range. The diagrams are reproduced from Electronic Industries, N.Y.
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