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Aviation History
1949
1949 - 0212.PDF
142 FLIGHT FEBRUARY 3RD, 1949 RADAR and the WEATHER Applications to the Science of Meteorology IT has been said that, without radar, Fighter Commandcould not have won the Battle of Britain, nor could theweight of Bomber Command have been brought to bearon the enemy; it has further been hailed as the greatest single invention oi the 1939-45 war. Apart from its normal functions in aviation, the applica-tions of radar have been few. Since the war, the Telecommuni- cations Research Establishment, in conjunction with theMeteorological Office, has been making a study of possible uses of radar to the science of meteorology, and a synopsis of theresults of these investigations was given in a paper. Radar as an Aid to the Study of the Atmosphere, before the Royal Aero-nautical Society on January 27th. The author, Dr. F. E. Jones, M.B.E./B.Sc, Ph.D., A.M.I.E.E., is Senior PrincipalScientific Officer at T.R.E., and is Chief of the Department ol Experimental Physics. With the advent of microwave radar in 1941, it soon becameapparent that echoes were obtained from precipitating clouds, rain and snow. Recent research has been directed to discoverthe possibility of employing microwave radar to give advance warning of approaching rainbelts. The work has brought tolight some new evidence concerning the formation of rain, and it now seems possible that radar will be able not only toindicate distant areas of precipitation, but also to give a quan- titative estimate of the amount of precipitation occurring. The measurements obtained at T.R.E. were made with aradar set working on a wavelength of 3.2 cm and directed vertically upwards towards the falling rain. Simultaneously,measurements were made of the precipitation rate with a recording raingauge. Although measurements were made atvarious heights, they were converted to equivalent values at 3,000ft on the assumption that, since the precipitating areacompletely filled the aerial beam, the power was inversely proportional to the square of the range. Measurements whichwere made only during conditions of steady rainfall confirmed that this assumption was valid and that precipitation wasuniform from the ground up to the height at which the measurements were made. When investigating echoes from rain with the radar directedvertically, it was noticed that, although echo intensity de- creased with height, as would be expected, a considerableincrease in power was observed from a thin layer at a certain height. This strong echo, termed the "bright-baud," hasbeen observed by workers in many parts of the world. Experi- ments have established that the bright-band is located a littlebelow the freezing level, and that the echo power is five to nine times greater than the echo from the rain jest belowthe bright-band. Above that level, the echoes are very much smaller and vary considerably in strength. „ „ History Of Cloud Warning With certain types of clouds, such as cumulo-nimbus, con- vection currents are sometimes so violent that they present a danger to aircraft. Such clouds are invariably associated with heavy precipitation, and it was therefore of interest to study the radar response obtained from them. In the summer of 1946, a party of scientists from T.R.E. flew to the Singapore area in a specially-fitted Lancaster to make an airborne study of the cumulo-nimbus cloud problem. The radar set operated on a wavelength of 3.2 cm and the performance was su»h that towering cu-nim clouds were seen up to ranges'of 100 miles, and measurements of the echo intensity indicated that the rainfall associated with them was of the order of 2in per hour, which is a normal value of the tropical monsoon areas. As a result of the Singapore experiments, a cloud warning radar for aircraft installation was designed at T.R,E., and subsequent development and production undertaken by E. K. Cole, Ltd., Malmesbury. The apparatus works on a wave- length of 3.2 cm. and the position of heavy rainclouds up to a range of about 40 miles is indicated. With the greater power available, it has proved profitable to use 10 cm as the operating wavelength for radar ground stations to indicate storm centres. Such high-performance equipment is capable oi detecting heavy rainstorms at raages in excess of 150 miles, and in tropical regions the use of radar in giving early warning of the approach of rainstorms is proving invaluable, particularly to the forecaster who is con- cerned with the operation of aircraft. It has proved possible to issue short-term forecasts of bad landing conditions, often to within five minutes, and to estimate the duration of the unserviceability of an airfield as a result of storm conditions. There is a requirement in meteorology, especially for use in weather ships but also on airfields, for a method oi measur- ing the height of a cloud base from a single station. J. W. Ryde has indicated that the echo power from reflecting particles is inversely proportional to the fourth power of the wavelength. Thus the power of a set on a wavelength of 6 mm, with an aerial 30m diameter, would have to be more than 300 kW in order to give an echo from an average non- precipitating cloud at a range of 10,000ft. Thus, attention has been given to the possibility of using the large peak power available in the visible light region of the spectrum obtained from electrical discharges in gases. Flash Light Radar The experimental cloud-base meter designed at T.R.E. uses as a source a high-voltage electric spark produced between aluminium electrodes placed at the focus of a searchlight mirror : the duration of light pulse is less than 1 microsecond. The pulse reflected from the cloud. base is received on the second searchlight mirror which focuses the light on to a photo-electric cell the resultant current pulse being amplified and. displayed on the cathode-ray tube in the usual radar manner. The peak power of the spark is about 11 megawatts, and the efficiency is such that, with the mirror system used, a flux of about 4 million lumens is obtained on a distant cloud. Much thought has been given to study of the upper atmo- sphere, particularly with a view to determining how changes at such high levels influence the weather on the earth. The radio-sonde equipment enables routine measurements of pres- sure and temperature to be made up to altitudes of about 60,000ft, but it is difficult to make measurements above this height. The author suggested in - 1946 a relatively simple method of exploring the upper atmosphere—the outcome of a proposal by Synge, who suggested measuring the molecular scattering from great heights by means of a modulated search- light beam. —--:. The apparatus proposed for this measurement consists of a high-power pulsed light apparatus, similar in arrangement to the cloud-base meter already described. As the measure- ments will be made at night in a clear atmosphere, a photo- multiplier can be used as a detector in place of the photo- electric cell. In addition, accurate range measurements are not required, and hence the pulse shape is not as important as in the cloud-base meter equipment. With the general tendency of aircraft to operate at higher altitudes, the problem of providing accurate wind data has t>ecome more difficult. However, a radar system of wind finding has been in use in this country for. some time. The measurement is made by following a balloon-borne radar target, taking the form of a corner-reflector, from a powerful 10 cm ground station. The problem of long-range accurate wind finding has been studied at T.R.E., and a system employing secondary radar (i.e., a pulsed ground station) working in conjunction with a responder has \x-rn designed and is in process of develop- ment. In this system, the outgoing transmission from the ground consists of two microsecond pulses on a wavelength of about 2 metres. The associated balloon-borne responder con- siste of a miniature 2-metre receiver, the output of which is fed, via a modulator valve, to a small triode oscillator work- ing on a wavelength of about 10 cm. The power supply of the balloon-borne system consists of small batteries especially developed for the purpose. The outgoing 2 microsecond pulses from the ground station are transmitted back on a wavelength of 10 cm and are received on a paraboloidal aerial of 5ft diameter. The aerial system has been designed to "lock-on" to the responder signal and to follow it during its motion through the atmos- phere quite automatically and with great accuracy. The "lock-follow" aerial system indicates the azimuthal angle, the elevation angle and, in addition, the radar display indi- cates the range. The differentials of these are also available, and it is intended to feed these parameters into a computer which will indicate values of the velocity and direction of motion of the responder, i.e., wind h\»ed and direction, on two dials. Early flight trials have indicated that the responder can D 10
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