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Aviation History
1953
1953 - 1385.PDF
FLIGHT, 16 October 1953 539 RADAR SOUNDING . . . measurement may be sufficiently accurate at fairly low levels, the accuracy obtainable with existing meteorological elements decreases with diminishing pressure, so that the error in heights derived from the pressure-measurement becomes greater as the balloon ascends. Today, the development of radar by the armed Services during the last war—and especially the development of centimetric tech niques—has opened up a completely new field of wind-finding, and now, by means of "rawin," we are able to obtain wind-velocity measurements to a very much higher degree of accuracy than before. Rawin is a coined word formed from the initials of radar wind-finding and is now internationally accepted. The centimetric radar set emits short, intense pulses of radio energy which are con centrated into a very narrow beam by means of a suitable reflector, much as in the case of a normal car headlamp. If a suitable target is attached to the balloon, a pulse echo is reflected back to the ground transmitter when the target intercepts tiiis beam. When the echo is received back at the radar set it is amplified and presented on indicators, of which there are many types. The exact nature of each indicator depends on the purpose for which the radar set is designed. For example, for general search work it is preferable to display pictures of the complete areas as the radar aerials are rotated, whereas for gunlaying and radar wind-observations it is preferable to display only the signal reflected from the balloon target, and to adjust the direction of the aerials to track this target. The round-trip travel time of the radio pulse is proportional to the distance (or "slant range") to the target, and by suitable electronic devices this range can be indicated directly at the radar set. Both die angular bearing, or azimuth, and the elevation in which the radar beam is pointed can be measured, so that by this method it is possible to pin-point the position of the balloon in space. A plan chart showing the track of the balloon during its ascent is drawn; and the wind- velocity, at any particular time and height of the balloon's pro gress, can then be estimated from this chart. Plotting Up-currents It is also possible to measure the height of the balloon directly at any time. Whilst this latter information is not of direct use in plotting the upper-air charts—the contour lines being plotted at standard pressure-levels—it does provide a means of detecting variations in the vertical air currents at different levels, since under non-turbulent conditions the rate of ascent of the balloon will be constant over the major part of its ascent and will not show any discontinuities. This is particularly valuable in tropical regions, where violent up-draughts, such as those occurring in heavy cumulo-nimbus clouds, constitute a real menace even to the larger aircraft now employed in commercial aviation. The maximum rate at which the balloon carrying the radar target rises is limited, bom by the economics of the balloon's design, and also by the response-time of die meteorological measuring unit incorporated in the radio-sonde transmitter equip ment carried. If the rate of ascent is too high, the finite time required for the meteorological elements to respond to the changes in atmospheric conditions introduces an unreasonable error in measurement. The normal upper limit to the rate of ascent has been found by experience to be between 1,200 and 1,400 ft/min, so that the balloon will take approximately an hour to reach its bursting-height of 60,000 to 100,000ft. The balloon is carried by the winds throughout the whole of its ascent, so that its range at its maximum height will, obviously, deoend on the average winds prevailing during the ascent. With light winds such as those experienced in this country during the summer months, the maximum range may not exceed 10 to 15 miles. At other periods of the year, however, when wind speeds of 50kts and more are quite frequent, the range will often reach 50 to 60 miles before the balloon has risen to 60,000ft. As a result, unless the performance of both the ground radar equipment and the reflector attached to the balloon is capable of ensuring a satis factory response at these large ranges, the balloon cannot be tracked to its maximum height, and valuable data on the wind- structure at the higher level is lost. This becomes particularly serious as the normal operating heights of aircraft are increased, since forecasts at 40,000 to 50,000ft must be based on data from levels both above and below these heights. The design of radar reflectors for attachment to meteorological balloons has been the subject of considerable development both in this countrv and America. Any metallic object will reflect to a certain extent, but the efficiency of a non-directional reflector is comparatively low, due to the fact that the energy reflected from the target is spread out in all directions, whereas the effective part of the radiation is that directed towards the ground equipment. The design of these reflectors has, therefore, been aimed at pro ducing reflectors in which the incident energy is entirely reflected back along its path. The principle adopted is one well known in the field of optics, in which a prism having three mutually perpendicular faces is used to reflect the incident light. This principle can be seen in the design of various reflectors used on the road and on the rear of vehicles. The efficiency of the radar reflector is entirely dependent on the physical size of the three planes, provided they are accurately maintained at right angles to each other. For some time reflectors were constructed from aluminium foil, having a paper backing and mounted on a thin balsa-wood structure, and the various planes were assembled on the station in dieir correct position. As the size of the reflector is increased, however, the wind-drag on the various solid planes becomes increasingly important, since it impairs the height- performance of the balloon and makes it difficult to release the reflector in high winds. It has been found, however that die efficiency of a plane is only reduced by approximately 5 per cent if the plane has a mesh construction, the hole-spacing being approximately four to the inch for radar wavelengths of the order of 10cm. A special nylon net metallized by the Sucal process to make it conductive has therefore been used in the design of the modern reflectors, and by this means it has been possible to con struct "Umbrawin" reflectors having dimensions of approximately 4ft 6in, but still having less aerodynamic drag than the older solid- plane type of approximately similar dimensions. This modern "Umbrawin" reflector (manufactured by the London firm of Chemring, Ltd.) can now be folded in a similar way to an umbrella and erected on the station in one simple movement; and by suitable construction the required degree of accuracy in normality of the planes has been achieved even with the com paratively light framework employed. With these reflectors, and using the standard G.L.3 radar equipment now in use with the Meteorological Office, maximum ranges of up to 100,000 yd are frequently obtained and with this performance wind data up to 60,000ft and more is obtained on the majority of occasions. Whilst this range-performance meets existing requirements, it must still be improved to meet future demands for information up to 100,000ft, and developments are now proceeding to achieve these results. Although greater ranges can be achieved by increasing the size of the reflector still further with the present form of construction, the weight becomes excessive and the matter of handling the reflector in anything but absolutely calm conditions becomes difficult. An experimental programme is being carried out by Chemring to develop a reflector having no rigid framework and being contained inside a balloon. In this way it is hoped to produce much larger reflectors which will self- erect as the balloon expands, so eliminating any handling difficulties on the ground. The internal reflector is contained in an inner balloon whose diameter does not exceed a size deter mined by the dimensions of the actual radar reflector. This envelope is then contained in a second balloon which is free to expand in the normal way. A suitable pressure-release valve for maintaining the dimensions of the inner balloon has been evolved and allows the hydrogen to escape into the outer envelope when the diameter of the inner has reached its optimum value. By diis design the wind-drag on the balloon and reflector assembly has been reduced to an absolute minimum, since there is no additional apparatus connected to the balloon, which is free to ascend to its bursting limit. This internal-reflector design has advantages in other fields not necessarily meteorological. In these applications the reflector may be fitted inside a constant-altitude balloon, stabilized so that it may be towed and fly in the same attitude, irrespective of the wind-speed, and diis device has many possible applications, such as in radar markers for air/sea rescue work, and also for high speed towed targets for use in die training of bodi Naval and Royal Air Force radar operators. We can, therefore, say that with the successful conclusion to this development many gaps in the field of radar reflectors for specialized applications will have been met. H.J.F. CONTROL BY JET THE Office National d'Etudes et de Recherches Adronautiques (O.N.E.R.A.) have sent us brief details of two British patents which they have held since 1948 and 1949. Both concern methods of control by jet deviation; this subject is one that has resulted in many patents, but O.N.E.R.A. do not state what further progress has been made in putting their ideas to the test. The first scheme involves profiling the engine tail-pipe and jet nozzle so that the final outlet has the shape of a cross, there being two flat gas streams intersecting at right angles. By deflecting the nozzles in die manner of a rudder and /or elevators stability and control about all three axes would be obtained. The second arrangement consists of the main engine compressor supplying air to two distinct sets of auxiliary combustion cham bers, each with its associated nozzle, one nozzle facing forwards and the other to the rear. In normal flight the rear nozzle would be used to supplement thrust; for landing, or other decelerating case, the tapped flow would be passed to the forward-facing chamber and nozzle.
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