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Aviation History
1952
1952 - 0585.PDF
FLIGHT, 7 March 1952 265 WINDS in the IONOSPHERE— — and Their Effect on Radio Transmission. Results of a Three-nation Research Programme ONE of the causes of fading in radio communications is a wind phenomenon in the ionosphere. Winds of the upper atmosphere are produced by the heating effect of radiations from the sun and by solar and lunar gravita tional tides. Because knowledge of their motions is necessary for correct evaluation of the role of ionization, de-ionization, and diffusion process in the upper atmosphere, the U.S. National Bureau of Standards has incorporated a study of winds in its ionospheric research programme. The Bureau's observations of ionospheric winds (writes a U.S. correspondent) are made on a regular monthly schedule, and the data are co-ordinated with similar observations made by scientists in Great Britain and Canada. The agreement that has been obtained between the records of the three participants suggests the possibility that the observed winds belong to a world-wide circula tory system of the outer atmosphere. Analysis of the results indicates that the azimuths of the winds are subject to seasonal variations. During June, July and August the winds blow principally to the east; in December, January and February their direction is generally south-westerly. During the spring and autumn, the winds often appear to rotate in a clockwise sense, completing two complete rotations in a 24-hour period. Speeds are also subject to variations; some are as high as 300 m/sec (660 m.p.h.), but the average is about 70 m/sec (150 m.p.h.). In the past, ionospheric-wind characteristics have been calcu lated from observations of slowly drifting meteor trails and lumin ous clouds occasionally seen during the night hours. However, because the data are derived from relatively rare phenomena, this method is not capable of giving a comprehensive picture of upper atmosphere winds. At present the Central Radio Propagation Laboratory of the Bureau is using a systematic technique that takes advantage of the reflecting property of the ionized layer of the upper atmosphere, upon which all long-distance radio propaga tion depends. When radio waves of constant amplitude are reflected from the ionosphere back to the earth, they usually return with an amplitude that varies erratically with time. The variation or fading of the radio signal is generally a nuisance factor in long-distance com munications, but aptly lends itself to an investigation of iono spheric winds. If the fading is observed at two receiving aerials separated by one or two wavelengths of the probing radio waves, the fading patterns are very similar, but often appear displaced in time. This time delay can be explained by assuming that the ionosphere is made up of patches of ionization; in other words, that some parts of the ionosphere are more densely ionized than others. For this reason, the radio waves that are scattered or reflected from the ionosphere produce a "diffraction pattern" of signal intensities on the ground. Thus, as the overhead layer of the ionosphere moves, the ground pattern also moves and the fading sequence appears first at one aerial and then the other. (Right) Comparison between data obtained at the Sterling, Va., station and the Cavendish Laboratory (Cambridge) field- station in this country. Measures of agreement such as these shown here lend support to the theory that the inonospheric winds may have a world-wide circulatory system. (Below) A typical record obtained at the Sterling station. The ground diffraction pattern is first intercepted by one aerial (voltage reading shown by solid line) and about 2 sec. later by the other (dotted line). The "pegs" are 4-sec calibrations. 150 100 50 0 50 100 150 200 150 100 50 0 50 100 150 Pictorial representation of the way In which the moving ground-pattern of reflected signals is plotted by three receivers in triangulation. The time delay between the arrival of various parts of the fading pattern at the two aerials will depend on the speed of the move ment. The direction and speed of the motion can be determined by locating three aerials at the corners of a triangle, and then employing triangulation principles. The ground pattern moves twice as fast as the ionosphere irregularities which give rise to it. The new ionospheric-winds equipment is located at the Bureau's radio-propagation field station at Sterling, Virginia. It consists of a transmitter operating at 2.3 Mc/s and three receiving aerials and their associated receivers. The transmitter produces 200-micro- second pulses, with a peak power of 10 kW, at a rate of 60 per second. The receivers are gated to respond only to those pulses that have been singly reflected from the ionosphere. "i 1 1 1 1 1 1 1 1 1 1 1—1—1—r Q 1 1 i—r—i—r?5T" bb^ ¥ »V8%^^4H>J^. ; STERLING, VIRGINIA ft * EST .Q_J—1—I—I__I 1 1 1 1 1 1 O <SR 1—1—1—1—m—1—1—1—1—1—r • JULY 17, 1950 O JULY 18, 1950 A JULY 19, 1950 -i I I I L 1 ' 6 ' ' ^ 00 r Q 4 oy^^+'l-^*^ o°~ 2-yZ-t, *Jb S,, ' CAMBRIDGE, ENGLAND GMT I—I—I—I—I I I I I I I I ''''' • • J I I I I L 8 10 12 14 HOURS 16 18 20 22 24
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