radar astronomy


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radar astronomy,

application of radarradar,
system or technique for detecting the position, movement, and nature of a remote object by means of radio waves reflected from its surface. Although most radar units use microwave frequencies, the principle of radar is not confined to any particular frequency range.
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 to the determination of distances and planetary features within the solar system, such as rotation rates. A short burst of radio waves is transmitted in the direction of the object under study. The object reflects the radio waves back to earth, where they are detected by the same antenna that sent the signal. The time between sending the signal and receiving the "echo" can be precisely measured electronically. Since radio waves travel with the speed of light, the roundtrip distance from the earth to the object and back is then easily computed. This technique differs from radio astronomyradio astronomy,
study of celestial bodies by means of the electromagnetic radio frequency waves they emit and absorb naturally. Radio Telescopes

Radio waves emanating from celestial bodies are received by specially constructed antennas, called radio telescopes,
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 in that the celestial object is here merely a passive reflector, rather than the actual source of the emission. The first yield of radar astronomy was a much improved value for the distance from the earth to the moon. Using more powerful transmitters, the distances to Venus and Mercury were also measured, as well as the planets' rotational periods and gross surface properties. Even greater precision is obtained by replacing the radio transmitter with a laserlaser
[acronym for light amplification by stimulated emission of radiation], device for the creation, amplification, and transmission of a narrow, intense beam of coherent light. The laser is sometimes referred to as an optical maser.
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. During the Apollo project, special reflectors were installed on the moon; subsequently, by bouncing laser light off the moon the distance from the earth to the moon could be determined within centimeters. Radar observations are also useful for asteroids and comets whose orbits take them relatively near the earth. Much of the surface of Venus has been mapped by unmanned probes using radar altimeters to penetrate the cloud cover.

radar astronomy

(ray -dar) The study of celestial bodies (as yet) within the Solar System by means of the faint reflections from them of powerful high-frequency radio transmissions aimed in their directions from the Earth. Some of the very large radio dishes such as the 305-meter dish at Arecibo, are equipped for radar work. The time interval between transmission of a signal and reception of the reflected signal is an accurate measure of intervening distance. The determination of planetary distances by this method led to a precise value for the astronomical unit. The transmitted frequency may be chosen to penetrate the atmosphere of a planet in order to establish a profile of the surface, which is otherwise hidden from view. In addition, rotation periods can be determined from the doppler shift produced in pulses bounced off different parts of an object. The rotation periods of Venus and Mercury were first determined by radar measurements.

Radar Astronomy

 

a branch of astronomy in which the bodies of the solar system are studied with the help of radio waves sent out from transmitters and reflected by these bodies. The methods of radar astronomy are used for the solution of problems in astrometry and astrophysics.

The use of radar has enabled the distance to celestial bodies to be measured by means of the time required for a radio signal to reach the object and return. Since the accuracy of these measurements (<1 km) significantly exceeds that of range finding based on astrometric observations, radar measurements are used for refining the values of fundamental astronomical constants, the parameters of motion of the bodies of the solar system, and the bodies’ dimensions. Radar astronomy increases the accuracy of probes sent to the nearest planets and makes possible a precise landing of space vehicles at specified regions on the planets’ surfaces.

The distance to point O of a planet’s surface (Figure 1), which is measured by radar and is the point closest to the earth, in combination with the distance to the planet’s center of mass C, the position of which may be determined from the laws of celestial mechanics, allows the distance between O and the center to be computed and consequently the height of O above a certain mean surface.

Figure 1. Lines of equal return (1) and equal Doppler shift (2) on the surface of a planet: (PP′) axis of rotation, (O) center of the disk, (C) center of mass, and (B) and (B′) discriminated regions on the planet’s surface

In radar measurements taken during the passage of a planet behind the sun, a delay has been detected in the reception of the echo signal. This is caused by a decrease in the speed of propagation of electromagnetic waves in the gravitational field of the sun and is in accordance with Einstein’s theory as it relates to gravitational phenomena. The detection of this effect served as one of the experimental tests of the general theory of relativity.

The solution of many astrophysical problems by radar astronomy is based on the study of the shift and broadening of spectral lines in the echo signal caused by the Doppler effect, which in turn is caused by the motion, relative to an observer, of the object reflecting the radio signals. This method is used to study the motion of meteors in the earth’s atmosphere, the motion of ionized regions in the sun’s corona, and the rotation of planets. A major achievement of radar astronomy has been the determination of the period and direction of rotation of Venus and Mercury.

The high penetrating capability of radio waves has enabled scientists to overcome the dense cloud layer of Venus, which is opaque to visible wavelengths, and obtain information about its surface. Measurements of the intensities of reflected signals, which depend on the magnitude of the reflection coefficient of the surface material, have shown that the surface of Venus is similar in its electrical properties to silicate-bearing rocks, which are widely distributed on the earth. A bright spot is observed at the center of Venus’ disk, while the edges are lost in shadow, as in a sphere with a smooth, mirrorlike surface. This phenomenon also appears at radio wavelengths for other planets with solid surfaces but not at visible wavelengths. Jupiter and Saturn, which have thick gaseous envelopes, do not provide a noticeable reflection. The rings of Saturn, however, prove to be good reflectors and scatter radio waves in the same way as clouds scatter visible light.

A method has been worked out in radar astronomy for determining the terrain of a planet’s surface by discriminating from the total reflected echo signals those parts that correspond to small portions of the planet’s surface. This method is based on the analysis of the intensity distribution in the echo signals with respect to the arrival time at the receiver and to the Doppler frequency shift. The signal’s return time and its frequency shift depend on the distance to the particular portion of the planet’s surface and on the radial velocity of this portion relative to the radar antenna; these values change from point to point in a regular manner. From Figure 1, we can see that all points lying on circle 1, which is in a plane perpendicular to the line of sight, are equidistant from the antenna. The circle therefore appears as a line of signals of equal return time. Points lying on circle 2, which is in a plane parallel to both the line of sight and the planet’s axis of rotation PP’, have the same radial velocity relative to the antenna. This circle appears as a line of signals having an equal Doppler shift. On the basis of the known motion of the planet, the return time and Doppler shift for the points on circles 1 and 2 can be calculated. These values can then be used to discriminate from the total signals the signals that are reflected by the portion of the surface near point B, which lies at the intersection of the circles, and to measure the intensity of these signals. The discrimination of signals reflected by the points B and B’, for which the distance and radial velocity are identical, is accomplished by means of the spatial selectivity of the antenna or by a radio interferometer.

The map of a section of the moon’s surface obtained through such radar techniques at the Massachusetts Institute of Technology (MIT) is found to differ little in quality from an actual photograph taken from the earth with the aid of an optical telescope. The reflected signal was received simultaneously by two antennas, and by measuring the phase differences in these signals, the deviation at each point on the lunar surface from a certain mean surface could be measured. On the MIT map, depressions in the surface appear dark and elevations appear light. The application of this method is especially promising for Venus, since its surface is inaccessible to direct photography. As of 1974, an image had been obtained of a small portion of Venus’ surface, on which craters could be seen.

Whereas from the moon and planets radio waves are reflected from the solid surfaces of these bodies, echo signals received from the sun are reflected by ionized gas in the sun’s corona. With the help of radar, formations have been observed in the corona moving with speeds of up to 200 km/sec, both toward and away from the sun’s center. When radar is used with meteors, the radio signal is reflected from the extended ionized trail, which arises upon the entrance of the meteor particles into the earth’s atmosphere.

Radar measurement of meteors and the moon was begun in the 1940’s. The first echo signals from the sun’s corona were obtained in 1959 in the United States, and the first ones from Venus, in 1961 in the USSR, the USA, and Great Britain. The major difficulty in radar observation derives from the proportional decrease in the intensity of the received signals with an increase in the distance (to the fourth power), which confines radar observation to the solar system.

REFERENCES

Kotel’nikov, V. A. [et al.]. “Uspekhi planetnoi radiolokatsii.” Priroda, 1964, no. 9.
Shapiro, I. “Radiolokatsionnye nabliudeniia planet.” Uspekhi fizicheskikh nauk, 1969, vol. 99, issue 2. (Article translated from English.)
Dubinskii, B. A., and V. I. Slysh. Radioastronomiia. Moscow, 1973.
Radar Astronomy. Edited by J. V. Evans. New York, 1968.

B. A. DUBINSKII and O. N. RZHIGA

radar astronomy

[′rā‚där ə′strän·ə·mē]
(astronomy)
The study of astronomical bodies and the earth's atmosphere by means of radar pulse techniques, including tracking of meteors and the reflection of radar pulses from the moon and the planets.
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