The radio receiver is much like the radiometers used in radio astronomy, and it should be able to withstand the very large signal that the transmitter puts into it without damage or impairment of sensitivity.
III METEOR ASTRONOMY
Quite simple radar equipment has been used to measure the reflections from the ionized trails formed as meteoroids enter and burn up in the upper atmosphere. Such trails reflect best when they are in a direction perpendicular to the beam of radio waves from the antenna. Observers can find the direction from which the meteor has come, and, by observing the way the echo from the meteor trail grows with time as the meteor enters the antenna beam, they can also measure the velocity of the meteor.
IV SOLAR SYSTEM ASTRONOMY
In tracking objects within the solar system, the transmitter sends out a series of pulses of radio energy, or sometimes a suitably modulated train, or sequence, of radio waves. The modulation characteristics of the transmitted signal and those of the signal returned from the object being studied are compared to obtain the travel time of the signal to and from the object. This time can be accurately measured to a few millionths of a second. Thus, by using the known velocity of radio waves, the distance to the object can be found.
The wavelength of the returned signal is different from that of the signal sent out, because the object being studied is moving towards or away from the Earth. This wavelength difference is carefully measured because, by applying the Doppler effect, the difference can be used to find the velocity of the object relative to the Earth. If the object is rotating, the signals returned from various parts of it will be changed in wavelength for that reason. The spread of wavelengths in the returned signal is thus used to measure the rotation rate of the object in relation to Earth. If such measurements are made over a few months, the direction of the object from Earth will have changed, and the direction in which the object is rotating about its axis can be determined. Another method of signal processing, especially with space-probe data, allows maps to be made of the radio reflectivity of the Moon or a planet. By selecting signals over suitable range and wavelength-change regions, the entire surface of a planet can be mapped (see Venus). One of the most impressive examples of this technique was the US space probe Magellan, which went into a polar orbit around Venus and, between 1990 and 1994, used synthetic aperture radar (making use of the spacecraft’s change of position between sending out and receiving the radar waves to increase image resolution) to produce a global map of the planet in three dimensions and at high resolution for the first time. A similar mission has been proposed for the early 21st century to map both the surface morphology and the depth of ice on Jupiter’s moon Europa.
Many interesting and valuable experiments in physics have became possible through the use of radio astronomy. For example, a radar signal passing through the gravitational field of the Sun should be bent if the general theory of relativity is correct. In the late 1970s, Viking spacecraft sent towards Mars were equipped with transponders for receiving and retransmitting radar signals from Earth to test this effect. By 1980 they had provided values that were within 0.002 per cent of those predicted by general relativity.
"Radar Astronomy",Microsoft® Encarta® Encyclopedia 2001. © 1993-2000 Microsoft Corporation. All rights reserved.
