Janice Breskow, age 15, of Bowling Green, Ohio, for her question:
HAS RADAR ASTRONOMY BEEN USED A LONG TIME?
Radar astronomy is a branch of astronomy in which celestial objects are studied by projecting radio waves against them and then analyzing the echoes. The first radar astronomical observations were made in 1946 when radio echoes were returned from the moon. Since the first observations, larger antennas and more powerful radio transmitters have become available. With better equipment, successful radar studies have been made of the planets Mercury, Venus and Mars as well as the corona of the sun.
The equipment used in earth based radar astronomy consists of a large single beam antenna (usually a large, steerable, parabolic reflector), a sensitive radio receiver and a data recording and data processing system.
The transmitter used in radar astronomy is capable of generating tremendous radio power. Average powers as much as 1 million watts have been used. Similar but smaller antennas are used on space probes.
Data recording and processing often require a large digital computer connected directly to the output of the radio receiver while observations are being made. 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.
Quite simple radar equipment has been used to measure the reflections from the ionized trails formed as meteors enter and burn up in the upper atmosphere. Such trails reflect best when they are in a direction perpendicular to the beam of radio wave 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.
In tracking objects within the solar system, the transmitter sends out a series of pulses of radio energy, or sometimes a suitable 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 which the radio waves have traveled, 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 toward or away from earth.
A radar signal passing through the gravitational field of the sun should slow down if the general theory of relativity is correct. In the late 1970s, Viking spacecraft sent toward Mars were equipped with transponders for receiving and retransmitting radar signals from earth to test this theoretical effect. By 1980 they had provided values that were within 0.002 percent of those predicted by general relativity.