SOLAR SYSTEM AND RADAR — GREEN AND PETTENGILL 275 



approach that has been used at rehitively low frequencies (30-60 mega- 

 cycles) is the Stanford solar radar installation, shown in plate 1. 



For the major part of the spectrum available to radar astronomy 

 (substantially the same region of interest in radio astronomy), how- 

 ever, the choice seems to favor a parabolic reflector illuminated by a 

 relatively simple antenna located at its focus. An instance of a very 

 large paraboloid under construction is seen in plate 2. 



To a certain extent, the value of an antenna of given size may be 

 improved by operation at a higher frequency (shorter wavelength). 

 As the wavelength is shortened, the reflecting paraboloid forms a nar- 

 rower beam, concentrating more of the transmitted energy on the 

 target. But the dimensional accuracy of the antenna and its mount 

 must be proportionately greater. Furthermore, above 10,000 mega- 

 cycles absorption in the earth's lower atmosphere becomes important. 

 And in some cases, as the sun, the reflection properties of the target 

 tend to place an upper limit on useful frequencies. 



Although much work may be carried on with the simple displays 

 that conventional radars use — such as oscilloscopes and cameras — a 

 digital computer of some sort is required for more advanced signal 

 processing. When the signal-to-noise ratio of the desired return 

 falls below unity, special processing is necessary to extend the detec- 

 tion sensitivity. But even where sufficient signal is available, com- 

 puters are needed if techniques such as those described for lunar map- 

 ping are to be attempted. 



Finally, a continued effort is required to reduce receiver noise tem- 

 peratures, a problem discussed by F. D. Drake in Sky and Telescope 

 for December 1959, page 87. Masers and variable-reactance amplifiers 

 appear promising, and in the future may be improved so much that the 

 residual noise level will be limited only by the background tempera- 

 ture of the sky or, sometimes, the target. This theoretical limit is 

 already near at hand in some cases, although the availability of reliable 

 amplifiers using these principles at all interesting frequencies is still 

 limited. 



THE VENUS EXPERIMENTS 



Initial astronomical use of such a device came in February 1958, 

 when Lincoln Laboratory employed a 440-megacycle solid-state maser 

 (pi. 3, fig. 1) in its first Venus observations, described on page 384 

 of the May 1959 issue of Sky and Telescope. When the experiment 

 was repeated, at the next inferior conjunction in September 1959, a 

 parametric amplifier was used. In both cases the background noise 

 at the receiver input was kept down to 170° Kelvin. 



The 440-megacycle transmitter sent a sequence of several thousand 

 pulses, each 0.002 second long and several hundred kilowatts in peak 

 power, into the 84-foot Millstone Hill antenna dish, which was pointed 



