276 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



at Venus. The pulse sequence lasted for the entire 5-niinute round- 

 trip travel time, so the last pulse was transmitted just before the echo 

 of the first was due. Then the antenna output was switched over to 

 feed the low-noise receiver, whose output was recorded on tape and 

 later processed in a digital computer. Each such 10-minute operation 

 constituted a "run." 



The processing had two purposes. First, the individual echoes 

 were much too weak to be distinguished from the background noise, 

 so it was necessary to add together all of the several thousand received 

 echoes-plus-noise to build up the signal-to-noise ratio. Since the 

 echoes have a more or less fixed structure, and the noise is different 

 from pulse to pulse, the former add up faster than the noise. 



A second function of the signal-processing equipment was to deter- 

 mine the correct value of the planet's distance. The transmitted 

 sequence of pulses was deliberately made nonperiodic, since otherwise 

 it would be impossible to tell which received pulse corresponded to 

 a particular transmitted one. By matching up the outgoing and 

 returning patterns, no ambiguities in time of travel will remain. 

 This matching is too lengthy a job for the computer to do wliile the 

 observations are in progress, so in the 1958 experiment the received 

 signals were recorded for later treatment. In the second Venus ex- 

 periment, a digital computer (pi. 3, fig. 2) located at the radar site 

 was programed to do part of the processmg during each actual run. 



At the 1959 Venus conjunction, an experiment similar to this was 

 carried out by J. V. Evans at Jodrell Bank in England. Our labora- 

 tory's 1958 work had produced four valid runs, of which two con- 

 tained large-output signals agreeing in range. Since it was thought 

 that the 25-million-mile distance to Venus had been measured to 

 better than 250 miles, this implied that the solar parallax had been 

 redetermined to within 1 part in 100,000. Over 150 rims were 

 made during the 1959 Lincoln Laboratory effort, yet no echoes as 

 strong as those of 1958 were observed, either in England or America, 

 though the former group did get weak indications for a distance 

 consistent with the solar parallax determined in the 1958 experiment. 



It is difficult to explain the disparity between the results obtained 

 at the two Venus conjunctions. Our current feeling is that the planet's 

 reflectivity may be highly variable with time, and that the two suc- 

 cesses in 1958 were observations made on very favorable occasions. 



SOLAR SYSTEM DISTANCES 



Astronomers, in specifying the mean distance of the earth from the 

 sun, ordinarily speak of the corresponding solar parallax — the angular 

 radius of the earth as seen from the center of the sun. Several pro- 

 posed values of the solar parallax, with their probable errors, are 



