<P> The maximum range of astronomy by radar is very limited, and is confined to the Solar System . This is because the signal strength drops off very steeply with distance to the target, the small fraction of incident flux that is reflected by the target, and the limited strength of transmitters . The distance to which the radar can detect an object is proportional to the square root of the object's size, due to the one - over-distance - to - the - fourth dependence of echo strength . Radar could detect something ~ 1 km across a large fraction of an AU away, but at 8 - 10 AU, the distance to Saturn, we need targets at least hundreds of kilometers wide . It is also necessary to have a relatively good ephemeris of the target before observing it . </P> <P> The Moon is comparatively close and was detected by radar soon after the invention of the technique in 1946 . Measurements included surface roughness and later mapping of shadowed regions near the poles . </P> <P> The next easiest target is Venus . This was a target of great scientific value, since it could provide an unambiguous way to measure the size of the astronomical unit, which was needed for the nascent field of interplanetary spacecraft . In addition such technical prowess had great public relations value, and was an excellent demonstration to funding agencies . So there was considerable pressure to squeeze a scientific result from weak and noisy data, which was accomplished by heavy post-processing of the results, utilizing the expected value to tell where to look . This led to early claims (from Lincoln Laboratory, Jodrell Bank, and Vladimir A. Kotelnikov of the USSR) which are now known to be incorrect . All of these agreed with each other and the conventional value of AU at the time . </P> <P> The first unambiguous detection of Venus was made by Jet Propulsion Laboratory (JPL) on 10 March 1961 . The Jet Propulsion Laboratory established contact with the planet Venus using a planetary radar system from 10 March to 10 May 1961 . Using both velocity and range data, a new value of 149 598 500 km was determined for the astronomical unit . Once the correct value was known, other groups found echos in their archived data that agreed with these results . </P>

Why did scientists use radio telescopes to bounce radar pulses off of venus