In order to complement the Mariner mission to Venus, the radar experiments were repeated from October to December, 1962 (during the Mariner mission), using improved equipment and refined techniques. As in 1961, the experiments were directed by W. K. Victor and R. Stevens.
The 1961 experiments used two 85-foot antennas, one transmitting 13 kilowatts of power at 2,388 megacycles, the other receiving the return signal after the round trip to and from Venus. The most important result was the refinement of the astronomical unit—the mean distance from the Earth to the Sun—to a value of 92,956,200 ±300 miles.
Around 1910, the astronomical unit, plotted by classical optical methods, was uncertain to 250,000 miles. Before the introduction of radar astronomy techniques such as those used at Goldstone, scientists believed that the astronomical unit was known to within 60,000 miles, but even this factor of uncertainty would be intolerable for planetary exploration.
In radar astronomy, the transit time of a radio signal moving at the speed of light (186,000 miles per second) is measured as it travels to a planet and back. In conjunction with the angular measurement techniques used by earlier investigators, this method permits a more precise calculation of the astronomical unit.
Optical and radar measurements of the astronomical unit differ by 50,000 miles. Further refinement of both techniques should lessen the discrepancy between the two values.
The 1961 tests also established that Venus rotates at a very slow rate, possibly keeping the same face toward the Sun at all times. The reflection coefficient of the planet was estimated at 12%, a bright value similar to that of the Earth and contrasted with the Moon’s 2%. The average dielectric constant (conductivity factor) of the surface material seemed to be close to that of sand or dust, and the surface was reported to be rough at a wavelength of 6 inches.
The surface roughness was confirmed in 1962. Since it is known that a rough surface will scatter a signal, the radar tests were observed for such indications. Venus had a scattering effect on the radar waves similar to the Moon’s, probably establishing the roughness of the Venusian surface as more or less similar to the lunar terrain.
Some of the most interesting work was done in reference to the rotation rate of Venus. A radar signal will spread in frequency on return from a target planet that is rotating and rough enough to reflect from a considerable area of its surface. The spread of 5 to 10 cycles per second noted on the Venus echo would suggest a very slow rotation rate, perhaps keeping the same face toward the Sun, or possibly even in a retrograde direction, opposite to the Earth’s.
In the Goldstone 1962 experiments, Venus was in effect divided into observation zones and the doppler effect or change in the returned signal from these zones was studied. The rate of rotation was derived from three months of sampling with this radar mapping technique. Also, the clear, sharp tone characteristic of the transmitted radar signal was altered on return from Venus into a fuzzy, indistinct sound. This effect seemed to confirm the slow retrograde rotation (as compared with the Earth) indicated by the radar mapping and frequency change method.
In addition to these methods of deducing the slow rotation rate, two other phenomena seemed to verify it: a slowly fluctuating signal strength, and the apparent progression of a bright radar spot across from the center of Venus toward the outside edge.