Venus has often been called Earth’s sister planet, mainly because the two are roughly the same size. And there the similarity ends. One significant difference is that Venus is covered by an opaque cloud layer that does not allow direct visualization of its surface. For this reason, early probes making flybys of Venus didn’t even carry cameras. In 1961, using Earth-based radar scientists were for the first time able to peer through the clouds and obtain very low resolution “images” of the surface of Venus. By the mid-1970s technology had improved so that Earth-based radars could provide surface resolution down to about 10 kilometers, but only of limited areas of the planet. The next step was to place a small radar instrument into orbit around Venus – what it lacked in size it more than made up for in proximity to the surface.
The first spacecraft to study Venus using radar was Pioneer Venus Orbiter, managed by the NASA Ames Research Center in California’s Silicon Valley, which entered a polar orbit around the planet in December 1978. Over the next 14 years, among other observations it mapped approximately 90% of Venus at a resolution of about 10 kilometers. Pioneer Venus found the planet to be almost perfectly spherical, unlike Earth which has a measurable bulge at the equator. Also, more than half of Venus’ surface is within 500 meters of the mean surface height. The radar imagery found two large continents, Ishtar Terra roughly the size of the United States and Aphrodite Terra about the size of Africa, as well as large-scale tectonic features such as rift valleys and volcanic calderas.
As good as the results returned by Pioneer Venus Orbiter were, scientists yearned for higher resolution radar imagery. The Soviet Union obliged by launching two spacecraft carrying Synthetic Aperture Radar (SAR) instruments with large antennas. Venera 15 and 16 entered polar orbits around Venus in October 1983 and during their eight-month missions mapped about a quarter of the planet’s surface to a resolution of 1 to 2 kilometers. During this time, NASA was planning its own mission called the Venus Radar Mapper, later renamed Magellan, with the capability to map the planet down to a resolution of 120 meters using SAR. Magellan’s prelaunch goal was to map up to 70% of the planet during one 243-day imaging period, equivalent to one Venusian “day.” The Jet Propulsion Laboratory in Pasadena, California, managed the mission.
Source: https://solarsystem.nasa.gov/news/930/30-years-ago-magellan-off-to-map-venus/