What measurement technique is used today to bounce radar signals off planets to refine the AU length?

Bouncing radar signals off nearby planets, like Venus

While early attempts to define the AU relied on historical optical methods like triangulation based on phenomena such as the transit of Venus, modern measurement techniques achieve far greater precision by utilizing the constant speed of light. Astronomers now bounce extremely accurate radar signals off nearby terrestrial planets, such as Venus or Mars. By timing the round-trip travel time of these signals with high accuracy, astronomers establish a fixed baseline distance. Since the speed of light is known precisely, this radar baseline allows for a highly refined calculation of the Earth's orbit, directly improving the accuracy of the AU value.