[RachelF]

You are correct, the "light" is the transmitted radar pulse from Goldstone, not from the sun.

They do all sorts of fancy signal processing to get this sort of resolution.

Because radar power received goes down with 1/(distance^4), the inverse square law, squared, this is hard to do at astronomical distances.

The power level differential between transmitted and received power can be in the order of ~10^15.

Goldstone transmits a 500kW radio pulse but will probably get nanowatts back, which is amplified millions of times by the big dish, amplifiers and signal processing.