How radio telescopes get "images" of asteroids

Every time I post a radio telescope image of a near-Earth asteroid, I get at least one reader question asking me to explain how radio telescopes take photos, so I'm hereby writing a post explaining the basics of how delay-Doppler imaging works.

To begin with, imaging of any kind done with radio telescopes (or radio antennae on spacecraft) is an active technique: the imaging requires that the antenna first broadcast a signal at the object of interest. The signal reflects from the object, and the antenna waits for the return signal.

The simplest sort of radio "imaging," then, is just radio ranging. Send out a ping, wait for the echo. Use a precise clock to time how long it takes the reflection to return to the antenna, and you know very precisely the range or distance to the target. That's RADAR, which is an acronym for Radio Detection and Ranging....


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[link to www.planetary.org]