What is the physical geometry of this apparent "eclipse" of a tiny moon of Asteroid Florence? Below is a small GIF made from a small subset of images contained in the original 36MB GIF found at https://cneos.jpl.nasa.gov/images/news/florence.p5us.1Hz.s382.sep01.gif as I just found in the Sky and Telescope article Asteroid Florence Has Two Moons.

Although it's tempting to interpret the image as an optical image, I belive this is pulsed-doppler data from the Goldstone Deep Space Network center. I am guessing that one axis is range (delay) and the other is doppler shift. 

Still I can't figure out the geometry that would produce an "eclipse" (in this case the moon passing behind the larger asteroid) with it disappearing so "far" from the asteroid.

Could someone help me out here? Is one axis delay (time) and the other doppler shift? One can start with this description but there is more to it than is discussed there.

below: "A radar image shows asteroid 3122 Florence and tiny echoes from its two moons. Here is an animation that shows them more clearly. The direction of the radar illumination (and thus the direction toward Earth) is at the top." From here. NASA / Jet Propulsion Laboratory. This is a small subset of the frames contained in the original 36 MB GIF, and the size has been decreased by a factor of 2 in order to fit in SE's 2 MB limit.
The vertical axis is signal delay, and therefore distance from the radar antenna; this is more or less true spatial information. As the caption indicates, delay is increasing from top to bottom in the image.

The horizontal axis is frequency shift (due to the Doppler effect). To the left are signals with a lower frequency relative to what was sent from the radar antenna, i.e. reflections from points moving away from the antenna; to the right, signals with a higher frequency, coming from points moving towards it.¹ This axis is more difficult to interpret.

For a single rigid body, which Florence presumably is, the only way it can have points moving in different directions at the same time is by rotating. On one edge of the body, points that have just come into view of the radar are moving quickly towards it. This relative movement slows down as the points approach the centre of the radar-visible disk; then, they move away increasingly fast until they disappear from view on the opposite edge. (The speed change isn’t linear, which means the body will be somewhat warped on the radar image.)

Only this effect allows the radar to distinguish between points that are at the same distance. If the asteroid wasn’t rotating at all, or if we happened to look exactly onto one of its poles, it would appear only as a thin vertical line in the image. For a rotating body, note that the radar cannot tell to which direction the rotation is: A mirror asteroid rotating the other way would look exactly the same to it.

Now for the moons. They are not rigidly connected to Florence, and apparently not tidally locked to it either, so they will not follow its rotation. With that, the starting point “moving in different directions at the same time = rotation” which worked on the main asteroid breaks down. When one of the moons is moving towards the radar, that tells us exactly nothing about its relative position to Florence; the visualization is misleading in that respect. Therefore, it is absolutely possible for a moon to be straight behind Florence even when it appears to its side in this particular visualization.

Emily Lakdawalla has a nice blog post on this topic, with animated drawings: How radio telescopes get “images” of asteroids



¹ After correcting for the movement of Florence – or the whole Florence-and-moons system – relative to Earth, of course.