Radar Guns Primed For Asteroid Spin Trap

... of the "rotating target" variety?

Back in the '60s I was working on a project involving using synthetic aperture radar to resolve rotating objects (including NON-rotating objects that were orbiting or flying by - giving you a viewpoint from progressively different directions, which is equivalent.)

Reflection of an illuminating "chirp" (long signal with a constant rate-of-change-of-frequency) gives you a return with a frequency shift that tells you the distance to each feature of the target, added to a doppler-shift term telling you how fast it's approaching or receeding. Coherently combining results from a series of chirps lets you separate the two, giving you distance from the unchanging part and cross-range position from the change. Shazam: A two-dimensional map of the object's radar reflectivity, from a viewpoint over the axis of (apparent) rotation. Right-angle inside folds and corners (and some other concave features) retro-reflect like roadside reflectors, too.

We were working on something that could resolve details of a small satellite in orbit, as if you were looking down on it and seeing every corner, fold, and "ding" as if it had a marker light attached. (The project didn't get followed-on and later I figured out why: A picture leaked during the first shuttle flight when they had lost some tiles and were worried about reentry problems: The spooks had a ground-based telescope camera that could read a license plate from that distance. B-b )

Such techniques can resolve objects to amazingly fine detail. Because phase information is preserved the solution is "analytical" - you can resolve far below the wavelength of the illuminating signal. Your resolution limit is more related to the accuracy of your signal data collection and stability of your electronics than illuminating wavelength (actually - frequency breadth of the "chirp").

The basic technique would give you a map as if you were hovering over the object on its axis of (apparent) rotation. You'd have the "northern" and "southern" hemisphere combined - but if the object's (apparent) rotation axis isn't aligned so you're exactly in the plane of its equator, continued observation through more than one rotation would let you separate the two.

Also: You only need ONE antenna, not a long-baseline array of them. The effective "baseline" is the length of the apparent path of the antenna, while it illuminated a given point, as viewed from the surface of the target. This would be FAR larger than the diameter of the earth - essentially the half-circumference of an "orbit" with the distance from the antenna to the target as the radius , i.e. pi * distance to the rock if it makes at least half a turn (from our viewpoint) during the flyby.

This was the sort of thing we could do in the '60s, with rudimentary equipment by today's standards. With half a century of technological advancement I'd expect much better results. B-)

So I hope they're applying this technique, or some improvement of it, to this flyby.

Source: http://rss.slashdot.org/~r/Slashdot/slashdotScience/~3/8vkiTDWD4RI/story01.htm

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