NASA's planetary radar captures detailed view of oblong asteroid
phys.org
phys.org
A 70 meter dish can emit and focus a powerful signal but probably can't outshine the Sun at these distances. Radar emissions also consist of short pulses while the Sun emissions are continuous.
Since the article mentions precise distance information being acquired, the radar system was able to detect it's own emissions (likely only from the unlit regions)
This is related to the periodic outages of geostationary satellites when the Sun is directly behind them. Ground receivers are essentially "jammed" by the microwaves emitted by the Sun itself.
Our eyes interpret it as a top–down view, or as a face–on view with lighting from one side, but it’s really more like a slice through the middle. Or rather, like a bunch of slices through the middle all stacked on top of each other, since the beam probably isn’t all _that_ narrow after traveling for a few million miles.
I wish I could find the paper though; press releases so often don’t bother linking to them. Maybe it hasn’t even been published yet? The observations were just 19 days ago.
The radar imaging process is complex, but suffice to say the shown "image" is not in physical coordinates. If it's a conventional radar image, the vertical coordinate of the shown image is "delay" (distance from observer). And also conventionally, the horizontal image coordinate is "doppler", which is the doppler shift given to the returned signal by the rotating asteroid.
So, stuff on the left side of the image was moving away from the observer, and stuff on the right side was moving towards the observer. And of course the brightness is essentially the "amount of stuff" at that delay-doppler locus.
The reason we can't plot an "image" in physical coordinates, and have to be content with the altered coordinates, is that all we get from the returned radar carrier signal is a delay, and a doppler shift. That's it - "delay-doppler" coordinates.
So any set of sites on the asteroid surface with the same distance and the same relative velocity (w/r/t the observer) will be binned into the same place in the radar image. There is no guarantee that these sites are near each other, and for complex geometries (rough asteroids), they often will not be.
If you want to get a real image in physical coordinates, you have a separate inversion problem to solve, and you'll probably need more images and some model constraints.
For much more, see this paper (http://mel.ess.ucla.edu/jlm/publications/Ostro02.AsteroidsII...), and in particular, see around Fig. 1, Fig. 4 (especially), and Fig. 6.
Delay-Doppler for planar surfaces, like a remote sensing radar zipping along Earth’s surface, is pretty straightforward, as you note…and we get to set up the system parameters, like beam width, pulse rate and ground speed, so it works out nicely.
Can you comment at all why a technique like Inverse Synthetic Aperture Radar (ISAR) is not used? That relies on the rotation of an object to generate cross-range resolution through sampling a diverse set of aspect angles (and is certainly useful for non-uniform, non-planar surfaces). If the rotation rates of the asteroids are known, then that minimizes one of the main challenges in forming quality ISAR imagery. For the use-cases I'm familiar with, we need to estimate the rotational motion because vehicles do unfortunate things like accelerate and turn while we're trying to look at them -- the nerve! And ISAR has certainly existed before the paper you linked in another comment was written (and also before the previous study the paper itself references).
As a side note, that paper by Ostro et al. is very interesting to me; it's like being familiar with Leibniz's notation for calculus and seeing something written using Newton's notation (or vice-versa). I skimmed the references and all of the ones I saw seemed to be from astronomy / astrophysics sources. It's almost as though we have two fields using similar methodologies to look at different objects that don't seem to talk at all and have developed different dialects.
Edit: maybe delay-Doppler imaging is akin to ISAR, like medical tomography and SAR were shown to be mathematically related?
ISAR sounds like its operating principle is the same as the radar imaging technique used for these asteroid results. In general, it’s all radar imaging so you get reflectances in delay/Doppler coordinates as your observable.
One possible difference is the poor SNR of the asteroid problem — you have to average many radar images to beat down the noise. The velocity of the scattering elements on the asteroid is assumed to be the same across all these images (as far as I know). (After correcting for a known offset due to the Earth’s changing motion.)
This seems to be a little different than the ISAR “swaying boat” type of application in which the body is indeed accelerating and your radar image must be adjusted for that, or else the mast of the swaying boat will smear across the image.
It may also be worth saying that there is no synthetic aperture in the OP - it’s a physical aperture.
From the perspective of the radar receiver it's just receiving a series of reflections over a period of time. These are processed for the above measures and then perspective transformed to show an "overhead" image that we see here.
But the other (left/right) coordinate is doppler, which does not map 1:1 to a physical location -- doppler will depend on the rotation and geometry of the asteroid.
The radar beam is not "scanning" across the object. The asteroid is too small to focus the beam on a part of it -- the beam illuminates the whole object. We get lucky that the object is spinning, so that different surfaces on the asteroid cast energy into different doppler offsets.
Anyway, the returned signal is then binned into (delay, doppler) coordinates. This is repeated for many separate pings to beat down the receiver noise.
During the observation window (series of pings), you have to compensate for the relative motion of the earth and the centroid of the target, because the relative velocity (zero-point of the doppler coordinate) is changing the whole time.
<https://en.wikipedia.org/wiki/Goldstone_Solar_System_Radar>
It seems possible to me that for a near-earth object it might be possible for a bistatic image to show a perspective difference between the side nearest the transmission and receiving antennae.
Though at five times Earth-Lunar distance (about 2 million km), any such parallax baseline would be minimal (13 thousand km).
The postprocessing explanation offered elsewhere seems far more likely.
I went looking for some details on how the picture was produced on the NASA page linked at the bottom of the article but I can't find much technical info on the process. However it was done I'm sure it's a cool technique!
Yes, that's right: they are using doppler shift of the rotating asteroid, and the delay of the propagating signal, to form the "image". See my other comment nearby.
(I've watched too many sci-fi movies, I'm just really curious)
So it would be equivalent to a pretty big nuke. More or less a city-killer. No effects more major than that, though.
https://www.visualcapitalist.com/wp-content/uploads/2020/08/...
Pessimist: "It's a space grenade"