[avianlyric]

Looking at the article, these images, a s your description of what happening. I get the impression that the imaging radar has no angular resolution, the only thing it captures is time of flight of the radar signal from chirp to return, and that this data is a very thin beam, 50 cm wide in this case.

So when the data is visualised, the perspective in the SAR image is actually 90deg offset from the imagining direction. This is done because the collected data only contains distance, rather than distance and angle (which you would get with LiDAR).

So places where building look transparent are caused by the fact that visualisation perspective is different to the imagining perspective. In an image that appears to have been taken with the camera south of a target, it was actually imaged from the north (this is a simplification). So you will always get overlapping data, because the imaging and apparent visualisation perspectives are different. And the reason for not aligning the perspectives is because there isn’t actually enough data to do that (no return or transmission angle is collected along side the distance data).

It’s important to note that I say “apparent” perspective. Because the imaging perspective doesn’t actually change, I think the perspective change is caused by our brains recognising shapes, and computing a perspective angle. But this angle is actually incorrect because the RADAR imaging process is completely different to how our eyes work.

For those still confused about this. Try thinking about how the shadows are being cast, they’re not from the sun, and likely the only thing admitting radar signals to “illuminate” objects is the satellite emitting the RADAR chirps, and also doing the imaging.

Could someone tell me if I’ve got this right?

[enriquto]

> Could someone tell me if I’ve got this right?

Yes, you got this right!

> So when the data is visualised, the perspective in the SAR image is actually 90deg

True. The extreme (and useless) case would be when the satellite is exactly in the vertical direction, then the whole image would collapse in a single point. The closer to the horizon, the best resolution you have (but then there are problems with occluding objects). As you say, there is a sort of "equivalence" between an optical image taken at an angle α from the vertical, and a radar image taken at an angle α-90.

> I get the impression that the imaging radar has no angular resolution,

True. All the antenna can do is to send a spherical wave and receive replicas of it. Conceptually, there is 0 angular resolution. In practice the beam is somewhat directed (to not waste energy), but the high resolution you see does not come from that.

[jcims]

That's the same conclusion I'm coming to. In the golf course image I think the imager is actually where the sun appears to be based on shadows and perspective. The 'evidence' I'm looking at that the imager can see through the roof may just be in open view of the imager and we're just seeing retroreflections from the corners of the window.

So i guess they take a lateral sweep of the image and let the return fill in the other axis of the image?

[enriquto]

> So i guess they take a lateral sweep of the image and let the return fill in the other axis of the image?

Exactly. Notice also that there is no "sun", these images may be taken at night or through the clouds, as far as you know. The "shadows" that you see are parts inaccessible from the radar beam itself.

[jcims]

(Sorry to drag this on, really appreciate your input)

Yep, it's starting to click. My guess would be that this (or the 180 degree opposite) is the flight path of the imager in the golf photo, basically opposite the perceived perspective of the photo:

https://i.imgur.com/MEYo12Z.jpg

So there isn't a sun, but it kind of looks like it because of how the signals accumulate or get shadowed.

This would also explain why we could see artifacts from the curved glass that appear to be 'through' the roof. The imager actually has direct line of sight to them from that flight path, and it's the layover effect from the roof/soffit that makes it appear that we are seeing through the roof.

[enriquto]

> Yep, it's starting to click. My guess would be that this (or the 180 degree opposite) is the flight path of the imager in the golf photo, basically opposite the perceived perspective of the photo

Yes! Typically SAR images are annotated with the direction of flight, otherwise they are very difficult to interpret (unless there are tress, like here).

You are right that, even if the image is acquired from the "upward" direction, it looks like a regular image taken from "below". But this correspondence is not exact when there are occlusions (typically the vertical walls of buildings). The side of the building that is visible is the one closest to the antenna, not the one closest to your imaginary point of view. Thus it looks as if it was transparent, because it is superimposed with the ground next to it.

[jcims]

Awesome, didn't expect to learn this much and definitely wouldn't have without your input. Thanks!