[jcims]

Why does it seem that press releases and public statements like this tend to overstate things, to the point of fallacy, in order to make a definitive point?

If you took a SAR image of my home you would absolutely see how many vehicles are in my garage.

Some examples from Sandia labs SAR image gallery - https://www.sandia.gov/radar/imagery/index.html

(I believe these are aerial vs. orbital but the technology is the same)

Ku band SAR

Airport Historical Site - Can see parked vehicles right through the roof of the building on the left - https://www.sandia.gov/radar/_assets/images/gallery/ku-band-...

Ka band SAR

Golf Course - Can see right through the roof of the clubhouse and golfers (eg. human bodies) on the greens - https://www.sandia.gov/radar/_assets/images/gallery/ka-band-...

[enriquto]

I don't think that you are interpreting these images correctly (but the screenshots do not indicate the viewing direction, so it's not your fault). The objects that you see "through the roof" may be in front of the building, not inside. Radar imaging is based on the return time of a chirp to the antenna. If two objects appear on the same pixel in the image it means that they are at the same distance to the antenna. For example, if the sensor is at a (common) elevation of 45 degrees, a 10m vertical wall will seem to cover the 10m of ground next to it, and any object there will be superimposed with the roof of the building.

EDIT: I have some professional experience with the interpretation of satellite SAR imagery.

[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.

[rmrfstar]

They should open up a rate-limited retail API that lets us verify for ourselves what is visible in our life and what is not. Otherwise we have to outsource our thinking to others who may have a conflict of interest.

[notahacker]

Many high value commercial applications of ultra high resolution SAR only involve small areas of land and irregular periods

Ultimately satellite providers don't have any more obligation to subsidise access to their services to people wishing to verify for themselves that the product doesn't contradict the laws of physics than 5G providers or microchip providers.

[rmrfstar]

Complex systems produce behavior that cannot be derived from first principles, which is why it is important to study the actual behavior of those systems. I studied E&M for four semesters at college, and it is far from clear to me that "the laws of physics" foreclose the possibility of this system unearthing new personal information about my patterns of life.

You might want to question your own confidence once in a while. Unless you independently predicted rowhammer, spectre, and meltdown.

[jcims]

I like this idea. There's already sales channels for this, they would just be another imaging provider.

[jcims]

You're right I probably blew it with that one. I found the building on google maps and there's no cars there.

BUT! What do you think about this one? - https://news.ycombinator.com/item?id=25482504

[jcrawfordor]

I have the local knowledge to tell you that there aren't parked vehicles in or near that part of the building. You can even see in aerials that it's that it's a lawn with walking paths and a prefab outbuilding. I think this SAR image may be from before the outbuilding was there, and historical imagery shows that it used to just be an extension of the lawn. I think the bright objects are a distorted reflection from elements of the building facade, perhaps. I don't know, I'm squinting at the SAR image having a hard time deciding if it depicts the outbuilding or not, and it perhaps says something about the limitations of SAR images that it's not clear to the untrained eye whether or not an entire building is there. I think it's possible the building is there and is just made of materials with very low reflectivity, there are sort of "ghost lines" that seem in the right place for its edges.

[jcims]

You're right. Here's the back of the building - https://i.imgur.com/JRcer7d.jpg

Why am I worried about SAR when Google is ten times creepier, lol.

Could definitely be some kind of artifact

[StavrosK]

I don't understand what you think is shown "through" the roof of the clubhouse (or cars, for that matter). That's just the roof:

https://goo.gl/maps/bsSQjADvWuynPLvq6

[jcims]

I just started to doubt myself but you saved me lol. OK take a look at the front of that building (found on flickr)

https://i.imgur.com/cDF8Fsr.jpg

Now take a look at this map image, and go back and forth to see if you feel that the X i marked is a reasonable approximation of the photographer's location

https://i.imgur.com/fvj0ZPj.jpg

Now take a look again at the SAR image and tell me if you can see any artifacts from the front of the building through the roof (hint, zoom in and look for the curved line of bright dots :) )

https://www.sandia.gov/radar/_assets/images/gallery/ka-band-...

[StavrosK]

Not really, the building windows are octagonal (you can see that they're straight) and the dots curve.

The brick wall at the front, though, has the exact same curvature as the dots, dead-on.

[CapriciousCptl]

I don't follow. Can you edit the ka band image and circle what you're seeing?

[jcims]

This part - https://i.imgur.com/CmtzOES.png

But now that I have a better understanding of the layover effect I think it might just be that the satellite is looking from where it looks like the sun is shining, and it's directly imaging that front window frame and brick wall. Need our SAR pro to comment :)

[Firerouge]

Interestingly it does seem to see right through the awning that's to the right of the roof.

There appears to be a couple tables that are hiden from Google's satellite view but clearly visible to the bottom right of the building in the ka band SAR image.

[StavrosK]

The SAR photo was probably taken before an awning was installed.

[Firerouge]

I thought that might of been the case as well, but based on Google Earth Pro's historic satellite data, it's been there since 2009.

[StavrosK]

That's odd, but you can see in the SAR photo that there isn't even an outline of the awning, nor the golf carts under it, nor the columns that hold the awning up, but there are trees which aren't there in the photos from the ground.

[Firerouge]

It certainly looks like you're right.

While I can't find the image online earlier than 2010, there's an ieee research paper titled TanDEM-X for High-Resolution SAR Interferometry from 2007, so it certainly could predate the awning.

[technick]

How long has this witchcraft been around?

[studius]

> Why does it seem that press releases and public statements like this tend to overstate things, to the point of fallacy, in order to make a definitive point?

While in my mind, things were better with Fairness Doctrine and Equal-time Rule restrictions, I think we could solve this in the US by greater tax-funded federal, state, and local government-funded media, where the government has no control over that media aside from that of regulation to promote fair and equal representation within that media.

Basically, put PBS, state, and local programming into a streamed source similar to Netflix/Hulu/Prime, produce content with quality surpassing other streaming services, and give the public air time both via meritocratic decision by randomly selected electorate as well as via lottery and availability. The BBC might be a good model to emulate also.

At that point, there would at least be an alternative to B.S. journalism and bad media behavior reinforced by viewership and advertising dollars.

[derstander]

> If you took a SAR image of my home you would absolutely see how many vehicles are in my garage.

Unless your garage was made of materials that are transparent to the frequency band of the radar or the collection geometry enabled the radar to image through a door or large windows, this is not true.

[craftinator]

Look into the physics of how this technology works. If you do that, and take the time to grok it, you'll have the information to understand that this type of radar cannot penetrate any of the materials that buildings are made.

[andi999]

10 Ghz is used in ground and wall penetrating radar

[jcims]

That's the disconnect here I think.

SAR is a technique for building up a two dimensional image by rasterizing one dimensional signal reflection data. Any type of radiant energy would presumably support SAR techniques, from sonar to x rays, although the 'R' in SAR presumably limits it to radio frequency range (~30Hz-300GHz). Some of which, as I mentioned originally, absolutely would see through buildings.

My point is that the headline and article are trying to make the case that 'SAR can't see through buildings', when it should be 'Why it looks like our satellites are seeing through buildings' because that is all that is explained.

[andi999]

Yes. And actually I wouldnt be surprised if their satellites actually can see through buildings (because the frequency band they use should allow that). Maybe just need to adjust the color-scale (or switch to logarithmic if it is not). I mean 10GHz SAR is capable: https://www.kurzweilai.net/seeing-through-walls-in-real-time (of course SNR might be too low for satellite, but this is nowhere written)

[nitrogen]

There's that old video of a police helicopter tracking someone who shined (shone?) ^W^W pointed a green laser pointer at them, even through buildings. Was that SAR?