from Hacker News

No, SAR Can't See Through Buildings

by Shelnutt2 on 12/19/20, 8:21 PM with 107 comments

• by molticrystal on 12/20/20, 3:58 AM

  If you do want something that can look inside a building and determine the positions of people and objects, look up wifi slam [0] which was acquired by apple. It can use the wifi signals to map out the inside of a building and determine the location of objects inside. About a decade later, thanks to neural networks and other techniques, they can use wifi to determine pose information, they can create a rough skeletal model of a person that moves as you move. [1] The 2020 enhancements are a decent upgrade. [2]

  [0] https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=wifi...

  [1] http://rfpose.csail.mit.edu/

  [2] https://cse.buffalo.edu/~lusu/papers/MobiCom2020.pdf

• by supernova87a on 12/19/20, 10:16 PM

  "... It helps to think of it just as your brain’s interpretation of a two dimensional representation of the coherent sum of backscatter responses from electromagnetic waves..."

  Well that clears it right up, thanks! Lol.

  I'm pleased that they think any general public person or even journalist is capable of seeing the difference between the left and right figure in the article.

• by jcims on 12/19/20, 9:39 PM

  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-...

• by elil17 on 12/19/20, 9:38 PM

  Actually, some specially designed SAR can see through buildings, but not the kind of SAR that’s in publicly available remote sensing data.

  One example (of many): https://www.kurzweilai.net/seeing-through-walls-in-real-time

• by danaliv on 12/19/20, 10:58 PM

  I don’t understand this at all. Is there a side-by-side comparison of radar and visible wavelength somewhere? Or maybe a version of the geometric diagrams with example objects and the imagery they’d produce?

  EDIT: Ok, after Googling this a little I think I get it. The skyscrapers are upside-down. (I think?) The radar is measuring slant-range distance, and due to the viewing angle, the tops of the skyscrapers are closer to the radar than the bases. So the tops of the buildings are closer to the bottom of the image.

  Since the skyscrapers are in the “wrong” place in the image, they get blended with ground features that are in the “right” place.

  Is that right?

• by chrisbolt on 12/19/20, 9:30 PM

  SAR = Synthetic Aperture Radar

• by JohnJamesRambo on 12/19/20, 10:20 PM

  They didn't follow the first rule of writing something. I don't know what SAR is.

  Synthetic-aperture radar (SAR)?

• by rmrfstar on 12/19/20, 9:20 PM

  Does anyone have a white paper about corner reflectors and X-band SAR?

  For what it's worth, the state very likely needs a warrant to use technologies capable of looking inside structures [1]. Not that the law applies to them, but hypothetically.

  [1] https://en.wikipedia.org/wiki/Kyllo_v._United_States

• by andi999 on 12/20/20, 4:06 AM

  Do I believe that. Not really. Frequency is around 9,7GHz which is the upper limit of ground and wall penetrating radar systems. Of course you cannot look through metal or conductive structures, but why not thin plastic tents or thin walls? For this we need to know the sensors sensitivity. Of course imaging gets a bit messier, since the imageing algorithm is based on the assumption that you only get an entry reflex (so basically it is 2D or lets call it 2,5D imaging). For full 3D you also need to scan/move orthogonal to the first satelite direction.

• by bitminer on 12/20/20, 4:02 AM

  There is a lot marketing bullshit in the Capella articles.

  For example, the Singapore image of buildings and other detections by the SAR is clearly overlaid on an optical image with trees and bushes and respective shadows. Trees and bushes are invisible to X-band (10GHz) SAR. It has to be an optical image underlaying the radar data.

  Look at it. The detections by SAR are bright white. Most of the image is grey-scale showing background items.

  It is not advertised as such. As an interpretable image, perhaps this is an improvement over notoriously hard-to-understand monochrome SAR imagery. BUT! It is not described as such. Hence my subjective evaluation as "bullshit".

  [0] https://ichef.bbci.co.uk/news/976/cpsprodpb/C172/production/...

  [1] https://www.bbc.com/news/science-environment-55326441

• by mikewarot on 12/20/20, 3:27 PM

  I know better than to worry about seeing through buildings. What I want to do is to explore the data, and see how the land rises and falls with the tides, the rain levels, temperature, etc. There have to be all sorts of new and beautiful images and movies to be made from this data. I look forward to getting my hands on it eventually.

• by mattlondon on 12/19/20, 9:17 PM

  Does anyone have any links to full-resoluton examples from Capella?

  All of the ones I have seen so far appear to be resized for use in their blog/press releases ... or are those images as high-res and as sharp as they get?

• by IshKebab on 12/20/20, 10:43 AM

  Wow that was a terrible explanation. As far as I can figure out, it is only 1D SAR - that is the X axis is spatial, but the Y axis is actually time. If you fire radio waves at the ground at an angle, and the ground is flat, then you can easily map time to position. But buildings and hills mess up that mapping. I always assumed SAR used 2D arrays.

  Again, really terrible explanation. It feels like they really didn't want to explain it at all.

• by the__alchemist on 12/20/20, 1:23 AM

  Anecdote you may or may not find relevant: SAR maps are used regularly by fighter aircraft (And have been for decades) to generate high-precision coordinates (Absolute or relative) that can be used to target weapons. They do this from relatively far away. Most 4th gen+ fighter radars are capable of doing this in hardware and software.

  The layover effect at the center of this article is due to the map being of skyscrapers.

• by ganstyles on 12/20/20, 6:00 AM

  I've heard of c band sar, and x band sar. Is x band higher resolution? I understand it has properties where x sees through fewer things, but is x fundamentally better resolution or something? If not wouldn't higher wavelength be better for seeing through and resolving objects?

• by Itsdijital on 12/19/20, 11:36 PM

  Can someone explain what the advantage of this is over a typical optical visible range satellite.

• by fnord77 on 12/20/20, 2:57 AM

  no where on the page or the site do they explain what "SAR" stands for.

  https://en.wikipedia.org/wiki/Synthetic-aperture_radar

• by fourthark on 12/20/20, 11:39 AM

  The blow-up picture doesn’t make sense either - hardly any difference in zoom and the little circle contains only a small part of the image in the big circle.

  Do they understand how these diagrams are supposed to work?

• by fithisux on 12/20/20, 10:19 AM

  Given this opportunity, anyone has any good tutorial on the mathematics of SAR image formation from electromagnetic scattering ??

  Thanks

• by zyxzevn on 12/20/20, 1:21 AM

  But SAR can see changes after earth-quakes

  https://maps.disasters.nasa.gov/arcgis/apps/MapSeries/index....

  So it can track your mum

• by arcticfox on 12/19/20, 9:26 PM

  Can someone edit in a comma to the HN title?

  "No, SAR Can't See Through Buildings" is the real title, which is completely different than the current (awkward) "No SAR Can't See Through Buildings"

• by 14 on 12/19/20, 10:20 PM

  It will be scary the day they can track every human movement at all times. But I think it is coming. One side and I think well we would know what happened to Michael Dunahee, a child that went missing when I was a kid, being able to reverse the video of what happened. But also it wasn’t that long ago that Canada punished a man for being gay so this is a very dangerous technology if it can track humans. Privacy is critical to a free society.