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Comment by excerionsforte | Hacker News Reader

Lol. Articles like this remind me of Silicon Valley where Pied Piper app is built for engineers rather than "Regular people": https://www.youtube.com/watch?v=QYBcLMiR9b0

and then Richard going in to the focus group trying to explain it science-speak rather than ELI5.

It seemed like this article's intended audience is regular people and it missed the mark. First question I had was... what is SAR?

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Synthetic Aperture Radar. What you're really seeing here is what happens when you try to put together a 3D image from an angled line scan from a moving scanner. There isn't enough information to do this perfectly, so you get artifacts on tall objects. You can see somewhat similar artifacts in Google Earth when looking down on tall objects for which they don't have full elevation data, like trees.

What's impressive is that they're getting 50cm resolution from orbit. Doing this from aircraft is nothing new; that's been going on for decades. But 50cm from a satellite? That's an achievement.

I read it is 50cm because it is the legal (not technical) limit.

In some sense achieving high resolution imagery from orbit is simpler in that SAR requires motion compensation on the order of a wavelength. Planes are subject to all sorts of motion like turbulence whereas a satellite’s motion is often far more predictable. Closing the link budget can certainly be more challenging given longer ranges and size, weight, power, and cooling constraints for satellites though.

Reading that long article where the writer didn't have awareness that they should define that term reminds me of reading in grad school. Please define the main term in your articles, people! Geeze.

Consider a simple radar as capable of locating objects in 3D spherical coordinates- range, azimuth, and elevation. Radar is typically really good at measuring distance, but because of the physics of wavelengths and antennas, it needs a relatively huge aperture to achieve good angular resolution.

SAR technique create a virtual or synthetic antenna by imaging coherently over a path that draws out the synthetic aperture.

Of course, time continues as the the platform (satellite in this case) is moving. SAR then requires precise spatial/temporal awareness to combine the returns from the ground into a coherent image.

It is similar to long exposure in photography leading to seeing dim objects far away.

In both cases, if objects in the imaged scene are moving during the scanning period, they become blurred in the final result.

Radar is good at ranging. So you can locate things precisely in that dimension. It’s not great at cross-range though because a beam is typically wide. SAR gets around this by essentially using the flight path or orbit of a radar sensor to synthesize a longer antenna than you’ve got. You make some assumptions that can be violated to varying degrees that may degrade the image (you need coherence over the synthetic aperture, you assume things you’re imaging don’t move, etc.)

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