Hacker News Reader

Comment by 0cf8612b2e1e | Hacker News Reader

It used to be a joke that satellites could read your license plate from space. Then I see commercial images like this and I am less sure.

View on HN

This is a SAR satellite which uses radar to produce altitude map. I thought the resolution would be bad but it is really good at up to 16cm. This isn't going to read anything since it isn't a camera.

The other thing is that commercial satellites are lower resolution than military ones. The giant ones are super expensive, and commercial users don't need to read license plates. They would much rather have multiple satellites that visit same spot every day.

You don't need a camera to read license plates since the lettering is embossed. You absolutely could read license plates in SAR images, just not in current space borne systems.

Checkout this paper to see what's possible with ground- and airborne SAR systems (scihub is your friend): https://ieeexplore.ieee.org/abstract/document/7461591

Your wavelength would have to be small compared to the size of the embossing. At that point you're getting into the infrared range.

Sure, some call it Infrared, some call it terahertz radar.

The linked paper talks about 300 GHz SAR, which is generally the lower limit of infrared.

"Those satellite photographs -- the landsat photographs -- are so darn good that when they re fully enhanced by computer, we can actually tell how high the waves are out in the middle of the Pacific; we can tell what the temperature of the ocean is 20 feet below.." - Grace Hopper in 1982 NSA lecture. YT link: https://www.youtube.com/watch?v=si9iqF5uTFk&t=1612s

I bet current spy satellite will be doing a lot things we can hardly imagine, may be we will know in 40-50 years down the line.

There are a few open-literature papers already on how a submarine's wake might show up in analysis of surface patterns. Doesn't seem hard to imagine that the classified literature is a few steps ahead.

We got a hint of how much better spy satellites are during Trump's presidency.

https://www.npr.org/2022/11/18/1137474748/trump-tweeted-an-i... / https://www.npr.org/2019/08/30/755994591/president-trump-twe...

Folks figured out the satellite that took it, too; a 2011 KH-11: https://www.npr.org/2019/09/02/756673481/amateurs-identify-u...

Just look at the image: https://x.com/realDonaldTrump/status/1167493371973255170/pho...

You can see lamp posts and fenceposts. Fenceposts are literally < 2" thick! So the resolution must be on the order of 2cm.

- " Fenceposts are literally < 2" thick! So the resolution must be on the order of 2cm."

That's not quite how it works. If you apply a Gaussian blur on the scale of 20 cm to a 2 cm imaged object, it will persist in some form, if the contrast ratio is very high. That doesn't mean you have 2 cm resolution. Spatial resolution is rather different: it asks something more like, can you distinguish *two* objects at a 2 cm separation distance? Can you distinguish the case there two such separated objects, from that where there is only one?

That's closer to what you need to answer the question "how small text can be read?"

(Some of the visible stars in the sky, by the way, are ridiculously small (angular size) compared to the human eye resolution—there's no contradiction there either! The angular diameter of (for instance) Rigel is smaller than 1/10,000th the resolution of a human eye!)

That's a point that far too few people in the remote sensing industry understand!!

Put another way, try measuring the width of small objects in the scene. You'll find there's a minimum width things will appear regardless of how small they actually are. Small, high-contrast objects will be visible, but will be wider than they actually are. Measuring the width of small bright objects is one way of estimating the spatial resolution (i.e. FWHM of the PSF) of optical imagery.

And with that said, those fenceposts are not that thin. They're likely on the order of 10cm. It's widely assumed from regulations/etc that "spy" sats can get below 10cm spatial resolution. In the US, commercial sats are not allowed to collect imagery better than 10cm spatial resolution. At the 10cm resolution point, things like atmospheric lensing due to temperature variations become major issues and need to be corrected for (e.g. that "shimmer" you see above the pavement on a hot day). That type of tech gets tightly regulated very quickly even in the US's current "let private industry image how they want" regulatory environment (used to be more restrictive not long ago).

So that imagery is likely somewhere on the order of 5cm to 10cm resolution in its native form. Which is pretty nuts. It's crazy what the NGA can do!

Just another example of impulsive and very poor judgment from Trump. Even though he had the right to declassify this image, his only reason to do so was to taunt the Iranians - Ha Ha, your rocket blew up. The Iranians likely corrected their issue and moved on whereas now our adversaries have clear evidence of our satellite capabilities and can adjust their behavior accordingly for years to come.

My father, back in the 80's, told me about satellites in the 70's that could read your license plate. Using imaging tubes before CCD technology no less. The technology limitation is not only the resolution, which was rather good btw, but rather the time. Early spy sats used film that had to be de-orbited and captured for processing. Then they started beaming down what was essentially slow scan television which only improved with time. Now you can get clusters of CCD's beaming down massive amounts of digital signals from wide swaths of area. None of this is secret, really. Just look up the Landsat program. They even used multispectral cameras to count things like tree growth and duck populations. In the 70's!

Well the Hubble is 70s tech, right?

Because Hubble was designed for in-orbit servicing by the STS, it had it's optics, CCD systems, and even the flight computers (not originally designed for in-orbit servicing!) replaced over the various servicing missions. The differences between WF/PC 1 and WFC 3 is the difference between going into space in 1990 with mid-80's hardware and going into space in 2009.

Since the 2009 Service Mission 4, Hubble tech has held steady, but until then it was regularly being upgraded to state-of-the-art.

State-of-the-art for what could survive space. It used 486's for quite a while. You are still very correct, I just think it's funny that the "best" is not always what people think.

What is released commercially is not the same thing the NRO uses.

Reply on news.ycombinator.com