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Just look at the image: https://x.com/realDonaldTrump/status/1167493371973255170/photo/1 https://x.com/realDonaldTrump/status/1167493371973255170/pho... You ca

Just look at the image: https://x.com/realDonaldTrump/status/1167493371973255170/photo/1 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.

perihelions 2y ago
- " 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!)
- jofer 2y ago
  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!

perihelions 2y ago

- " 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!)

jofer 2y ago
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!