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What capabilities will those (supposedly) satellites have ? read the small handwriting on the shopping list I have made through the apartment window at night ?

by 2rsf 7y ago

marsRoverDev 7y ago
Perhaps SAR with very high resolution too - allowing them to image at night and through cloud cover.
habnds 7y ago
I saw something once that explained how the powerful magnification seems silly until you account for the angles that things are at. The example they used was that license plates are vertical so to read them the satellite can't be directly above the car so it's actually a much further distance and greater magnification is necessary because of the atmosphere and the distortion of reading something at an angle.
- hwillis 7y ago
  More that the satellites are in fixed orbits that are relatively low (250 km) and moving at thousands of km per second. Even if the target is directly below them, they need to rotate quite a lot to see it for more than a few seconds at a time. If the target isn't directly below, they need to look much farther sideways than down.
  - habnds 7y ago
    most spy satellites aren't geostationary?
    - kragen 7y ago
      No.
  - ubertakter 7y ago
    Slight typo there: "thousands of km per second", you probably meant "thousands of meters per second". Low orbit is on the order of 7-8 km/s. I did find the typo humorous.
hwillis 7y ago
The picture Trump tweeted was from a satellite with a resolution of at least 4 inches. They can definitely see your cell phone, but they can't read it- at least not from space.
kragen 7y ago
Keep in mind the Airy disk limit 1.22λ/d. Unless you're doing optical interferometry between satellites, 100mm resolution from 100km altitude, 1 microradian, requires an aperture about 800'000 wavelengths across. For blue light, that's a 300-mm-wide telescope; doable, although getting the optics to be that good is a serious challenge, motion blur is another issue, and such small sensor pixels don't get many photons and can be noisy. Now if you want to get to 10mm resolution, you need a 3-meter aperture; 1mm, like you're suggesting, requires 30 meters. These are much more challenging to launch. There are some interesting things you can do with indirect light, but so far they don't help with this.

What capabilities will those (supposedly) satellites have ? read the small handwriting on the shopping list I have made through the apartment window at night ?

by 2rsf 7y ago

What capabilities will those (supposedly) satellites have ? read the small handwriting on the shopping list I have made through the apartment window at night ?

marsRoverDev 7y ago

Perhaps SAR with very high resolution too - allowing them to image at night and through cloud cover.

habnds 7y ago

I saw something once that explained how the powerful magnification seems silly until you account for the angles that things are at. The example they used was that license plates are vertical so to read them the satellite can't be directly above the car so it's actually a much further distance and greater magnification is necessary because of the atmosphere and the distortion of reading something at an angle.

hwillis 7y ago
More that the satellites are in fixed orbits that are relatively low (250 km) and moving at thousands of km per second. Even if the target is directly below them, they need to rotate quite a lot to see it for more than a few seconds at a time. If the target isn't directly below, they need to look much farther sideways than down.
- habnds 7y ago
  most spy satellites aren't geostationary?
  - kragen 7y ago
    No.
- ubertakter 7y ago
  Slight typo there: "thousands of km per second", you probably meant "thousands of meters per second". Low orbit is on the order of 7-8 km/s. I did find the typo humorous.

hwillis 7y ago

The picture Trump tweeted was from a satellite with a resolution of at least 4 inches. They can definitely see your cell phone, but they can't read it- at least not from space.

kragen 7y ago

Keep in mind the Airy disk limit 1.22λ/d. Unless you're doing optical interferometry between satellites, 100mm resolution from 100km altitude, 1 microradian, requires an aperture about 800'000 wavelengths across. For blue light, that's a 300-mm-wide telescope; doable, although getting the optics to be that good is a serious challenge, motion blur is another issue, and such small sensor pixels don't get many photons and can be noisy. Now if you want to get to 10mm resolution, you need a 3-meter aperture; 1mm, like you're suggesting, requires 30 meters. These are much more challenging to launch. There are some interesting things you can do with indirect light, but so far they don't help with this.