Satellite photos of license plates?

[Jinx]

As I recall, in the 1980’s, there was talk of “spy satellites” having such grandeous hi-tech capabilites to actually photograph car license plates. I WAG maybe this was, at best, a way to express the high resolution of the cameras into practical, layman’s terms? I mean, unless license plates are on car roofs, how could this really be true?

What’s your WAG?
-Jinx

[Duck_Duck_Goose]

Don’t need a WAG when I’ve got the Washington Post on the job.

http://www.washingtonpost.com/ac2/wp-dyn?pagename=article&node=&contentId=A33942-2000Jul1

A LOOK AT . . . Spy Satellites & Hollywood
By Dwayne A. Day
Sunday, July 2, 2000;

In the ridiculous “Shadow Conspiracy,” the evil NSA guys find White House adviser Sheen by reading the license plate of his car. But remember: Satellites take their best images while directly overhead. Therefore, they would have a hard time seeing license plates from above. Think about it: If you stood on a very tall ladder on the roof of your car, could you read its license plate?

Even if a satellite had the right angle on the license plate, it wouldn’t be able to make out the letters and numbers. The best resolution of an American spy satellite, achieved by an older series no longer in use, was reputed to be about 2 1/2 inches. This means that the smallest visible object would be the size of a baseball, not the thin letters and numbers on a license plate. And smoke, haze, smog or clouds would all reduce the quality of the resolution, as would the distance required to see the license plate from an angle. Memo to future filmmakers: License plates cannot be read from satellites.

[Squink]

A 1.8 meter (70") mirror can theoretically resolve two points of (green) light that are separated by 1.54X10[sup]-5[/sup] degrees. At a spy satellite height of 250 km (150 m) this works out to a resolution of 0.0675 meters, or ~3". That’s not good enough to read license plates.
Atmospheric turbulence will always make the actual performance of a satellite mirror worse than the above theoretical number. Turbulance isn’t as big a problem for scopes looking down, rather than up, but it’s doubtful that it will allow satellites to get down to the ~1/2" range of resolution needed to make out license plate numbers.

http://www.hia.nrc.ca/moffatt/eng/whybig/whybig.html

Home | Physics & Astronomy

[El_Kabong]

My tiny little brain would explode if I tried to make the calculations, but surely the fact that the satellite is whizzing overhead at several thousand miles per hour relative to the ground has some effect on resolution as well; i.e. blurring of the subject. Seems one would need an incredibly high shutter (or CCD capture) speed to eliminate this effect entirely.

[CurtC]

El_Kabong, a satellite wouldn’t need very high shutter speed. Think of taking a picture of a sign while you’re driving by in your car. If you just held the camera in a fixed postion (w/ respect to the car), the image would be blurred. But if you were holding the image in the same spot in the viewfinder, you could have a relatively long exposure time and the image would be sharp.

[Balthisar]

*Originally posted by El_Kabong *
**My tiny little brain would explode if I tried to make the calculations, but surely the fact that the satellite is whizzing overhead at several thousand miles per hour relative to the ground has some effect on resolution as well; i.e. blurring of the subject. Seems one would need an incredibly high shutter (or CCD capture) speed to eliminate this effect entirely. **

Or maybe the satellites are geostationary? Then they don’t move relative to the ground.

[CurtC]

The only way that a satellite can be geostationary is for it to be 24000 miles up. Since low-earth orbit is just a few hundred miles, satellites can take much clearer pictures in LEO.

[cmburns]

Maybe not a satellite, but what about a SR-71, or Prowler or other type of unmanned spy plane? My feeling is if it’s flying unseen over my head taking pictures, it doesn’t make much difference if it’s a satellite or a plane.

Anyone know the best resolution for a flying spy platform, whatever form it’s in?

[Squink]

A 70" mirror on an SR-71 flying at 80,000 feet (15miles) would theoretically have a resolution ten times better than that of a 70" mirror on a satellite 150 miles out -(see my previous post).So it’d be able to resolve two points 0.3" apart. Which is likely good enough to read a license plate. Turbulant movement of the aircraft would make that resolution impossible to achieve in practice.

Most Spy satellites are in low earth N-S orbits. There is no problem with blurring due to the satellites velocity because blurring is caused by large changes in visual angle with respect to time, not actual speed. You can verify this for yourself by looking out the window of a speeding car. Objects up close are blurred. Those further off are not.