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herocomplex
2002-Oct-30, 08:04 PM
Hello everyone, I'm a major lurker on this board...just never had much to say I guess. /phpBB/images/smiles/icon_smile.gif

I'm wondering if there is a theoretical maximum on resolution capabilities of a "spy" satelite, or if it's just a matter of magnification? For instance would it ever be possible for a satelite in orbit to read a newspaper on the ground, or is there a fundamental limit based on the wavelength of light or other fancy-sounding terms?

Thanks everyone...this board is an endless source of entertainment. I should have my language translation software for decoding the HUb'-bot's code available soon...I'll let you all know.

Zathras
2002-Oct-30, 08:28 PM
This issue concerns the Rayleigh criterion for resolution. The Rayleigh criterion states that the best possible angular resolution of a lens is given by:

1.22 lambda/d

where lambda is the wavelength of light, d is the diameter of the lens, and the angle is given in radians.

To read a newspaper, let us say the satelite is in near earth orbit, ~250 miles up=~400,000m. You would need an angular resolution of .001m/400,000m (assuming you need a 1mm resolution to read the paper) or 2.5e-9 radians. In order to resolve this you would need a lens of
d=1.22*5e-7m/2.5e-9
or d=~2.4e2 m or 240m wide lens!
So not likely.

<font size=-1>[ This Message was edited by: Zathras on 2002-10-30 17:20 ]</font>

Russ
2002-Oct-30, 08:37 PM
I can't say much about what is possible now but I can tell you what was possible in 1974.

I was in Viet Nam and periodicly got intellegence photos for my missions that show the target area. One time we got a photo that showed a man sitting in a captured jeep. He was looking up at the sky and you could see the rank insignia on his collar. When we got to the target we could recognize him among the captives.

I would assume it has gotten better since then.

herocomplex
2002-Oct-30, 09:19 PM
Thanks for the equations. I thought there was something to do with the wavelength of light. I ran the math for the 1m resolution photos that are released to the public and it results in .244m lens. What a huge difference. Any idea on what the maximum resolution of current satellites might be? Or perhaps the maximum sized lens that could be put in orbit? Are there other factors that might affect resolution besides lens diameter (computers, digital imaging, etc).

I really have no idea why I'm on this topic today. I have a very intelligent roommate that leans towards conspiracy theories and hearing him say "Well everyone knows the govt. has spy satellites that can read your newspaper" is just begging for a rebuttal. /phpBB/images/smiles/icon_smile.gif

I originally found this board after seeing an apollo hoax site and finding it pretty convincing. I went looking for the other side and it took about 5 minutes of research before I realized how incredibly stupid it was. Since then I've developed an immense amount of pleasure watching pseudo-science get utterly demolished. Keep up the good fight guys.

CJSF
2002-Oct-30, 09:27 PM
On 2002-10-30 15:37, Russ wrote:
I can't say much about what is possible now but I can tell you what was possible in 1974.

...

I would assume it has gotten better since then.

I know you say you know what you saw, but would bet is isn't from a satellite. It perhaps COULD have been an airphoto.

The satellite would have to be HUGE to have the equipment necessary to get that kind of resolution from space. Just the lens or mirror would have to be hundreds of meters in diameter.

CJSF

Chuck
2002-Oct-31, 12:18 AM
So they can't tell my blood type by looking at a mosquito bite on my arm?

Donnie B.
2002-Oct-31, 12:26 AM
Only if you scratch it and spread the blood around a little... /phpBB/images/smiles/icon_smile.gif

Jetmech0417
2002-Oct-31, 07:25 PM
What if you get the mosquito before she can fly away?

Colt
2002-Oct-31, 08:39 PM
The Guy was probably looking up at the FAC taking the photo. /phpBB/images/smiles/icon_smile.gif -Colt

Edit: FAC= Forward Air Controller, small airplanes used to orient ground forces and for recon./phpBB/images/smiles/icon_biggrin.gif
_________________
"I'm afraid. I'm afraid, Dave. Dave, my mind is going. I can feel it. I can feel it. My mind is going. There is no question about it. I can feel it. I can feel it. I can feel it. I'm a...fraid." http://www.geocities.com/wandererofthewastes/index.html

<font size=-1>[ This Message was edited by: Colt on 2002-10-31 15:39 ]</font>

Babtoo
2007-Apr-02, 12:42 AM
I was researching stuff on the web and came across this post. The Rayleigh Criterion, is quite well done, however as the devil's advocate, a smaller lens could be used if you considered UV "light" say in the region of 10nm, then you're calculation works out to a more managable 4.88 metres.

Now don't ask me if or how you can convert UV into a visible image (I don't work in area 51:) ) but thought I'd toss this out as an idea to consider.

cjl
2007-Apr-02, 01:17 AM
The biggest problem with that is that the atmosphere isn't especially transparent to UV, especially when compared to visible light (which is lucky for us, or we'd all have terrible sunburns 24/7)

Squink
2007-Apr-02, 03:57 AM
Hubbles 2.4 meter mirror achieves a resolution of about 0.04 arcseconds. That's equivalent to 58 mm at a distance of 300 km. The spy satellites are roughly the same size as Hubble, but they're looking down through a turbulent atmosphere.

KH-11 (http://en.wikipedia.org/wiki/KH-11) has a 2.3 meter primary.

According to an estimate by the private Federation of American Scientists (FAS), three satellites operated by the U.S. National Reconnaissance Office (NRO) have resolutions as sharp as 10 centimeters (3.93 inches) -- in other words, the satellites can discern a softball-sized object from several hundred miles away.Spy Satellites: Still a Few Steps Ahead (http://www.space.com/news/gov_imagery_990921.html)

djellison
2007-Apr-02, 08:20 AM
The similarity there between the size of the Hubble mirror and the KH-11 mirror is not coincidental :)

Doug

Ronald Brak
2007-Apr-02, 08:25 AM
Sounds like someone unloaded a cruddy mirror left over from a spy satellite program on NASA.

hhEb09'1
2007-Apr-02, 09:47 AM
Sounds like someone unloaded a cruddy mirror left over from a spy satellite program on NASA.No, they were making so many of them that they got sloppy :)

RGClark
2007-Apr-02, 09:43 PM
It is possible that the new "negative refractive index" materials will allow imaging beyond the diffraction limit in astronomy. If so, the size of the lens or mirror required will be dramatically smaller than is the case now.
These "superlenses" have beat the diffraction limit in microscopy where the lens can be put very close to the imaged objects in experiments. But it is not proven they can be used in astronomy where the lens will be far from the imaged object.

New superlens opens door to nanoscale optical imaging and high-density optoelectronic devices.
By Sarah Yang, Media Relations | 21 April 2005
"BERKELEY – A group of scientists at the University of California, Berkeley, is giving new relevance to the term "sharper image" by creating a superlens that can overcome a limitation in physics that has historically constrained the resolution of optical images."
...
"In the long run, this line of research could lead to even higher resolution imaging for distant objects, the researchers said. This includes more detailed views of other planets as well as of human movement through surveillance satellites."
http://www.berkeley.edu/news/media/releases/2005/04/21_superlens.shtml

Bob Clark

joema
2007-Apr-03, 01:45 PM
...is a theoretical maximum on resolution capabilities of a "spy" satellite...would it ever be possible for a satellite in orbit to read a newspaper on the ground, or is there a fundamental limit based on the wavelength of light or other fancy-sounding terms?...

Yes there's a fundamental limit, based on currently-understood optical physics.

Under perfect conditions and assuming lowest possible perigee, current optical spy satellites might resolve about 1.5 to 2 inches. This assumes perfect adaptive optics to cancel atmospheric distortion.

The typical resolution is probably worse than this because the perigee isn't normally that low, the atmosphere isn't perfectly clear and adaptive optics don't work perfectly. They definitely can't read a newspaper or license plate, but under ideal conditions might crudely identify a person by body shape.

Unlike the movies, optical spy satellites don't hover overhead in geostationary positions. They typically operate in very low orbits, thus are only overhead a couple of times per day for maybe 20 minutes per pass. That plus cloud cover is the real limitation.

Details: KH-11/12 optical spy satellites are somewhat larger and heavier than the Hubble Space Telescope. They are about 4.5 meters in diameter, 15 meters long, and weigh about 18 tons (36,000 pounds, or 16,300 kg). About 14,000 pounds of this is maneuvering propellant.

The optics are thought to be about 2.5 meters in diameter, roughly the size of Hubble's primary mirror. They likely have adaptive optics of some type.

The orbit perigee is variously reported as 95 miles (153 km) or 170 miles (274 km). Given the fuel capacity, it's likely they can maneuver substantially.

http://www.fas.org/spp/military/program/imint/kh-12.htm
http://www.astronautix.com/craft/impystal.htm

From this we can calculate the theoretical linear resolution. Angular resolution at 500 nanometers is:

resolution = 250000 * wavelength / diameter, or 0.05 arc seconds

Linear resolution at distances of 153 and 274 km is given by:

res = tan (angle) * distance, or 3.7 cm (1.4 in) at 153 km and 6.6 cm (2.6 in) at 274 km.

The Lacrosse/Onyx radar-based recon sats use synthetic aperture radar to image though clouds. Their resolution might be about 1 meter:

http://www.fas.org/spp/military/program/imint/xlacrosse.htm

Practical use of negative refractive index optics to exceed diffraction-limited resolution is a new development. Current use is for microscopes. Whether it's applicable to telescopes is being researched. Discussion: http://www.bautforum.com/showthread.php?t=54814

NEOWatcher
2007-Apr-03, 04:33 PM
Yes there's a fundamental limit, based on currently-understood optical physics.
Great post, but I couldn't resist the opening for this...

[Sarcastic puppet mode]
Then why not move the Hubble to the ISS?
You say:
"They typically operate in very low orbits."
So its clear that Hubble can too!
"spy satellites are somewhat larger and heavier than the Hubble Space Telescope." AND "About 14,000 pounds of this is maneuvering propellant." AND Given the fuel capacity, it's likely they can maneuver substantially."
So if that can be done with these telescopes, then there is no issue that can't be easily resolved with HST.
[/Sarcastic puppet mode]
Oh by the way...I saved you from bolding and colors.:D

pghnative
2007-Apr-03, 05:00 PM
No, they were making so many of them that they got sloppy :)But sloppy in a very precise way, right?

mantiss
2007-Apr-03, 05:03 PM
The spy satellites are roughly the same size as Hubble, but they're looking down through a turbulent atmosphere.

Pardon the naive question but surely adaptive optics can work both ways? If it is possible for ground equipment it must be feasible for space-based even if it has to be much hardier and probably less fancy, I am sure some basic AO can be used?

Doodler
2007-Apr-03, 06:42 PM
Pardon the naive question but surely adaptive optics can work both ways? If it is possible for ground equipment it must be feasible for space-based even if it has to be much hardier and probably less fancy, I am sure some basic AO can be used?

Situational, I guess. If you think about it, the bulk of your adaptive optics scopes are situated as high up as possible to minimize the atmosphere between them and space. With spysats, you don't have the luxury. If what you're looking at is 50 feet below sea level in the Netherlands (as an example), you've got all the atmosphere an astronomer could never want between you and it, and that's going to play hell on your resolution capability.

hhEb09'1
2007-Apr-03, 08:35 PM
Unlike the movies, optical spy satellites don't hover overhead in geostationary positions. They typically operate in very low orbits, thus are only overhead a couple of times per day for maybe 20 minutes per pass. That plus cloud cover is the real limitation.

Details: KH-11/12 optical spy satellites are somewhat larger and heavier than the Hubble Space Telescope.That's not really true. The KH satellites typically are in Molniya orbits, which can "hover". See The BA's article about it (http://www.badastronomy.com/bad/news/geosynch.html).

pghnative
2007-Apr-03, 08:52 PM
Situational, I guess. If you think about it, the bulk of your adaptive optics scopes are situated as high up as possible to minimize the atmosphere between them and space. With spysats, you don't have the luxury. Unless, of course, your intention is to spy on the astronomers;) .

Doodler
2007-Apr-03, 09:33 PM
Unless, of course, your intention is to spy on the astronomers;) .

Oh, I imagine that gets fun, a stargazer and a spook playing peekaboo tag through their respective 'scopes.

pghnative
2007-Apr-03, 09:39 PM
Oh, I imagine that gets fun, a stargazer and a spook playing peekaboo tag through their respective 'scopes.Each trying to blind the other, one with an artificial star, and the other with an artificial astronomer.

joema
2007-Apr-03, 10:38 PM
That's not really true. The KH satellites typically are in Molniya orbits, which can "hover". See The BA's article about it (http://www.badastronomy.com/bad/news/geosynch.html).
I don't think that's correct. A Molniya orbit is a specific type of highly elliptical orbit. A Molniya orbit has:
(1) an orbital inclination of 63.4 degrees
(2) an orbital period of 12 hours
(3) No perigee precession (orbital mechanics ensure perigee stays at the same place)

If the orbit doesn't have those exact characteristics, by definition it's not a Molniya orbit: http://en.wikipedia.org/wiki/Molniya_orbit

Molniya orbits typically have apogee in the northern hemisphere and an apogee altitude of about 40,000 km (24,850 miles).

Molniya orbits do have long "apogee dwell". This is useful for communications satellites and missile warning satellites, since they can work from high altitude. It would tremendously handicap an optical spy satellite by vastly reducing the resolution.

A KH-12 satellite's max theoretical resolution at apogee in a Molniya orbit would be reduced to 31 feet (vs about 2 inches in a low orbit). So while a Molniya orbit allows a certain amount of "hovering", it's at a very high altitude that would make an optical spy satellite almost useless.

Satellites (inc'l optical spy satellites) can be visually tracked by amateurs and the orbital elements computed. From this we know KH optical spy satellites have an orbital inclination of about 97-98 degrees (vs 63.4 degrees for a Molniya orbit), an orbital period of roughly 90 minutes (vs 12 hr for a Molniya orbit): http://www.fas.org/spp/military/program/imint/andronov.htm

It's true the KH spy satellites have somewhat elliptical orbits, but this is very mild relative to the 40,000 km apogee of a Molniya orbit.

A key purpose of a Molniya orbit is there's no perigee precession, so no manuvering fuel is required to maintain it. By contrast the KH satellites have a huge amount of manuvering fuel and likely change orbital elements quite a bit.

hhEb09'1
2007-Apr-04, 02:32 PM
I don't think that's correct. A Molniya orbit is a specific type of highly elliptical orbit.Maybe not precisely, but here is another source (http://www.spacetoday.org/Satellites/YugoWarSats.html), that says "[KH] satellites are in egg-shaped elliptical orbits ranging from a low of 175 miles to a high of 625 miles above Earth. Each passes over its assigned observation target on the ground twice a day."

I'm not sure what they mean by "egg-shaped" :)
A KH-12 satellite's max theoretical resolution at apogee in a Molniya orbit would be reduced to 31 feet (vs about 2 inches in a low orbit). What values were you using for that calculation?

swansont
2007-Apr-04, 04:56 PM
This issue concerns the Rayleigh criterion for resolution. The Rayleigh criterion states that the best possible angular resolution of a lens is given by:

1.22 lambda/d

where lambda is the wavelength of light, d is the diameter of the lens, and the angle is given in radians.

To read a newspaper, let us say the satelite is in near earth orbit, ~250 miles up=~400,000m. You would need an angular resolution of .001m/400,000m (assuming you need a 1mm resolution to read the paper) or 2.5e-9 radians. In order to resolve this you would need a lens of
d=1.22*5e-7m/2.5e-9
or d=~2.4e2 m or 240m wide lens!
So not likely.

<font size=-1>[ This Message was edited by: Zathras on 2002-10-30 17:20 ]</font>

You could, in principle, use smaller lenses that were separated by that (or greater) distance. It doesn't have to be a single lens, but at optical wavelengths I imagine you have a host of technical difficulties in making that work.

Amber Robot
2007-Apr-04, 06:09 PM
you've got all the atmosphere an astronomer could never want between you and it, and that's going to play hell on your resolution capability.

Is it though? Looking down through the atmosphere is a different game than looking up.

pghnative
2007-Apr-04, 07:01 PM
What values were you using for that calculation?It appears he was using 40,000 km, which is the value for apogee of a Molniya orbit, according to the 2nd link in the BA's article. I get 34 ft, not 31, but clearly close enough. This assumes we all agree that to either:

- hover (theoretical true geosynchronous)
- essentially hover (real-life geosynchronous where some wobble is allowed)
- psuedo-hover (Molniya orbit, which is nearly geosynchronous at apogee)

that you must be ~ 40,000 km above the earths surface, AND we all agree that joema's posted calculation for theortical resolution of visible light is correct.

pghnative
2007-Apr-04, 07:02 PM
Is it though? Looking down through the atmosphere is a different game than looking up.Why? Turbulence is turbulence when it comes to hurting resolution, isn't it?

Amber Robot
2007-Apr-04, 07:21 PM
Why? Turbulence is turbulence when it comes to hurting resolution, isn't it?

I think a lot of people assume that, but I'm not convinced this is true. I think it matters where the turbulent medium is relative to the source and the observer. Imagine looking up from the bottom of a swimming pool versus looking down at the bottom from a diving board.

A given angular scale subtends a different size portion of the atmosphere (the size of the turbulent cells being an important parameter in seeing) when you are close to the atmosphere than when you are far.

If you hold a diffuser close to your eye everything is blurry, but if you place a diffuser right against a sheet of paper you can read it perfectly.

Unfortunately, I have not been able to dig up a great description of what seeing would be like when viewing down through the atmosphere. The best thing I did find was the following paper:

"Limiting Resolution Looking Down Through the Atmosphere", by D. L. Fried, Journal of the Optical Society of America, Volume 56 (1966).

In this paper the author implies that the limiting resolution effectively becomes asymptotic at about 4.6 cm at altitudes of 10 km or higher. This paper is 40 years old though and none of the papers that reference it pertain to looking down.

I'd love to know the definitive answer to this, but I don't think it is as simple as saying the limiting resolution looking down is the same as looking up.

joema
2007-Apr-04, 11:16 PM
...[url=http://www.spacetoday.org/Satellites/YugoWarSats.html] says "[KH] satellites are in egg-shaped elliptical orbits ranging from a low of 175 miles to a high of 625 miles above Earth. Each passes over its assigned observation target on the ground twice a day."

That's correct and those are not Molniya orbits. The imaging pass is made during perigee, then the satellite swings up to a somewhat higher apogee to reduce atmospheric drag, which reduces propellant consumption to maintain the orbit. But the orbital period is typically about 90 minutes -- very fast. IOW it swings around the apogee fast, because the apogee isn't very high. By contrast a satellite in a Molniya orbit loiters up near its 40,000 km apogee for many hours.

Due to earth's rotation under the orbital track, an optical spy satellite only overflies the same point on earth's surface about twice per day, and only for a brief period each time.

The key point is it's a low orbit, hence during the imaging pass the satellite streaks rapidly overhead in about 20-30 min or less. It's as far opposite the "hovering spy satellite" movie depiction as possible.

...What values were you using for that calculation?

Angular resolution of a 2.5 meter mirror at 500 nanometers is about 0.05 arc seconds.

Linear resolutions is given by the formula:

s = tan (a) x d, where:

s = linear resolution in units determined by d
a = angular resolution in degrees
d = distance to object

Thus at the 40,000 km apogee of a Molniya orbit, linear resolution would be:

s = tan (1/(3600/.05)) x 40,000,000 meters
s = 9.62 meters, or 31.56 feet