Wednesday, March 07, 2007

Satellite Imagery - The propagated myths

Recently I was asked to speak at BYU-I about remote sensing in agriculture and some of the new technologies that are available for farmers and ranchers. It was a good opportunity for me to do some research and see what has happened in satellite imagery and other technologies over the past couple of years. I learned a lot, and hopefully there is something here that you may not have known either.

Most of us have seen satellite imagery and been amazed at the amount of detail that can be gathered from a photo taken hundreds of kilometers above the earth. In fact, if you are like me, you have probably thought to yourself... "wow, that is pretty neat. But I bet it is nothing compared to the stuff the CIA/NSA or Russia military can get." I would even bet that you have some idea in your mind how advanced 'their' stuff is compared to the average stuff that you and I can see on Google Earth.

Well, I think there is some technology in the modern surveillance satellite that might just blow your mind. But a lot of the people that I talk with have a false sense as to what a satellite is capable of doing. This misconception can probably be placed right at the feet of Hollywood writers and production teams.

Take a look at this movie file if you want to get an idea of what Hollywood would have us all believe the NSA is doing to us today.

A quick objective look at that movie clip shows some common misconceptions about satellite technology and what is possible with surveillance. The first misconception I want to get rid of is the obvious: Full motion/real-time video feed from an orbiting satellite. This is simply not possible now... and may never be realized to the level that we see in that movie. There are some serious limitation brought upon us by the laws of physics.
  1. Imaging satellites are (for a very good reason) in sun synchronous orbits. They are moving across the sky very quickly and they could only view an area briefly during a single orbit.
  2. The bandwidth required to take high-resolution full motion (~15-30 fps) images is prohibitive.
  3. The processing/data storage/and sheer mass of force required to pull that off is not really realistic now. It may be in the future, but it is not now. To get an idea of how modern (this includes most military) satellites work, watch this quick movie clip (sorry that it is a .wmv).

The second problem I saw on the movie was the clouds. Anyone who has worked with imagery knows that visible (and infrared) light does not travel through clouds, smoke, haze, fog, or any other type of obstruction.

The last problem with that great movie clip was the satellite they were controlling. My thoughts tell me that the special effects crew looked at this satellite and tried to copy it. But that satellite uses Synthetic Aperture Radar (SAR). And it is a really cool technology, but that is not the purpose of this post. You can read about it here, and here (small spoiler to make you want to read about it... you can take "pictures" of things in the dark with SAR and even see through smoke and clouds). But that satellite is anything BUT an imaging satellite that will give you pretty color photos.

Because of movies like that one, it isn't hard to imagine a public that has crazy ideas about the ability of satellites. Here are a couple other satellite myths I want to put an end to:
  1. Satellites can see faces and recognize people. They can even read the newspaper print off your newspaper. Truth is, there is a limit to the resolution of images taken from space. It is limited by the wavelength of the light, the height of the satellite, and the size of the objective (diameter) of the telescope. The shortest wavelength of light that penetrates the atmosphere is blue/ultraviolet. The lowest that a satellite usually operates is around 200 km. And the largest telescope is probably around 250cm. The formula for the best resolution of a telescope is the " Rayleigh resolution limit here
    (arc sec) = 140 / Aperture Diameter (mm). You put all that information into the equation and you get roughly 5cm resolution. It can only resolve things that are 5cm in size... there is no way a face could be resolved with that resolution. Don't even think it will ever come close to reading the typeface off your newspaper. No wonder Osama is still running around, they can't see him.
  2. Satellites can read your license plate on your car and track you. OK, this is sort of a continuation of the last one, but this one really gets me. In order for a satellite to "read" a license plate, it would have to be looking perpendicular to it (just like we look at them). This would require the satellite to look either way in front of it, way behind it, or way to the side of it, and through all sorts of atmosphere to get a good look at a license plate. Not realistic at all.

Now a look at what a satellite can really do. It can really take nice photos with a large field of view. It can look at areas that aerial operations can not get into. These 2 capabilities make them invaluable for wartime operations because those commanding operations must know what the whole field looks like, not just little pieces available through aerial imagery. It also makes them ideal for dangerous reconnaissance where an airplane will be shot down (like Russia did to the U-2 during the cold war). Look at some specifications of these satellites below.

Here is a list of some non-spy satellites and their known resolution:

  1. Landsat (Thematic Mapper) 60 m
  2. Landsat (Multispectral Scanner) 120 m
  3. SPOT 1-2 m
  4. NOAA IR System 900 m
  5. NASA/AVHRR1 1-14 km
  6. NASA ER2/AVIRIS Airborne System 11 m

Here are the known resolutions on some spy satellites:

  1. Air force and CIA Discoverer 13 Series Spy Satellites 12 Inch (30 cm)
  2. KH-11 -1984-88 (CCD imager) 4 inch (10 cm)
  3. KH-7 -1966 (film based) 2 inch (5 cm)
Here is a list of satellites and their orbit heights (approximately) and the time it takes to orbit:

  1. Space Station : 217-226 miles (350-364 km) -- 91mins

  2. Imaging Satellites : 350-600 miles (600-1000 km) – 96-110mins

  3. GPS : 12,550 miles (20,200 km) – 11hrs 58mins

  4. Geosynchronous (GEO) (weather/communication/radio surveillance): 22,240 miles (35,786 km) -- 23hrs 56mins

A list of US spy satellites and their capabilities can be found here. So next time you see a movie that shows amazing satellite footage, just remember that you are just watching a movie. You may be able to sleep better at nights.

This is a photo of Landsat...


chartie said...

Some people will post about anything to stay out of my junk pile . . .

I don't believe anything you just said. I think you're working for "The Man" to convince me that they're not spying on me from space. Nice try, Big Brother.

Troy said...

Can I get my own private geosynchronous satellite with good enough resolution to check water for me? That's really all I want. Checking lines on Google Earth would be pretty cool.

Anonymous said...

In Bourne Ultimatum, Bourne uses a "pocket" telescope and appears to be able to "almost" read a document from across the street.

I'm not an expert in optics but I have had a little exposure.

It seems that the more powerful a regular telescope is the further the minimum focusing point is.

How close is the Bourne thing to being realistic?