Spy satellites – IMINT (Imagery Intelligence)
30 cm resolution satellite image (DigitalGlobe)
Satellites fly across the sky unhindered by borders. With a camera on board, they can take photographs of the ground below (if there are no clouds in the way). Many satellites carry cameras for benign reasons such as weather forecasting or environmental monitoring, but some are explicitly seeking military information, and these we call spy satellites.
They can track the movements of individuals on the ground, identify cars, and probably even read a license plate provided clear skies and good viewing angle—all while operating covertly hundreds of miles above in space & transmitting images in real time.
These satellites provide intelligence information on the military activities of foreign countries, and can even detect Missile launches or Nuclear explosions in space. They can pick up and record radio and radar transmissions while passing over a country and play a role in the negotiations of the SALT arms control treaties. Finally, they can be used as an orbital weapon by placing warheads on a low orbiting satellite to be launched at a ground target.
Spy satellites used for photo reconnaissance or Imagery Intelligence (IMINT) travel at speeds of 28,000 Km/hr across the scene of interest. They normally use Low Earth Orbits (160-600kms) as they are able to see the surface of the Earth more clearly as they are not so far away. The cameras in the satellites take pictures of the strip of land below, as it travels around the globe.
Need for spy satellites
Spy planes have limitations. Even the highest-flying airplane cannot fly above the atmosphere (there isn’t enough oxygen in the air to continuously burn the jet fuel required to stay up there), and can, therefore, view only a limited amount of ground at any one time. Spy planes are intrinsically illegal in times of peace—they must violate national airspace to do their job (chances of getting shot down by enemy air defense)—and therefore, a political liability.
For wildlife photography, we use a digital camera with a telescope to spot the details of an object too distant to see in detail with the naked eye.
Modern spy satellites are like a camera-equipped smartphone—but with a very long telescope attached. The telescope has a mirror, more than 2 meters in diameter.
Note: The resolving power of a telescope or the image resolution of satellite gets better (proportionally) as the diameter of its primary mirror gets bigger. This means a 10-meter telescope has a light gathering power 25 times that of the 2-meter telescope.
Once you have taken photos using the built-in camera of your smartphone, you send it to whoever is interested in it via the cell phone network, in other words by radio link.
The number of pixels in the image taken by the camera dictates its graininess or resolution—that is, the ability to blow it up and see further detail. The more pixels in the camera the more you can magnify it without it becoming grainy.
Sometimes there is no network coverage where you have taken the picture and you have to wait until your journey brings you to an area with network coverage before you can send it. If this is a frequent occurrence you may consider switching to another network that offers better coverage.
The same applies to a modern spy satellite, leaving aside the fact that it takes images at a distance of 200 miles or more through a telescope. It captures images automatically to a predetermined schedule rather than when a human touches a button.
Early spy satellites used conventional photo film cameras to record images and drop photos in ‘Film Buckets’ from space for Airplanes to catch in Mid-Air. With the invention of CCD (charge-coupled device) cameras, the images were stored digitally & sent via encrypted radio links instead of Film recovery buckets.
Charge-Coupled Device (CCD)
Digital cameras store images on CCD. CCD is thus the equivalent of the photo film in a conventional camera, with the difference that a CCD can be reused indefinitely. A CCD is a microchip which contains an array of thousands of microscopic electronic devices that record light impacts as electrical messages. The CCD assigns different digital number values to represent varying light levels in the image. Digital enhancement techniques are used to further sharpen the images & remove the background noise. The images are stored in computer memory on the satellite, which can only hold a certain number of images before it reaches its capacity. The images may be radioed to the ground immediately, but frequently there isn’t a friendly ground station within sight of the satellite, so it waits until its orbit brings it within the coverage of its ground network.
The US and Russia have installed extra network relay stations to improve the coverage and thus get images back to the ground from more or less anywhere in the world. The relay stations are actually satellites located at suitable, very high orbits which relay the images from the spy satellite to the relevant ground network. These satellites, therefore, have no intrinsic limitation as to the number of images they take, provided the radio link has enough bandwidth to carry them. Other spy satellite operator (nations) have to wait until their satellite appears over their ground stations before receiving the recorded images.
Range and Resolution
The range of a satellite is a measure of the ability to cover an area.
Resolution in satellite imagery is defined as the ability to distinguish between two objects. In simple terms, it is the relationship between a pixel on your screen and its size on the ground. (For example, a 3-meter resolution would allow one to identify two tanks parked side by side).
In smartphones to describe the resolution of a camera, we use the term megapixels. Pictures are made up of millions of tiny square dots called pixels. Pixel stands for PICture ELement. Put enough of them together and you have a picture. They are arranged horizontally and vertically.
The Megapixel count is arrived by multiplying the number of horizontal pixels by the number of vertical pixels. A 3 MP camera has 2,048 (horizontal) x 1,536 (vertical) pixels, or 3,145,728 pixels. We call this simply 3 MP.
Resolution is how many pixels you have counted horizontally or vertically when used to describe a stored image. The resolution of satellite images varies depending on the instrument used and the altitude of the satellite’s orbit.
For a satellite resolution of 50cm, each pixel on image represents 50cm by a 50cm square surface of the ground, like the size of your arm. This effectively means that objects larger than 50 cm on the ground will be detected by the satellite.
In other words, if two objects on the ground are more than 50cm apart, then they will look like two different objects in the satellite imagery. But, if they are 25cm apart, then the satellite will recognize it as a single larger object instead of two separate objects.
25cm is the highest resolution allowed by US government for commercial satellite operators like DigitalGlobe which sell satellite imagery to Google (Google maps & Google Earth).
Military and intelligence satellites may be getting photos with a resolution of less than 10cm or even better.
The first spy satellite, Corona, during the 1960’s sent images having a 12-meter resolution. However, an 80cm resolution would be sufficient for a range of strategic intelligence needs, such as counting the number of fighter jets, tracking the location of an aircraft carrier, or monitoring tunnel construction activity.
Can spy satellites read a license plate from space?
The pictures below show two newspaper front pages, a license plate and a golf ball in images of various resolutions.
The first image is of 10cm resolution, probably the best that spy satellites can do. With that resolution, you can’t read the headlines or the number plate nor even detect a golf ball.
The second image with 3cm resolution (from Aerial imaging systems like drones, planes) probably can detect a golf ball but can’t read a license plate or newspaper headlines.
Now check the third image, it is of 1cm resolution, and allows you to read large tabloid headlines, but not the license plate or normal headlines.
The last image with 1mm resolution shows what the other images miss— the golf ball is distinguished as circular and not a small white box, the license plate is revealed and headlines, & text in both newspapers are visible.
The newest US spy satellite can distinguish objects less than 10 centimeters, which is less than the length of some smartphones. The exact resolution and other details of Modern spy satellites are classified. But if we even assume resolution to be 1cm, as we discussed above, that isn’t sufficient to read a license plate or newspaper headlines.
Some sources on the internet say “WorldView-3 satellite with a resolution of 31cm can grab shots of license plates”. Is that true?
The image below shows 0.25cm or 25mm (millimeter) Resolution Aerial photography, which has a much higher resolution than worldview-3. Due to low altitude acquisition, aerial data doesn’t suffer from atmospheric effects. From the image, I can count the number of people in the crowd, identify the car, also a car make and model and even identify the numbers on the license plate, but can’t read it.
Reconnaissance satellites can be further subcategorized into the following four types, depending upon their applications:
Missile Early warning:
Missile early warning (EW) satellites are military spacecraft used to detect the launch of ballistic missiles and immediately relay that information to the ground, enabling the targeted country to retaliate, take cover, or possibly shoot down the missile. Information from these satellites would provide the first alert of the start of a major missile attack, and is used to track the long-term patterns of foreign nations space programs
Nuclear explosion detection:
Identifies and characterizes nuclear explosions in space. A nuclear explosion releases electromagnetic energy across the entire spectrum, from radio waves through infrared, visible and ultraviolet light to x-rays and gamma rays. All of these provide a means of detection.
Satellites used for nuclear explosion detection carry sensors like Bhangmeter which detects the optical (light) flash generated by a nuclear explosion fireball, an X-ray sensor that discriminates between atmospheric & exoatmospheric (space) detonations. These satellites also monitor compliance with the Nuclear Test Ban Treaty.
Provides imaging of earth from space. Photo reconnaissance systems take pictures in the visible and infrared light. Present generation satellites use a new technology called radar imaging (SAR) which can image earth at night or through cloud cover.
Electronic-reconnaissance (ferret) satellites pickup and record radio and radar transmissions while passing over a foreign country.
Bachelor in Electrical Engineering, PGDC Thermal power (O&M) under Ministry of Power