Reconnaissance satellites use high resolution CCD cameras and large lenses to take pictures of the ground. They return images via encrypted radio transmissions. The Rayleigh criterion is used to calculate the resolution of an optical image. Commercially available high-resolution color images of the world are available, but military reconnaissance satellites update their images every 45 minutes. The US-built Mars Reconnaissance Orbiter returns high-resolution images of the Red Planet.
You may have wondered how reconnaissance satellites take detailed, close-up photographs of our home, the Earth. The answer is that they use high resolution CCD cameras coupled with large lenses to take pictures of the ground just below them as they pass. Photos taken in bad weather are likely to be filtered. As late as the mid-1980s, reconnaissance satellites reported their images back to Earth using flimsy film canisters mounted on parachutes and picked up by mid-air planes. Today they return images via encrypted radio transmissions.
Most of the information on modern spy satellites is highly classified. Much of our information about how these devices might work is based on assumptions and may be speculative.
A speculative calculation of spy satellite performance uses the Rayleigh criterion, a way of calculating the resolution of an optical image. The equation implies sinθ = 1.22 λ/D, where is the wavelength of light, is the angular resolution, and D is the diameter of the lens or mirror. Assuming a satellite operating in low Earth orbit at about 300 km altitude, with a Hubble-sized lens 2.4 m in diameter, looking at light of a typical visible wavelength of about 550 nm, we get an angular resolution of 229 nanoradians, which at an altitude of 300 km translates into a resolution of about 7 cm per pixel. This doesn’t account for atmospheric occultation or lens imperfections, but it seems like a fair estimate.
Much to the chagrin of the international intelligence communities, commercially available high-resolution color images of the world have recently become available using services such as Google Earth. These commercial services offer resolutions of around 20cm per pixel or better for some areas, possibly in the vicinity of spy satellites. The main difference is that the images on such services tend to be around 1-3 years old, whereas military reconnaissance satellites probably update their images every time they orbit the Earth, which is approximately every 45 minutes. The difference in intelligence between the two is, of course, huge.
Recently, the US-built Mars Reconnaissance Orbiter was deployed in orbit around the Red Planet, also returning high-resolution images of that body. Google Earth and Google Moon already exist, it’s only a matter of time before we start seeing Google Mars and Google Asteroids.
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