서울흥신소 Many people think of satellites as spying tools based on action-adventure movies and spy novels. But satellites are regulated and a company must meet strict government requirements to launch one.
Spy satellites crisscross the globe in secret orbits, grabbing digital snapshots of areas that the CIA and its policymakers want to see. They are overseen by the National Reconnaissance Office headquartered in Chantilly, Virginia.
Reconaissance Satellites
Unlike communications satellites and even weather satellites, which are mainly for civilian purposes, reconnaissance or spy satellites carry special sensors designed for military use. Spy satellites can track the deployment of enemy forces, keep tabs on weapons development and assess damage from bombs. They are primarily operated by the National Reconnaissance Office, and data from them are interpreted by the CIA and other agencies in centralized facilities such as the U.S. National Photographic Interpretation Center in Washington, D.C.
In their earliest days, the two competing programs for spy satellites pursued by the U.S. Air Force and the CIA used film to capture images, then sent TV scans back to Earth after a mission. But those old satellites often died after just three or four weeks, and they were limited by the amount of film they could carry.
By contrast, today’s most sophisticated satellites can last decades and work around the clock. And they’re no longer limited by the amount of information they can carry.
The latest satellites can, for example, use radar to see through clouds 서울흥신소 and even underwater. They can also use signals-intelligence or “ferret” satellites to pick up radio and microwave transmissions emitted from any country on Earth. And they can relay satellite data much faster by transmitting images from one spy satellite to other orbiting satellites, which then relay them to stations on Earth.
Visible Light Satellites
In visible light satellite imagery, passive satellite sensors gather information about physical phenomenon based on their interaction with energy that exists in the visible portion of the electromagnetic spectrum. This energy is reflected from the surface of the Earth, and visible light satellites use this to create images.
Visible light satellites are typically only useful during the local daytime since they require sunlight to create their images. Visible light satellites can see clouds, which appear white on the image, and they can see bodies of water, which are normally grey (since rivers have very low albedo).
The quality of an optical image from a visible-light satellite is measured by its spatial resolution (also called pixel capability). A 10m/pixel satellite can display an area of about 10 meters in size for each picture it makes. In contrast, a sensor on the satellite Pleiades Neo can capture data at a rate of nearly 0.3m/pixel, and is capable of showing individual trees and other objects.
Optical satellites collect this visible-light data through scan mirrors that move from north to south and east to west across the Earth. Each set of data from the scanner is assigned a color on the electromagnetic spectrum, and scientists then construct an image based on these colors. See the chart below for an illustration of how this works.
Radar-Imaging Satellites
Unlike optical imaging systems, radars do not use light but rely instead on electromagnetic signals of a specific microwave wavelength. Consequently, radars can operate at night and see through clouds. The images acquired by radars reflect the geometric and dielectric properties of a surface or volume being studied. The image resulting from these measurements can therefore be used to identify the kind of material (eg metal, concrete) and its moisture content.
Radar-imaging satellites can also be used to monitor vegetation, rivers and flooded land. They are particularly appreciated by icebreakers, which use the radar data to find an optimal track through Arctic or Antarctic sea ice.
The image quality depends on the radar wavelength or band, the kind of surface and the terrain type. For example, an L-band radar signal with a wavelength of about 23 cm can penetrate deeper into a forest, whereas a C-band radar has much less penetration capacity.
The polarization of the radar signal can be controlled. When the polarization of the transmitted signal is orthogonal to the received signal, it results in a bright spot at one location and a dark area at another location. This effect is called decorrelation.