Is Google Earth a satellite photo?

Yes, it was taken by satellite, with the highest resolution reaching 1 meter, and this is only the masterpiece of American civilian satellites. It is said that the highest resolution of its military satellites reaches centimeter level, and Russia can be seen clearly Screws on the aircraft carrier

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How "magical" are spy satellites

The images of military spy satellites (also called reconnaissance satellites) of various countries have always been top secret. The resulting sense of mystery has led to the spread of myths about satellite images. For example, a report by American journalist Dole Meidel stated: An American reconnaissance satellite operating in an orbit at an altitude of 160 kilometers photographed a Russian warship. After the photo was enlarged, the bolts on the deck could be clearly seen. A sailor nearby was reading a newspaper, and the words "Pravda" on the masthead were still vivid in his mind. A lieutenant general with a background as a novelist said that under the 24-hour strict surveillance of high-tech U.S. spy satellites, there are no secrets that can be hidden in the world today.

This description shocked the world. Is this true?

The "Eye" of the Satellite

In the past, imaging satellites used special film cameras. After taking pictures, the film was loaded into the return capsule and sent back to the ground, so the cycle of acquiring images was very long. . These photos are commonly known as satellite photos.

However, most of the imaging satellites delivered to users in the past 20 years use special digital (digital) cameras. Most of the images acquired are digital information, which can be directly sent back to the ground using radio waves. Therefore, the cycle of acquiring images has changed from digital to digital. Months and days are greatly shortened to hours or even less, and it is also easy to process and distribute using computers.

However, these digital information must be viewed on a computer or other related equipment to see the corresponding images, so today they are generally called satellite images and no longer called satellite photos. The corresponding reconnaissance satellites were also renamed imaging satellites.

Ground resolution is an important indicator to measure the technical level of optical imaging satellites. Resolution refers to the smallest distance that separates two objects in an image, not the smallest size of an object that can be seen. For example the resolution on Google "Satellite Maps" is about 1 meter. That is to say, when two people are more than 1 meter apart, they can be seen separately in the image. When the distance between two people is less than 1 meter, the images of the two people will merge into one, and only one person can be seen in the image. point.

A target with a plane size of about 1 meter is just one image point on an image with a ground resolution of 1 meter. Generally speaking, no matter how many times the image is magnified, it is just one image point. However, the ground resolution does not reflect the minimum size of ground objects that can be "seen" from the image. In the visible light satellite image with a resolution of 1 meter, people whose vertical projection size is much smaller than 1 meter can be seen, but the size cannot be seen. A green tent in a green space much larger than 1 meter.

In most cases, people use optical imaging satellites to obtain ground information. These satellites have ultra-long focal length cameras with focal lengths of more than 2 meters. The current maximum focal length is up to 6 meters, which is equivalent to a large-magnification telescope. The resolution of civilian high-tech imaging satellites such as the "Quickbird" of the United States is 0.6 to 1 meter. They can see tourists with a vertical projection of less than 0.5 meters in Tiananmen Square and lanes with a width of less than 0.2 meters on the streets of Beijing. Under good lighting and weather conditions, Sometimes you can even see thin high-voltage wires.

Of course, the performance of U.S. military high-tech imaging satellites is much higher than that of civilian satellites. There is indeed no problem in seeing the large bolts on the deck of Russian warships.

Optical imaging satellites generally adopt elliptical orbits. For example, the US military's "KH-12 (Advanced Keyhole)" visible light spy satellite operates in a sun-synchronous orbit with a perigee of 322 kilometers and an apogee of 966 kilometers. When an imaging satellite moves to the perigee of its orbit, the image resolution of the ground directly below is the highest, and its value is the highest resolution of the imaging satellite. However, most target areas are not located exactly on the ground directly below the perigee of the imaging satellite's orbit, so imaging satellites often have to tilt to capture ground images of the target area at a longer distance. As a result, the resolution of most images captured by imaging satellites is much lower than its highest resolution.

At present, the United States' "KH-12" spy satellite is the world's most advanced imaging satellite, with a resolution of 0.01 to 0.05 meters. It is known as the "extreme orbital platform."

However, this is only its highest resolution, and in fact most of its images simply fall short of this specification.

From "seeing" to "seeing clearly"

Obviously, it is not enough to just find the target from satellite images. It is often necessary to specifically identify the military or civilian identity of the target. As well as the target's model and even current condition. Military ground reconnaissance is divided into four levels. The first level is discovery, which can only determine the presence or absence of a target from the image, such as whether there is a ship on the sea, or whether there is a suspicious object on the ground; the second level is identification, which can roughly identify the type of target, such as a person or a vehicle. Is it a cannon or an aircraft; the third level is confirmation, indicating its type from the same type of target, such as whether the vehicle is a truck or a bus, and whether the sea ship is a tanker or an aircraft carrier; the fourth level is description, which can identify the target. features and details. For example, it can point out the models of aircraft and cars and the types of missiles equipped on ships. Among these four levels, "discovery" (seeing) requires the lowest ground resolution, and "describing" (seeing clearly) requires the highest ground resolution.

In fact, a reconnaissance satellite with a resolution of 30 meters can detect larger targets such as ports, bases, bridges, highways or ships sailing on the surface. With a resolution of 3 to 7 meters, smaller targets such as radars, small units, missile bases, and command posts can be discovered. On the Google "satellite map" with a resolution of 1 meter, city buildings, roads and cars can be clearly "identified". The house of the editorial office of this magazine can be found and the location of our office can be pointed out. Of course, aircraft carriers, aircraft, and tanks can also be "confirmed." As for the images of the most advanced military spy satellites in the United States, which have a maximum resolution of 0.01 to 0.05 meters, they are enough to "describe" the model of the gun in the hands of soldiers on the ground and "see" the headlines of newspapers. However, the sensational reports that American satellites can see everything on the ground today, such as whether soldiers have shaved or "clearly" read the headlines of newspapers on the ground, are exaggerated. If you want to clearly see (not just "see") newspaper titles from satellite images, the ground resolution must reach 0.003 meters, which is currently impossible.

In addition, current imaging satellites can take pictures vertically downward, or tilt left, right, forward and backward, so it is easy to take pictures of the sides of vertical objects such as buildings, and other places. Relatively speaking, the south side of the building has much better lighting than the north side. It is not surprising that satellites can photograph objects in front of the south window of the house. However, due to resolution limitations and the influence of the window glass, it is still difficult to see clearly the secrets in the south room. Some troops put many unnecessary flowers and plants on the ground and walls of military camps and camping sites, as well as eye-catching slogans and graphics, which will indeed lead imaging satellites to easily detect their locations.

Imaging satellites are not omnipotent

Today’s imaging satellites are generally capable of acquiring images of multiple spectra at the same time. Among them, infrared images can reveal camouflage and identify the authenticity of targets. For example, some shallow underground fortifications cannot be seen in visible light images, but are clearly visible in infrared images. Old-fashioned inflatable tanks are also easily distinguished from real tanks in infrared images. However, the current resolution of infrared images is generally lower than that of visible light images.

Radar imaging (imaging) satellites can work in a completely lightless night, can also penetrate clouds and smoke to find ground targets, and can even penetrate a certain thickness of stratum to find underground targets. They are the current new generation of imaging. satellite. However, its current resolution is far lower than that of optical imaging satellites, which is not conducive to target identification.

Currently, there are nearly 10 high-resolution imaging reconnaissance satellites still operating in orbit in various countries. Optical imaging satellites generally can only obtain ground images within a certain range below the satellite; at the same time, imaging satellites cannot hang over a certain location like communication satellites, but orbit the earth. For example, the famous American "KH-11" high-tech imaging reconnaissance satellite has a rotation period of about 97 minutes around the earth. The orbits of imaging satellites are generally predictable. Therefore, when no imaging satellites pass overhead, military actions will not be seen by imaging satellites.

When an imaging satellite passes through a target area, if there are clouds or fog or severe atmospheric pollution in the sky, it will be difficult for the imaging satellite to obtain clear visible light images of the ground. Clouds and fog or severe atmospheric pollution also have a certain impact on infrared photography. Making full use of clouds or severe atmospheric pollution, as well as artificial smoke obscuration, can greatly reduce the chance of military operations being seen by imaging satellites.