Shulou(Shulou.com)11/24 Report--

When it comes to taking pictures, we naturally think of the camera and the shutter, pointing the camera at what needs to be observed, and pressing the shutter will get an image of data filled with trichromatic coordinates on two-dimensional pixels.

But for satellite observation of the earth (surface / atmosphere), the specific imaging principle will be a little more complicated.

Scanning satellites can be divided into geostationary satellites and polar orbiting satellites according to their orbits. We know that the angular velocity of the earth's rotation is fixed. If we want to use a satellite to monitor the earth constantly, then the altitude of the satellite orbit is fixed, which is the geostationary satellite. The height of the geostationary satellite is 36000km, and the radius of the earth is about 6371km. It is conceivable that the angle of the earth seen on the satellite is only 18 °. How can the effect of taking pictures directly be good? If we want to have a very high resolution image, we need to use the "scan" method.

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Specifically, "scanning" is to obtain data by adjusting the angle of the observation instrument and pointing it at different positions of the earth. For example, the Fengyun-2 meteorological satellite itself rotates around its principal axis, so that the sensor can scan banded observation data on the earth; and then make the satellite adjust its attitude and tilt upward and downward, it will be able to scan the "parallel" observation data on the earth.

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Picture source: CCTV news client + the author indicates the scanning path and the rotation direction of the satellite. Of course, different satellites can improve the observation utilization rate by using different attitude adjustments. As far as the Fengyun-2 meteorological satellite is concerned, in the 360 °rotation of the satellite itself, only a few dozen degrees are facing the earth, and its utilization rate is obviously not high; while the Fengyun-4 satellite adopts a three-axis stable way to actively control the orientation of the satellite and make the probe face the earth all the time in the process of rotation, so the observation efficiency is obviously greatly improved.

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Image source: Google Picture "Fengyun 4 Weather Satellite" so some people can't help but ask, why not use three-axis stabilization to control the satellite orientation earlier?

Because the way of three-axis stabilization is not that simple.

When the satellite moves around the earth, the spin stabilization mode can easily achieve its own attitude stability, the key point is that the satellite itself is axially symmetrical to its own principal axis, and if the three-axis stabilization mode wants to keep the detector facing the earth, how to stabilize the attitude of the satellite is a big problem.

Secondly, the spin-stabilized satellite is uniformly exposed to solar radiation, while the three-axis stabilized satellite is always facing the earth, and the difference between the two sides of the satellite will cause the satellite to deform. The development and use of materials and the arrangement of satellite instruments all need to be carefully considered.

As the geostationary satellite is far away from the earth, the observation accuracy will inevitably be limited. If we want to obtain more accurate observation data, we need polar orbiting satellites. Polar-orbiting satellites (also known as solar synchronous satellites) have a lower orbit than geostationary satellites, about 840km, orbiting the earth's north and south poles.

Its observation mode can be imagined as scanning when doing CT, and the data can be obtained wherever the instrument / satellite scans; therefore, the satellite orbit and the longitude and latitude of the observed data can be directly matched, so the term "one-orbit data" can often be heard. For example, the CloudSat satellite swept out of Earth in 20070722 days like this:

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You can see that its coverage is still relatively small. Of course, there are also satellites in the process of moving, the detection instrument will be added to the wobble perpendicular to the direction of the orbit, so as to detect the data of a surface. For example, the left picture shows the surface precipitation measured by the GPM satellite. Draw a profile at the red line to get the precipitation rate profile at the red line (right):

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The problem is obvious. There is a time difference in the observed data of each orbit. According to the satellite flying close to the earth, it is roughly estimated that the period of the satellite orbiting the earth is about 84 minutes. We pick out a track of data analysis, in fact, it is difficult to say that these 84 minutes of data are observed at the same time; pick one of the data analysis should also take into account the difference in time.

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If we need to analyze global / regional data, we need to consider aggregating many days of data into a single graph to represent the long-term average. In fact, any point in the "average state" we get is not the average state obtained after observing the point continuously for a long time, but the point is observed again after a long time; and it is precisely because we cannot observe "simultaneous data" that the average of different points on the map is actually the average of different times. It's a bit of a circle, anyway, I just want to express that in addition to the spatial resolution, the improvement of time accuracy is also a very important thing.

This article comes from the official account of Wechat: stone popular Science Studio (ID:Dr__Stone), author: Mai Liting, American editor: Qijia cc

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