Weather satellites, as a rule, are launched either into a sun-synchronous orbit, or into a geostationary one. A geostationary orbit is an orbit in which the satellite’s period of revolution around the earth is exactly one day. Thus, it turns out that the satellite, as it were, hangs over the same point at the equator. The distance from the Earth's surface to the geostationary orbit is about 35,800 km. From such a height, a satellite can view about 1/3 of the entire surface of the Earth. Meteosatellites are located in this orbit: GOMS (Russia), GOES-15 (USA), Meteosat 10 (EU). A sun-synchronous orbit is an orbit in which a satellite passes over any point on the earth’s surface at approximately the same local solar time. Thus, the angle of illumination of the earth's surface will be approximately the same at all passages of the satellite. Such constant lighting conditions are very suitable for weather satellites. To achieve such characteristics, the parameters of the orbit are selected so that the orbit precesses eastward by 360 degrees per year (approximately 1 degree per day), compensating for the Earth's rotation around the Sun. Meteosatellites are located in this orbit: Meteor-M 2 (Russia), NOAA 15, 18, 19 (USA). Meteosatellites located in a sun-synchronous orbit do not immediately capture the entire surface of the Earth, they scan the image on one line and transmit it at a frequency of about 137 MHz in analog format (APT). Special computer programs collect ready-made images from these lines.
The resolution of images obtained from satellites series NOAA is 4 km per pixel. The image is made up of two IR channels, using geometric correction of perspective distortions. Thus, we get a certain average idea of the real temperature at a specific point on the surface.
Since our planet has the shape of a ball, the pixels located in the center of the image have a size of 4 km by 4 km, and the pixels located at the edges of the image have an elongated shape - 4 km by 8 km or even 4 km by 12 km.
Weather satellites photograph the surface in the visible range and in the infrared (IR) range. Infrared radiation is not visible to the human eye, but is detected by a satellite camera. Infrared radiation is also called “thermal radiation”, since infrared radiation from heated objects is perceived by human skin as a sensation of heat.
The infrared range is divided into several subranges.
* frequency ν (nude) - up to 4 · 1014 Hz * wavelength λ (lambda) - from 730 nm
Mid IR * frequency ν - up to 6 · 1013 Hz * wavelength λ - from 5 microns
* frequency ν - up to 1013 Hz * wavelength λ - from 30 microns
The Earth’s surface and clouds absorb visible and invisible radiation from the Sun and re-emit most of the energy in the form of infrared radiation back into the atmosphere. Some substances in the atmosphere, mainly water droplets and water vapor, absorb this infrared radiation and re-emit it in all directions, including back to Earth. Thus, the greenhouse effect keeps the atmosphere and surface in a warmer state than if there was no water vapor in the atmosphere. Sunlight illuminating the Earth is partially absorbed by the atmosphere, surface and clouds, and is partially reflected from the surface, clouds and aerosols. A satellite camera receives this radiation and forms an image in the visible range. In the infrared range, the satellite’s camera also receives radiation from the surface, clouds and the atmosphere. Moreover, surface radiation is partially absorbed by the atmosphere.
As a result of interactions with the atmosphere, the intensity of solar radiation at the surface of the Earth in comparison with its value in the upper layers of the atmosphere decreases by more than half.
Solar radiation coming to the surface of the Earth is not completely absorbed by it. Part of the radiation is reflected by the surface, and only the upper layer of the earth's surface is involved in the reflection, in which radiation is absorbed and converted. Such a layer includes the entire grass and vegetative mass of the forest, the first tens of meters of clear water and decimeters of muddy water, as well as decimeters of snow, a few centimeters of sand and a fraction of millimeters of dark soil. The reflectivity of the Earth's surface depends on the kind of bodies, their physical properties, color and condition. The ratio of reflected radiation to the total radiation of the Sun and the atmosphere is called albedo. Albedo values are most often expressed as a percentage. The albedo of the earth's surface varies widely. This is due to the type of landscape zones, and in temperate and high latitudes also with the change of seasons. So, in the central parts of the polar regions the reflectivity is great and varies little in the annual course: in Antarctica - about 85%, in the central Arctic - about 80%. In July, a decrease in the albedo in the Arctic (up to 65%) is associated with more intense melting of snow than in December in Antarctica. Average albedo values for various types of land surface (in%):
The albedo of the water surface is on average smaller than most natural land surfaces and depends on the angle of incidence of the rays, on the height of the Sun, the ratio of direct and scattered radiation, and the waves of the sea surface. With the position of the Sun at the zenith of the calm sea albedo for direct radiation is 2%. With a decrease in the height of the Sun, the albedo increases. With great ocean turmoil, when foam and lamb are formed, the sea albedo increases.
The following sims show that some objects are better visible in the IR range. For example, thin clouds.
Some objects, on the contrary, are better visible in the visible range.
The cloud temperature can determine the height of the clouds, because the higher the clouds, the colder they are.
The range of infrared radiation with a wavelength of 10 microns. The wave range of the Earth’s strongest radiation with little absorption and re-radiation (atmospheric window).
The range of infrared radiation with a wavelength of 6.2 microns. Wavelength range with significant water vapor absorption (Water vapor channel)
The range of infrared radiation with a wavelength of 3.9 μm. It transmits information simultaneously about solar radiation and Earth radiation. Allows visualization of low clouds and fog at night.
Also, the IR range allows you to see various warm objects - cities, and forest fires. Moscow city.
Wildfire on the island of Kalimantan. In the visible range, only smoke is visible.
In the IR range of 10 microns, too, only smoke.
In the IR range of 4 μm, fires are clearly visible.
In order to see the fog at night, the method of subtracting infrared signals is used. From a signal with a wavelength of 4 μm, a signal with a wavelength of 10 μm is subtracted.