Meteorological satellites are usually launched into either a sun-synchronous orbit or a geostationary one. A geostationary orbit is an orbit based on which a satellite orbits the Earth for exactly one day. Thus, it turns out that the satellite is set over the same point on the equator. The distance from the Earth's surface to the geostationary orbit is about 35,800 km. Applying this height the satellite can check about 1/3 of the Earth's entire surface. The following meteorological satellites are located on 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 value. Thus, the angle of illumination of the Earth's surface will be approximately the same during all passages of the satellite. Such constant illumination conditions are very suitable for weather satellites. In order to achieve desired characteristics, the orbit parameters are chosen in a way when the orbit precesses eastward by 360 degrees per year (approximately 1 degree per day), compensating for the Earth's rotation around the Sun. The following meteorological satellites are located on this orbit: Meteor-M 2 (Russia), NOAA 15, 18, 19 (USA). Meteorological satellites provided in the sun-synchronous orbit do not capture the entire surface of the Earth at once, rather scanning the image in the mode “one line at a time” and transmitting it at a frequency of about 137 MHz using the analog format (APT). Special computer software connects all ready images based on this data.
Resolution of images received from NOAA series satellites is equal to 4 km per pixel. The image is composed of two IR channels, applying geometric correction of perspective distortions. In such a way, we get a kind of averaged representation of the real temperature at a particular point of the surface.
Since our planet is shaped like a ball, all pixels located in the center of an image are equal to 4 km by 4 km, while pixels located on edges of the image turn to be elongated, reaching the value 4 km by 8 km or even 4 km by 12 km.
Meteorological satellites take photos of the surface in the visible range and in the infrared (IR) range. Infrared radiation is not visible to the human eye rather being detected by the satellite's camera. Infrared radiation is also called “thermal” since infrared radiation taken from heated objects is perceived by the human skin as sensation of heat.
The infrared range is divided into several subranges.
Near infrared range: wavelength λ = 0.74-2.5 microns
Middle infrared range: wavelength λ = 2.5-50 microns
Far infrared range: wavelength λ = 50-2000 microns
The Earth's surface and clouds absorb visible and invisible radiation from the Sun and re-radiate most of the energy as infrared radiation back into the atmosphere. Some substances contained in the atmosphere, mainly - water droplets and water vapor - absorb this infrared radiation and re-radiate it in all directions, including back to the Earth. Thus, the greenhouse effect keeps the atmosphere and surface warmer than if there were no water vapor in the atmosphere.
The 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 case of the infrared range, the satellite camera also receives radiation data from the surface, clouds and atmosphere. It is necessary to consider that surface radiation is partially absorbed by the atmosphere.
As a result of interactions with the atmosphere, the intensity of solar radiation at the Earth's surface is reduced by more than half compared to its value in the upper layers of the atmosphere.
Solar radiation arriving at the Earth's surface is not fully absorbed by it. Parts of the radiation spectrum are reflected by the surface, and only the upper layer of the Earth's surface, where radiation is absorbed and transformed, is involved in the reflection process. Such layer includes all the grass and plant (green) mass of forests, the first tens of meters of transparent and decimeters of turbid water, as well as decimeters of snow, centimeters of sand and fractions of millimeters of dark soils. The reflectivity of the Earth's surface depends on the type of its bodies, their physical properties, color and condition.
The ratio of reflected radiation to the total radiation taken from the Sun and atmosphere is called “albedo”. Albedo values are most commonly expressed as the percentage. The albedo of the Earth's surface varies within a wide range. It is connected with a type of landscape zones and in case of temperate and high latitudes - with seasonal changes. Thus, reflectivity is large and changes little in the annual course in the central parts of the polar regions: in Antarctica it is about 85 %, in the central Arctic - about 80 %. In July, the decrease of albedo in the Arctic region (up to 65 %) is connected with more intensive snow melting in comparison with December in Antarctica. Average albedo values determined for different types of the land surface (in %):
The albedo value of the water surface is less in average than most natural land surfaces and depends on the angle of incidence of rays, height of the Sun, ratio of direct and scattered radiation and sea surface excitement levels. When the Sun is at its zenith, the albedo value of calm sea is 2% for direct radiation. The albedo increases as the height of the Sun decreases. The albedo value of the sea increases when waves are very rough and have a foamy form. The following similes show that some objects are better to be detected in the infrared spectrum. For example, thin clouds.
Some objects are better seen in the visible range, on the contrary.
In case of infrared pictures, the white color corresponds to cooler areas and black - to warmer areas. The clouds shown in these pictures are white not because they are white, but because they are cold.
The cloud temperature can determine the height of the clouds, since the higher are the clouds, the colder they are.
An infrared radiation range with the wavelength of 10 microns. The wavelength range of the Earth strongest radiation with little absorption and over-radiation (atmospheric window).
Infrared radiation range with the wavelength of 6.2 microns. Wavelength range with significant water vapor absorption (water vapor channel)
Infrared radiation range with the wavelength of 3.9 microns. The data on solar radiation and Earth radiation is transmitted simultaneously. The instrument allows visualization of low clouds and fog at night.
The IR range also allows users to consider various warm objects - cities, and forest fires. Moscow.
A forest fire on the island of Kalimantan. Only smoke can be seen in the visible range.
The smoke is also seen in the infrared range of 10 microns.
Fire hotspots are clearly visible in the 4 microns infrared range.
The method of subtraction of IR signals is used in order to see fog at night. A signal with the wavelength of 4 microns is subtracted from the signal with the wavelength of 10 microns.