On the reception of images of the Earth from meteorological satellites
At present, there are about 10 satellites used for meteorological purposes in Earth orbit. These satellites continuously scan the surface and atmosphere of the Earth and carry out a direct information dump on the earth to the appropriate scientific centers, laboratories and everyone who can receive. The receiving station, which is located in the radio visible area of ​​the satellite, sees in real time what the satellite sees. Data from it comes directly at the time of shooting. Hardware image is taken not only in the visible spectrum, but also on some frequencies of the infrared range. It is even more correct to say that all the main channels are infrared, there are many more of them. These channels are much more important for practical purposes, because they can emit water vapor, smoke, heat radiation from forest fires or determine the temperature of the planet's surface. With the help of such images it is possible to determine even the ripening of the crop on the collective farm fields. This article tells about the technique of practical reception of images from Earth remote sensing satellites, thanks to which any person can go to such sites as meteosputnik.ru and see those real satellite images.
According to the World Meteorological Organization (WMO) open sky concept, meteorological information is distributed free of charge, and even you can receive a non-encrypted satellite signal in real time while you are flying over the horizon. Now they are constantly flying around the Earth and transmitting an image at a frequency of 137 MHz: NOAA15, NOAA18, NOAA19 satellites, at 1.7 GHz frequency: NOAA15, NOAA16, NOAA18, NOAA19, MetopA, MetopB, FENGYUN, Meteor-M1, at 8 GHz: Terra, Aqua, Aura and Calipso.
Fig.1. APT format.
The AVHRR (Advanced Very High Resolution Radiometer) on the NOAA satellite is a scanner with a resolution of 1 km / pixel, it forms an image in 5 IR channels. The Russian MSU-MR (Small-Resolution Multichannel Scanning Device) on the Meteor-M1 satellite forms 6 channels. On Terra and Aqua, a MODIS (MODERATE RESULTING Imaging Spectroradiometer) device scans in 36 spectral channels with 12-bit resolution in the visible, near, middle and thermal infrared bands.
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At a frequency of 137 MHz in the analog format ( APT ) a picture with a resolution of 4 km / pixel is transmitted, consisting of two IR channels, obtained as a result of geometric correction of perspective distortions and reduction of scale. At a frequency of 1.7 GHz, the HRPT (High Resolution Picture Transmission) digital format is already used (in NOAA, the Manchester code, and in Metop, the Reed-Solomon code and the Viterbi algorithm ) - 1 km resolution / pixel; at 8 GHz, the digital code is also used for transmission data with a maximum resolution of 250m / pixel.
A data stream in a format generally consists of rows, each of which contains:
• sync bytes
• satellite data
• time stamp for each line of the bitmap
• calibration values
• we need information about the brightness of the pixel in each channel
• The latest formats provide for the transfer of GPS coordinates of the satellite’s position in orbit.
The digital signal is formed by 10-bit bytes (in MODIS 12 bits), i.e. 10 bits per pixel of each channel. The lines go one after another, in accordance with the direction of flight of the photo-sensor, i.e., if the satellite flies from north to south, then the picture is built from top to bottom, and if it returns to us from Antarctica, then before watching, the picture should be turn over The sensor is designed in such a way that the lines are scanned one by one, continuously. He constantly looks at the center of the Earth, and the scanned line is perpendicular to the motion vector, i.e. the satellite is filming nadir. The projection is clear to the eye, but far from cartographic. In order to overlay a map of cities and country borders, one has to use rare, unknown, and unnecessary programs in ordinary life. The orbit on which the weather satellites fly is called sun-synchronous. The objects launched in such an orbit retain their position relative to the Earth and the Sun at each turn, i.e. if the satellite flew over Moscow in the evening, then on the next round it will fly over Europe too in the evening and in general it falls in the evening, and on the back side of the Earth, it means that it always happens in the morning, i.e. The earth actually scrolls beneath it inside its orbit.
Knowing the orbit of a radio transmitter flying in space, it is necessary to calculate its position relative to the receiving antenna. For this, there is the Orbitron program, under which the driver of the rotating device of the directional antenna is written. The input data for the program are: exact time (accuracy 1s) and keplers (Keplerian orbit elements) in TLE format - they are available on the Internet and they need to be updated periodically. The output angles: elevation and azimuth - how much to raise the plate and which way to turn. As long as we accept L-band satellites (1.7 GHz), the angles are adjusted once a second and that is enough. To receive these satellites, you need a plate from 1.5 m in diameter. To ease the weight you can use a mesh antenna. The focus of the parabola is an irradiator with circular polarization, a filter and a down converter that converts the frequency of 1.7 GHz to the frequency of 150 MHz. The radio signal comes from the head to the receiver via a coaxial cable, and then logically decoded by the decoder, which allocates 10-bit sync bytes, synchronizes with the stream, and by USB data gets into the program, which tracks the presence of the useful signal and writes it to a disk. If someone wants to know the details, he can read about PSK modulation, Manchester code, binary NRZ coding. In the computer we have ready-made HRPT Minor Frame Format structures, etc. This is where the hardware ends.
Video 1. Receive data from NOAA satellite.
The data received from different satellites must be re-assembled into a single format so that a universal program can open them. Therefore, the program ( BMsat ), which receives the digit from the decoder, repacks the data on the fly and writes to the disc in such a format so that it can be opened by the HRPT Reader program, which can build color images, overlay the map and save it in BMP or JPEG! This is the second program in the chain, which also needs to be pumped from the Internet orbit files (TLE). So that images obtained in infrared spectra look adequate enough for a blind person in this range of RGB-viewer, i.e. for you, there is a special algorithm named in the False color program. Thus, you should be clear that those meteorological images that are on the Internet are not real colors, but modeled on the basis of strips of the IR spectrum. In general, different channels can serve as the basis for the False color algorithm - here the choice is by operator. The functionality of the program still includes the possibility of rectifying perspective distortions. Those pixels that were scanned from the Earth's surface directly from under the satellite display 1km, and those that are closer to the horizon cover several kilometers, and that the Earth seemed not round, but flat, like on maps, you can pull the edges of the picture wide. At the same time, the image quality suffers, the size of the JPG file grows, an even smaller fragment is placed on the screen. But one way or another, everything is sent via FTP to the site, and you already know everything about it yourself. Is it possible to mention the splitting of a large picture into tiles (small squares) that would be downloaded for those places that the site user is looking at. How this is done in Yandex and Google maps.
Video 2. Data processing from meteorological satellite.
The equipment and programs that are used for reception, starting with two-axis turns, ending with HRPT Reader, are very specific and are not on the shelves in stores. All this should be developed on our own, using their experience and the experience of those who are already engaged in it, or pay for the development of specialists. In general, any equipment, of course, already exists, but its prices are space. Thanks to ebay, you can try to buy a retired copy of one or another device at your own peril and risk, try to fix it and modify it. To be able to receive 8 GHz satellites with a resolution of 250m / pixel, you need to increase:
• positioning accuracy of a parabolic antenna up to 10 '(10 angular minutes)
• positioning frequency - the plate should move smoothly
• diameter up to 3m
In addition, it is necessary to completely replace the hardware. Switch to other frequencies, etc. The images generated by such satellites are no longer line by line formed, but in strips of appreciable width, therefore it is not possible to look at them normally, since perspective distortions duplicate pieces of images close to the horizon. Those. here, too, there is a mathematical problem that must be solved.
To expand the functionality of the project “Photo from a satellite in real time” (it is presented on the website www.meteosputnik.ru ) it is necessary to solve various tasks, for example, converting a projection into Mercator , merging our services with others, for example, connecting to a free SAS program map .Planet, installation of new equipment and the opening of new receiving stations to increase the quantity and quality of images. To speed up this and everything else, it would be nice to expand the group of enthusiasts who are ready to participate in the project for free and find sponsors.
Our project currently provides open access to many unique and useful data. Its meaning is very many-sided. Predicting cloudiness and weather changes is sometimes much easier and more interesting when looking at weather images than at forecasts of various weather websites. You can also understand whether you get to the center of the cyclone, to the thunderstorm front, or the weather will change smoothly in a few days. In particular, one of my friends used these images three years ago, when everything was in smoke from forest fires, to make a decision where to leave the Moscow region for the weekend: in the pictures it was clear that transparent air began then in Belarus. But this is an example of short-term use of technology. The data we obtained are used in teaching students of meteorological faculties. It is very important to understand that this data is not just operational data, but a unique area of ​​technology. Such things may interest schoolchildren and stimulate them to learn at once in several subjects, besides physics, knowledge about nature, atmospheric, water processes, meteorology, geography, astronomy, geometry, computer science and radio electronics is included. You can try to test your knowledge of geography by mentally presenting the current state of the dried-up Aral Sea and comparing it with a picture from space. You feel the difference between theoretical and practical knowledge, would it be possible to simply take and look at the Aral Sea, being content with rare printouts from textbooks that are not present and with old maps? Or remember the tragedy in Krymsk - a city where, at one time, with the participation of D.I. Mendeleev opened the first oil-producing installation. Now, with this level of technical development, there was not a single person in that area to create a situation in which people would at least be warned about the approaching catastrophe. Those. For some reason nobody is dealing with survival issues, not even someone there, who should, but nobody else, probably a thousand years ago was the same situation. No one knows how sharply the science of the Earth will be able to soar, if not only Ph.D. students, but at least 1% of schoolchildren will be plunged into such knowledge. Would you like to have a computer at your time at school, a satellite reception complex? Probably, even now half of you are dreaming, if not of a telescope, then of communication with satellites, the reception of some completely new information for the mind. With the most modest desire, you can implement technology in education . But we all know that in general they are now being introduced into education.
Fig.2. HRPT format.
Immediate benefit to mankind from the pictures, except the colossal scientific, is very versatile:
• the photographs show various fires, their centers, the direction of propagation, this information is formed in the relevant fire services as soon as the satellite has flown
• in the countryside on the expanses of our Motherland, according to the color information of the fields, it is possible to determine the degree of ripening of crops, monitor harvest and the amount of snow in winter
• monitoring of floods and ice conditions is also carried out, so a couple of years ago it was possible to observe the frozen Sea of ​​Azov and ice drift. And in the White Sea, knowledge of the ice situation is vital for shipping.
• in the world there are countless natural phenomena, besides the observation itself, it is possible to assess the degree and scale of their influence.
The age of computerization so far walks around the planet in the form of the economy of sales of household appliances and electronic toys with glass, which you can touch with your finger. If you look at it from the outside, it can even become a little embarrassing, as humanity, having received such advanced technologies, uses them. On the other hand, in the scale of public services, there are global projects, but the example of our space industry also becomes a little embarrassing when you see that rockets built for public money and satellites flying on them that collect information belong, as it were, not to us, but to narrow closed a circle of people, which is almost impossible to get for a complex of reasons. A mere mortal Russian can see a normal space image only due to foreign sites of those countries where at least some participation in educational processes is provided for this, and in real time you can only see a picture with us. The situation is such that while the benefit of any meteorological information received in a timely manner may be comparable to the benefits of Yandex traffic jams, those government services that are directly involved in receiving images consider it inappropriate to upload pictures to the public. Those. they exist, build rockets and satellites at our expense, and then they offer us to buy our own pictures from them.
Now we all have the opportunity to work constructively in parallel to the state. It is necessary to open new receiving stations, develop, manufacture and install new equipment, write new programs to expand the coverage of the earth's surface. The other day we agreed to cooperate with an enthusiast from Japan, who also has his own receiving station, now we get his source data, process them and publish images from Baikal to Kamchatka. Among the users of our site there are many residents of the Far East. Thus, the coverage of photo data has expanded from Lisbon to Vladivostok and even further. Now absolutely everything: the manufacture of equipment, operation, maintenance of the receiving station, the work of the operator is not financed by anyone or anything - everything is done at our own expense. We do it for free and openly. For the maintenance and sustainable development of the project, interested people and funding are needed.
We have pictures of clouds in the Black Sea and the Caucasus of those days when thousands of people died in Krymsk. If at least 10 people in the country were genuinely interested in such phenomena, starting from school or from a conscious age, it is quite possible that hazardous phenomena (HL) would be predicted not just as “HL”, but “the level of flooding in some places will be more than 2 meters”. By the way, the weather service, according to the most pessimistic forecasts, gave up to 100mm of precipitation that night, but actually it was under 300mm. Here you have the effectiveness of the approach of hiding weather data, albeit not physical, but ideological.
According to the World Meteorological Organization (WMO) open sky concept, meteorological information is distributed free of charge, and even you can receive a non-encrypted satellite signal in real time while you are flying over the horizon. Now they are constantly flying around the Earth and transmitting an image at a frequency of 137 MHz: NOAA15, NOAA18, NOAA19 satellites, at 1.7 GHz frequency: NOAA15, NOAA16, NOAA18, NOAA19, MetopA, MetopB, FENGYUN, Meteor-M1, at 8 GHz: Terra, Aqua, Aura and Calipso.
Fig.1. APT format.
Formats.
The AVHRR (Advanced Very High Resolution Radiometer) on the NOAA satellite is a scanner with a resolution of 1 km / pixel, it forms an image in 5 IR channels. The Russian MSU-MR (Small-Resolution Multichannel Scanning Device) on the Meteor-M1 satellite forms 6 channels. On Terra and Aqua, a MODIS (MODERATE RESULTING Imaging Spectroradiometer) device scans in 36 spectral channels with 12-bit resolution in the visible, near, middle and thermal infrared bands.
')
At a frequency of 137 MHz in the analog format ( APT ) a picture with a resolution of 4 km / pixel is transmitted, consisting of two IR channels, obtained as a result of geometric correction of perspective distortions and reduction of scale. At a frequency of 1.7 GHz, the HRPT (High Resolution Picture Transmission) digital format is already used (in NOAA, the Manchester code, and in Metop, the Reed-Solomon code and the Viterbi algorithm ) - 1 km resolution / pixel; at 8 GHz, the digital code is also used for transmission data with a maximum resolution of 250m / pixel.
A data stream in a format generally consists of rows, each of which contains:
• sync bytes
• satellite data
• time stamp for each line of the bitmap
• calibration values
• we need information about the brightness of the pixel in each channel
• The latest formats provide for the transfer of GPS coordinates of the satellite’s position in orbit.
The digital signal is formed by 10-bit bytes (in MODIS 12 bits), i.e. 10 bits per pixel of each channel. The lines go one after another, in accordance with the direction of flight of the photo-sensor, i.e., if the satellite flies from north to south, then the picture is built from top to bottom, and if it returns to us from Antarctica, then before watching, the picture should be turn over The sensor is designed in such a way that the lines are scanned one by one, continuously. He constantly looks at the center of the Earth, and the scanned line is perpendicular to the motion vector, i.e. the satellite is filming nadir. The projection is clear to the eye, but far from cartographic. In order to overlay a map of cities and country borders, one has to use rare, unknown, and unnecessary programs in ordinary life. The orbit on which the weather satellites fly is called sun-synchronous. The objects launched in such an orbit retain their position relative to the Earth and the Sun at each turn, i.e. if the satellite flew over Moscow in the evening, then on the next round it will fly over Europe too in the evening and in general it falls in the evening, and on the back side of the Earth, it means that it always happens in the morning, i.e. The earth actually scrolls beneath it inside its orbit.
Receiving equipment.
Knowing the orbit of a radio transmitter flying in space, it is necessary to calculate its position relative to the receiving antenna. For this, there is the Orbitron program, under which the driver of the rotating device of the directional antenna is written. The input data for the program are: exact time (accuracy 1s) and keplers (Keplerian orbit elements) in TLE format - they are available on the Internet and they need to be updated periodically. The output angles: elevation and azimuth - how much to raise the plate and which way to turn. As long as we accept L-band satellites (1.7 GHz), the angles are adjusted once a second and that is enough. To receive these satellites, you need a plate from 1.5 m in diameter. To ease the weight you can use a mesh antenna. The focus of the parabola is an irradiator with circular polarization, a filter and a down converter that converts the frequency of 1.7 GHz to the frequency of 150 MHz. The radio signal comes from the head to the receiver via a coaxial cable, and then logically decoded by the decoder, which allocates 10-bit sync bytes, synchronizes with the stream, and by USB data gets into the program, which tracks the presence of the useful signal and writes it to a disk. If someone wants to know the details, he can read about PSK modulation, Manchester code, binary NRZ coding. In the computer we have ready-made HRPT Minor Frame Format structures, etc. This is where the hardware ends.
Video 1. Receive data from NOAA satellite.
Software preprocessing.
The data received from different satellites must be re-assembled into a single format so that a universal program can open them. Therefore, the program ( BMsat ), which receives the digit from the decoder, repacks the data on the fly and writes to the disc in such a format so that it can be opened by the HRPT Reader program, which can build color images, overlay the map and save it in BMP or JPEG! This is the second program in the chain, which also needs to be pumped from the Internet orbit files (TLE). So that images obtained in infrared spectra look adequate enough for a blind person in this range of RGB-viewer, i.e. for you, there is a special algorithm named in the False color program. Thus, you should be clear that those meteorological images that are on the Internet are not real colors, but modeled on the basis of strips of the IR spectrum. In general, different channels can serve as the basis for the False color algorithm - here the choice is by operator. The functionality of the program still includes the possibility of rectifying perspective distortions. Those pixels that were scanned from the Earth's surface directly from under the satellite display 1km, and those that are closer to the horizon cover several kilometers, and that the Earth seemed not round, but flat, like on maps, you can pull the edges of the picture wide. At the same time, the image quality suffers, the size of the JPG file grows, an even smaller fragment is placed on the screen. But one way or another, everything is sent via FTP to the site, and you already know everything about it yourself. Is it possible to mention the splitting of a large picture into tiles (small squares) that would be downloaded for those places that the site user is looking at. How this is done in Yandex and Google maps.
Video 2. Data processing from meteorological satellite.
The equipment and programs that are used for reception, starting with two-axis turns, ending with HRPT Reader, are very specific and are not on the shelves in stores. All this should be developed on our own, using their experience and the experience of those who are already engaged in it, or pay for the development of specialists. In general, any equipment, of course, already exists, but its prices are space. Thanks to ebay, you can try to buy a retired copy of one or another device at your own peril and risk, try to fix it and modify it. To be able to receive 8 GHz satellites with a resolution of 250m / pixel, you need to increase:
• positioning accuracy of a parabolic antenna up to 10 '(10 angular minutes)
• positioning frequency - the plate should move smoothly
• diameter up to 3m
In addition, it is necessary to completely replace the hardware. Switch to other frequencies, etc. The images generated by such satellites are no longer line by line formed, but in strips of appreciable width, therefore it is not possible to look at them normally, since perspective distortions duplicate pieces of images close to the horizon. Those. here, too, there is a mathematical problem that must be solved.
To expand the functionality of the project “Photo from a satellite in real time” (it is presented on the website www.meteosputnik.ru ) it is necessary to solve various tasks, for example, converting a projection into Mercator , merging our services with others, for example, connecting to a free SAS program map .Planet, installation of new equipment and the opening of new receiving stations to increase the quantity and quality of images. To speed up this and everything else, it would be nice to expand the group of enthusiasts who are ready to participate in the project for free and find sponsors.
Our project currently provides open access to many unique and useful data. Its meaning is very many-sided. Predicting cloudiness and weather changes is sometimes much easier and more interesting when looking at weather images than at forecasts of various weather websites. You can also understand whether you get to the center of the cyclone, to the thunderstorm front, or the weather will change smoothly in a few days. In particular, one of my friends used these images three years ago, when everything was in smoke from forest fires, to make a decision where to leave the Moscow region for the weekend: in the pictures it was clear that transparent air began then in Belarus. But this is an example of short-term use of technology. The data we obtained are used in teaching students of meteorological faculties. It is very important to understand that this data is not just operational data, but a unique area of ​​technology. Such things may interest schoolchildren and stimulate them to learn at once in several subjects, besides physics, knowledge about nature, atmospheric, water processes, meteorology, geography, astronomy, geometry, computer science and radio electronics is included. You can try to test your knowledge of geography by mentally presenting the current state of the dried-up Aral Sea and comparing it with a picture from space. You feel the difference between theoretical and practical knowledge, would it be possible to simply take and look at the Aral Sea, being content with rare printouts from textbooks that are not present and with old maps? Or remember the tragedy in Krymsk - a city where, at one time, with the participation of D.I. Mendeleev opened the first oil-producing installation. Now, with this level of technical development, there was not a single person in that area to create a situation in which people would at least be warned about the approaching catastrophe. Those. For some reason nobody is dealing with survival issues, not even someone there, who should, but nobody else, probably a thousand years ago was the same situation. No one knows how sharply the science of the Earth will be able to soar, if not only Ph.D. students, but at least 1% of schoolchildren will be plunged into such knowledge. Would you like to have a computer at your time at school, a satellite reception complex? Probably, even now half of you are dreaming, if not of a telescope, then of communication with satellites, the reception of some completely new information for the mind. With the most modest desire, you can implement technology in education . But we all know that in general they are now being introduced into education.
Fig.2. HRPT format.
Immediate benefit to mankind from the pictures, except the colossal scientific, is very versatile:
• the photographs show various fires, their centers, the direction of propagation, this information is formed in the relevant fire services as soon as the satellite has flown
• in the countryside on the expanses of our Motherland, according to the color information of the fields, it is possible to determine the degree of ripening of crops, monitor harvest and the amount of snow in winter
• monitoring of floods and ice conditions is also carried out, so a couple of years ago it was possible to observe the frozen Sea of ​​Azov and ice drift. And in the White Sea, knowledge of the ice situation is vital for shipping.
• in the world there are countless natural phenomena, besides the observation itself, it is possible to assess the degree and scale of their influence.
The age of computerization so far walks around the planet in the form of the economy of sales of household appliances and electronic toys with glass, which you can touch with your finger. If you look at it from the outside, it can even become a little embarrassing, as humanity, having received such advanced technologies, uses them. On the other hand, in the scale of public services, there are global projects, but the example of our space industry also becomes a little embarrassing when you see that rockets built for public money and satellites flying on them that collect information belong, as it were, not to us, but to narrow closed a circle of people, which is almost impossible to get for a complex of reasons. A mere mortal Russian can see a normal space image only due to foreign sites of those countries where at least some participation in educational processes is provided for this, and in real time you can only see a picture with us. The situation is such that while the benefit of any meteorological information received in a timely manner may be comparable to the benefits of Yandex traffic jams, those government services that are directly involved in receiving images consider it inappropriate to upload pictures to the public. Those. they exist, build rockets and satellites at our expense, and then they offer us to buy our own pictures from them.
Now we all have the opportunity to work constructively in parallel to the state. It is necessary to open new receiving stations, develop, manufacture and install new equipment, write new programs to expand the coverage of the earth's surface. The other day we agreed to cooperate with an enthusiast from Japan, who also has his own receiving station, now we get his source data, process them and publish images from Baikal to Kamchatka. Among the users of our site there are many residents of the Far East. Thus, the coverage of photo data has expanded from Lisbon to Vladivostok and even further. Now absolutely everything: the manufacture of equipment, operation, maintenance of the receiving station, the work of the operator is not financed by anyone or anything - everything is done at our own expense. We do it for free and openly. For the maintenance and sustainable development of the project, interested people and funding are needed.
We have pictures of clouds in the Black Sea and the Caucasus of those days when thousands of people died in Krymsk. If at least 10 people in the country were genuinely interested in such phenomena, starting from school or from a conscious age, it is quite possible that hazardous phenomena (HL) would be predicted not just as “HL”, but “the level of flooding in some places will be more than 2 meters”. By the way, the weather service, according to the most pessimistic forecasts, gave up to 100mm of precipitation that night, but actually it was under 300mm. Here you have the effectiveness of the approach of hiding weather data, albeit not physical, but ideological.
Source: https://habr.com/ru/post/169873/
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