On the reception of Earth images from meteorological satellites

Currently, about 10 satellites used for meteorological purposes are located on Earth’s orbit. These satellites continuously scan the surface and atmosphere of the Earth and directly dump information on the earth to the appropriate scientific centers, laboratories, and anyone who can receive it. The receiving station, located in the radio-visibility zone of the satellite, sees in real time what the satellite sees. Data from it comes directly at the time of shooting. A hardware image is received not only in the visible spectrum, but also at some infrared frequencies. It’s even more correct to say that all the main channels are infrared, there are much more of them. These channels are much more important for practical purposes, because they can emit water vapor, smoke, heat from forest fires or determine the surface temperature of the planet. With the help of such images, you can even determine the ripening of the crop on collective farm fields. This article talks about the technique of practical reception of images from Earth remote sensing satellites, thanks to which anyone can go to sites like meteosputnik.ru and see those same real satellite images.

According to the concept of “ open skies ” of the World Meteorological Organization (WMO), meteorological information is distributed free of charge, and even you, at your dacha, can receive an unencrypted satellite signal in real time while the satellite flies over the horizon. Now they constantly fly around the Earth and transmit images at a frequency of 137 MHz: satellites NOAA15, NOAA18, NOAA19, at a frequency of 1.7 GHz: NOAA15, NOAA16, NOAA18, NOAA19, MetopA, MetopB, FENGYUN, Meteor-M1, at a frequency of 8 GHz: Terra, Aqua, Aura and Calipso.

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Fig. 1. APT format.

Formats


The AVHRR (Advanced Very High Resolution Radiometer) device 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 (Multi-Channel Low Resolution Scanning Device) on the Meteor-M1 satellite forms 6 channels. On Terra and Aqua, the MODIS device (MODerate resolution Imaging Spectroradiometer) scans in 36 spectral channels with 12-bit resolution in the visible, near, middle and thermal infrared ranges.

At a frequency of 137 MHz in analog format ( APT ), a 4 km / pixel picture is transmitted, consisting of two IR channels, obtained as a result of geometric correction of perspective distortions and zooming out. At a frequency of 1.7 GHz, the digital HRPT format is already used(High Resolution Picture Transmission) (in NOAA - Manchester code, and in Metop - Reed-Solomon code and Viterbi algorithm ) - resolution 1km / pixel, at a frequency of 8GHz a digital code is also used for data transmission with a maximum resolution of 250m / pixel.

The data stream in the format generally consists of lines, each of which contains:
• sync bytes
• satellite data
• exact time stamp for each line of the bitmap
• calibration values
• the information we need about the pixel brightness in each channel
• the latest formats include GPS transmission - the coordinates of the satellite in orbit.

The digital signal is generated in 10-bitbytes (in MODIS 12 bits), i.e. 10 bits per pixel of each channel. Lines go one after another, in accordance with the direction of flight of the photo sensor, that is, 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 you look, the picture must be turn over. The sensor is designed so that the lines are scanned one after another, 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 shooting in the nadir. The projection is clear to the eye, but far from cartographic. In order to map the cities and borders of countries, one has to use rare, unknown and unnecessary programs in everyday life. The orbit in which meteorological satellites fly is called solar-synchronous. Objects launched in such an orbit maintain their position relative to the Earth and the Sun at each turn, i.e. if a satellite flew over Moscow in the evening, then at the next round it will fly over Europe in the evening, too, and generally all the time it falls in the evening, and on the other side of the Earth, that means it always happens in the morning, i.e. Earth actually scrolls beneath it inside its orbit.

Reception 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 rotary 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 must be updated periodically. At the output, the angles are obtained: elevation and azimuth- how much to raise the plate and in which direction to turn. While we are receiving L-band satellites (1.7 GHz), the angles are adjusted once a second, and that’s enough. To receive these satellites, you need a plate from 1.5 m in diameter. A mesh antenna can be used to alleviate weight. A circular polarized irradiator, a filter and a step-down converter that converts a frequency of 1.7 GHz to a frequency of 150 MHz are installed in the focus of the parabola. From the head via a coaxial cable, the radio signal arrives at the receiver, and then it is logically decrypted by a decoder that selects 10-bit sync bytes, synchronizes with the stream, and via USB data gets into a program that tracks the presence of a useful signal and writes it to a file on disk. If someone wants to know the details, they can read about PSK modulation, Manchester code, binary NRZ coding. In the computer we have ready-made HRPT Minor Frame Format structures, etc. This ends the hardware.


Video 1. Receiving data from NOAA satellite.

Software processing.


The data received from different satellites must be re-compiled into some single format so that some universal program can open them. Therefore, the program ( BMsat ) that receives a digit from the decoder, repackages the data on the fly and writes to disk in such a format that HRPT Reader can open it , which can build color pictures, overlay the map and save it in BMP or JPEG! This is the second program in the chain, which also needs to download orbit files (TLE) from the Internet. So that the images obtained in the infrared spectra look adequately enough for a RGB viewer to be blind in this range, i.e. for you, there is a special algorithm called in the program False color. Thus, it should be clear to you that those meteorological images that are on the Internet do not have real colors, but are modeled on the basis of IR strips. In general, different channels can serve as the basis for the False color algorithm - here the choice is made by the operator. The functionality of the program also includes the ability to straighten perspective distortions. Those pixels that are scanned from the Earth’s surface directly from under the satellite display 1 km, and those pixels that are closer to the horizon span several kilometers, and so that the Earth does not appear round, but flat, as on maps, you can stretch the edges of the image in width. 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. Unless you can mention the splitting of a large picture into tiles (small squares), which would be downloaded for those places that the site user is looking at. How is this done in Yandex and Google maps.


Video 2. Processing data from a weather satellite.

The equipment and programs that are used for receiving, starting with two-axis turns, ending with HRPT Reader, are very specific and do not lie on shelves in stores. All this must be developed on their own, using their experience and the experience of those who are already involved in this, or specialists should pay for the development. In general, all equipment, of course, already exists, but the prices are cosmic. Thanks to ebay, you can try to buy a decommissioned copy of one or another device at your own peril and risk, try to fix it and modify it. In order to be able to receive 8 GHz satellites with a resolution of 250 m / pixel, it is necessary to increase:

• the positioning accuracy of the parabolic antenna up to 10 '(10 arc minutes)
• positioning frequency - the plate should move smoothly
• diameter up to 3m

In addition, the hardware must be completely replaced. Switch to other frequencies, etc. Pictures generated by such satellites are no longer formed line by line, but by strips of tangible width, and therefore it is not possible to look at them normally, because perspective distortions duplicate pieces of images close to the horizon. Those. there is also a mathematical problem that will need to be solved.

To expand the functionality of the project “Real-time Satellite Photos” (it is available on the website www.meteosputnik.ru ), it is necessary to solve various problems, for example, converting a projection into a Mercator, merging our services with others, for example, connecting to the free SAS.Planet program card, installing new equipment and opening 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.

Currently, our project provides open access to many unique and useful data. Its meaning is very multifaceted. Predicting cloudiness and weather changes is sometimes much simpler and more interesting looking at weather images than at forecasts of differentweather sites. You can also understand whether you get to the center of the cyclone, to a thunderstorm front or the weather will change smoothly over several days. In particular, a friend of mine used these images three years ago, when everything was in smoke from forest fires, to decide where to leave the Moscow Region for the weekend: it was clear from the images that the transparent air started back then in Belarus. But this is an example of a short-term application of technology. The data obtained by us is used in the training of 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 students and stimulate them to study in several subjects at once, in addition to physics, knowledge about nature, atmospheric, water processes, meteorology, geography, astronomy, geometry, computer science and radio electronics. You can try to test your knowledge of geography by mentally imagining 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 just to take and look at the Aral Sea, content with rare printouts from textbooks that do not exist and ancient maps? Or remember the tragedy in Krymsk - a city where, in due time, with the participation of D.I. Mendeleev's first oil production unit was opened. Now, at this level of technical development, not a single person was found in that locality to create a situation in which people would at least be warned of an approaching disaster. Those. for some reason, no one deals with survival issues, not even someone there who should, but in general no one, probably a thousand years ago, was the same situation. Nobody knows how earth science can soar if not only piece doctors of sciences, but at least 1% of schoolchildren will plunge into such knowledge. Would you like to have a computer, a satellite reception complex at your time at school? Probably, even half of you now dream, if not about a telescope, then about communication with satellites, receiving some completely new information for the mind. With the most modest desire, you can implement technology in and now half of you dream, if not about a telescope, then about communication with satellites, receiving some completely new information for the mind. With the most modest desire, you can implement technology in and now half of you dream, if not about a telescope, then about communication with satellites, receiving some completely new information for the mind. With the most modest desire, you can implement technology ineducation . But we all know that they are currently introducing education into education.

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Fig. 2. HRPT format.

The immediate benefit to mankind from the images, except for the colossal scientific, is very versatile:
• the photos show various fires, their foci, the direction of distribution, this information is generated in the relevant fire services as soon as the satellite flew
• in the countryside in the vast expanses of our homeland by color field information you can determine the degree of ripening of crops, track the harvest and the amount of snow in winter
• Flood and ice conditions are also monitored, 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
• countless natural phenomena occur in the world, besides observing them themselves, you can evaluate the extent and extent of their influence

The age of computerization is still striding 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 side, it can even become a little ashamed, as humanity, having received such advanced technologies, uses them. On the other hand, on the scale of public services, there are global projects, but with the example of our space industry, it 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 a narrow, closed the circle of people into which it’s almost impossible to get into for a variety of reasons. A mere mortal Russian can see a normal space image only thanks to the foreign sites of those countries where at least some participation in educational processes is provided for this, but in real time the picture can only be seen with us. The situation is such that while the benefit to any person of timely received meteorological information can be comparable with the benefit of Yandex traffic jams, those public services that are directly involved in the reception of images, consider uploading pictures in the public domain unreasonable. Those. they exist, they build rockets and satellites at our expense, and then they offer us to buy our pictures from them. consider uploading pictures to open access unreasonable. Those. they exist, they build rockets and satellites at our expense, and then they offer us to buy our pictures from them. consider uploading pictures to open access unreasonable. Those. they exist, they build rockets and satellites at our expense, and then they offer us to buy our pictures from them.

Now we all have the opportunity to work constructively in parallel with the state. It is necessary to open new receiving stations, to develop, manufacture and install new equipment, to write new programs to expand the coverage of the earth's surface. The other day we agreed on cooperation with an enthusiast from Japan, who also has his own receiving station, now we get his source data, process it and publish pictures 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: manufacturing equipment, operation, maintenance of the receiving station premises, the work of the operator is not funded by anyone or anything - everything is done at your own expense. We do this for free and openly.

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 events (OA) would be predicted not just as “OA”, but “the level of flooding in places will be more than 2 meters”. By the way, weather services, according to the most pessimistic forecasts, gave up to 100 mm of rain that night, but actually it was under 300 mm. So much for the effectiveness of the concealment of weather data, albeit not physical, but ideological.

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