Ondro (zawin) is the author of the series of articles on the reception of NOAA meteo images, the original article can be found at www.svetelektro.com. Thx!
V tejto prvej časti sa budem venovať problematike satelitov NOAA, ktoré využívam na prijem meteorologických snímkov zeme. Zameriam sa na ich rozdelenie, vlastnosti, kódovanie dát, moduláciu a vysielacie frekvencie. V dalších článkoch sa budem venovať anténe na príjem NOAA satelitov a samotný príjimač z hladiska charateristiky a konštrukcie.
I hope that the issue will interest you and expand your horizons of electrical engineering 🙂
Distribution and reception of satellites
The area of satellites that interested me are the so-called WXSAT (Weather satellite - satellites for weather monitoring). These satellites can be divided into 2 basic groups:
– Geostationary – they do not change their position relative to the Earth (METEOSAT 7, GMS-5, GOES-E ...)
– orbital satellites with a polar orbit - are in motion relative to the Earth (NOAA, Meteor and others..)
The NOAA (National Oceanic and Atmospheric Administration) group became the subject of my interest. These satellites move in polar orbits around the Earth at a distance of 800 to 1200 km, they fly over the same place at approximately the same time every day. Each time they go around the earth, they pass close by the north and south fields – hence the name polar poles. The time to go around the entire globe is approximately 102 minutes.
We can determine the exact flight time over a given location by calculating the "Keplerian elements", which describe the current path of the selected satellite. Today, a number of PC programs are used to calculate the flight and the current position of the satellites. I myself prefer to use the program Orbitron in version 3.71 (fig. 1). It displays the current position of the satellites and can also predict their next orbits, further determine whether the satellite will be illuminated during the flyby or not, and much more.
fig. No. 1: Sample of the Orbitron program
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When transmitting data from NOAA satellites, it contains no beginning or end at our latitudes. Broadcasting takes place for the entire time of the satellite's flight. First, when the satellite appears on the horizon, the edge of the received image is noisy and gradually the signal grows. At the end, when the satellite falls below the horizon, the signal gradually weakens until it is completely lost. The duration of the flight over our location is approximately 10 minutes.
There are currently 4 active NOAA satellites that transmit data at a frequency of 137MHz - NOAA-15, NOAA-17, NOAA-18, NOAA-19. The last NOAA-19 satellite, which was launched on February 6, 2009, showed us that these satellites are still needed today.
The broadcasting frequencies of the satellites are as follows:
NOAA 15 137.50 MHz
NOAA 17 137.62 MHz
NOAA 18 137.10 MHz
NOAA 19 137.9125 MHz
When transmitting data from NOAA satellites, it contains no beginning or end at our latitudes. Broadcasting takes place for the entire time of the satellite's flight. First, when the satellite appears on the horizon, the edge of the received image is noisy and gradually the signal grows. At the end, when the satellite falls below the horizon, the signal gradually weakens until it is completely lost. The duration of the flight over our location is approximately 10 minutes.
Modulation and data format
Broadcast images from NOAA satellites consist of lines lasting 0.5 seconds, corresponding to sensor data. They provide one image of the earth's surface containing data from two channels. On channel A, an image in the visible part of the spectrum (VIS) is broadcast, and on channel B, an image in the infrared part (IR). Each line contains data from both channels (time multiplex) and consists of a sequence of separator tones translated by frame modulation.
Data in channel A is preceded by a short pulse of 1040 Hz and similarly data in channel B is preceded by a short pulse of 832 Hz. Each row also contains a calibration sequence. Thanks to this, the software used for decoding can then display only the selected type of image or synchronize the image to the edge of the screen.
The polar satellite uses APT type modulation and geostationary WEFAX. These encoding methods are very similar. The only significant difference is that the reception from the polar satellite has no beginning or end, the transmission of the signal to the Earth is continuous. We will receive the signals in the form of black-and-white image information (pseudo-coloring takes place only in the PC), through a standard audio channel, where the change in the amplitude of the subcarrier 2400Hz expresses the level of brightness. The maximum of the subcarrier modulation is defined as the amplitude indicated by the number 8 on the gray scale, which corresponds to (87 ± 5)% (must not exceed 92%) of the modulation and corresponds to the white color in the image. The AM signal created in this way is further modulated to the main carrier in the 137 to 138 MHz band (FM band).
APT format: The APT image format is shown in Fig. 2, and both A and B channels can be seen here, divided into 16 parts, which serve to calibrate the image. Each part is composed of eight consecutive image lines. So let's note that parts 1 to 14 are identical in the images from both channels. In fig. 3 already see this APT format "in practice", as I received it from the satellite 🙂
fig. No. 2: APT format
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fig. No. 2: APT format in practice
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Source: www.svetelektro.com