Receiving weather images (section 1.)

The author of a series of articles on the reception of NOAA meteorological images is Ondro (zawin), The original article can be found at www.svetelektro.com. TNX!

In this first part, I will focus on the issue of NOAA satellites, which I use to receive meteorological images of the earth. I will focus on their division, Properties, data coding, modulation and transmission frequencies. In other articles, I will focus on the antenna for receiving NOAA satellites and the receiver itself from the point of view of characteristics and construction.
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 (WEATHER 7, GMS-5,GOES-E …)
– orbital satellites with a polar orbit - are in motion relative to the Earth (NOAA, Meteor and more..)
The NOAA group became the subject of my interest (National Oceanic and Atmospheric Administration). These satellites move in polar orbits around the Earth at a distance 800 up to 1200 km, they fly over the same place at about 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 round-the-world time is approx 102 minutes.

We can determine the exact flight time over the given location by calculating from "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 Orbitron program in the version 3.71 (Fig. No. 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 flyover or not and much more.

Giant. no. 1: A sample of the Orbitron program

When transmitting data from NOAA satellites, it contains no beginning or end at our latitudes. Broadcasting takes place for the entire time the satellite is in flight. First of all, when the satellite appears on the horizon , the edge of the received image is noisy and the signal gradually increases. 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.

Active NOAA satellites that transmit data on a frequency of 137 MHz are currently available 4 – NOAA-15, NOAA-17, NOAA-18, NOAA-19. The tone, that these satellites are needed even in the present, the last NOAA-19 satellite that was launched convinced us 6. February 2009.

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 the satellite is in flight. First of all, when the satellite appears on the horizon , the edge of the received image is noisy and the signal gradually increases. 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 of duration 0.5 seconds, corresponding with sensor data. They provide one image of the earth's surface containing data from two channels. On channel A, a picture in the visible part of the spectrum is broadcast (VIS) and on channel B of the image in the infrared part (IR). Each line contains data from both channels (time multiplex) and consists of a sequence of parting tones translated by frame modulation.
Data in channel A is preceded by a short pulse 1040 Hz and similar data in channel B is preceded by a short pulse 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, that reception from the polar satellite has no beginning or end, signal transmission on Earth is continuous. We will receive signals in the form of black and white image information (pseudo-coloring takes place only in the PC), standard audio channel, where the change in the amplitude of the 2400Hz subcarrier expresses the brightness level. The maximum of the subcarrier modulation is defined as the amplitude indicated by the number 8 in gray scale, which corresponds to (87 ± 5)% (must not exceed 92%) modulation and corresponds to the white color in the image. The AM signal created in this way is further modulated onto the main carrier in the band 137 up to 138MHz (amazement FM).

Format APT: The format of the APT image is shown in Fig. 2, and both A and B channels can be seen here, divided into 16 parts, which are used 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 images from both channels. On the picture. 3 already see this APT format “in practice”, as I received it from the satellite 🙂

Giant. No.2: Format APT

Giant. No.2: APT format in practice

Source: www.svetelektro.com