Looking at earth from space...

Satellites NOAA POES ( NOAA POES (National Oceanic and Atmospheric Administration Polar Operational Environmental Satellites, USA ) and METEOR, OKEAN, RESURS (Russia) are the focus in this article. They are flying on polar orbits around the Earth at the distance of approximately 800 - 1200 kilometres passing over the same place at approximately same time every day [23]. Satellites pass the North or South pole on each orbit, that is why their orbits are called polar.

Satellite constellations [ movie_1 &  movie_2  wav_1 ]
 

It is possible to determine their trajectory precisely using "Keplerian elements", which describe the current orbit of the given satellite. Calculation of the exact time of a satellites orbit, from the moment when it appears at the horizon till the moment when it disappears behind the horizon, can be made nowadays using many programs for personal computers. I most frequently use a simple Windows program called SatWin [10, 24]. A version of SatWin was also written for MS-DOS and can be run on older personal computers of the DX486 type. Both these programs can be downloaded free of charge together with up-to-date Keplerian elements at the following address: http://www.emgo.cz/ . You will also find other information about the activities of satellites plus the signals that you can receive and decode using the receiver described in the following article.

Pictures are transmitted continuously from polar satellites without beginning or ending.

When the satellite appears over the horizon, the edge of the pictures is slightly cramped, gradually resolution of details in the picture improves. At the end of orbit the signal gets weaker and the picture begins to disappear in noise as the satellite slips behind the skyline.
 

Inclination is the angle made by the plane of satellites orbit and equatorial plane. A satellite that passes over both poles (on so called polar orbit) has the inclination of 90°. The inclination of American satellites NOAA 10-18 is 98°, their period is approximately 102 minutes and height of satellite is approximately 820 -850 kilometres.

Signals from the satellites are in WEFAX - APT format (Weather Faximile - Automatic Picture Transmission). This is an old, but still useful, system for transmission of black and white visual information using a standard audio channel where a change of amplitude of the 2400Hz sub carrier represents the level of the video signal brightness. Maximum modulation (black) is not zero, but approximately 5%, white is then approximately 87%. This audio signal is frequency modulated on the main carrier, e.g. 137.50MHz for the satellite NOAA 15.

APT is an analogue format: information is transmitted by means of AM/FM modulation of the carrier frequency in 137 MHz radio wave band with a bandwidth of about 40 KHz. Transmitted in the APT format are data of only two channels, with lowered spatial (to 3.5 km) and radiometric (to 8 bits) resolution.

After demodulation by the FM receiver we therefore obtain an amplitude modulated tone of 2400Hz. This signal is sent to the input of standard sound card in a PC and processed by a software decoder such as JVComm32 which can be downloaded from http://www.jvcomm.de/ . JVComm32 even handles bad quality demodulated signals due to the efficient digital filters.

The result of this processing is shown in Fig as picture displayed on a computer monitor.

APT (Automatic Picture Transmission) format
 

Transmission of images from NOAA satellites are composed of lines lasting 0.5 second, which correspond with data from sensors. They provide one picture of the Earth surface containing data from two channels. Channel A transmits picture in the visible spectrum (VIS) and channel B transmits picture in the infrared spectrum (IR). Each line contains time multiplexed data from both channels and is composed of separation tones interlaced with picture modulation. Data from channel A is preceded by and short impulse of 1040Hz and similarly data from channel B are preceded by and short impulse of 832Hz. Each line also contains a calibration sequence. Thanks to this the decoding program can display only the chosen type of picture. You will find more detailed information at http://www.noaa.gov/  .

You will find up-to-date information about Russian satellites METEOR, OKEAN, RESURS at http://sputnik.infospace.ru/  . These satellites have higher orbit than that of NOAA satellites (1200 km). For example inclination of satellites METEOR is 82° and their period is 115 min. The system of picture transmission from METEOR satellites is compatible, however slightly different, from that of NOAA satellites. Modulation is similar, but pictures contain only one photo with higher resolution. Edges of lines contain sets of phasing lines (alternately black and white), the lines mark end of picture and greyscale. Pictures in the infrared spectrum do not contain the greyscale. The pictures are also inverted as in comparison with NOAA pictures. Photos from NOAA satellites show warmer places by darker shade and colder places are brighter. The pictures from METEOR use inverse scale warm seas are white and cold cloud formations are black.

Summary of Russian Meteorological METEOR Satellites

Meteor 2-21
This is the last of the second generation Meteor satellite series. This and similar satellites are in a non sun-synchronous orbit, so the visible channel imagery is useful for only a portion of the year.

This spacecraft transmits on the 137.850 MHz channel with normal 120 line per minute visible mode APT. It can alternately transmit on 137.400 MHz. It continues to show very decreased signal strength during the approaching part of each orbital pass. This would be consistent with a improperly deployed VHF antenna or perhaps some damage to the antenna during launch. The second part of each pass (as the spacecraft passes away from the observer) has a much stronger signal, but is still weaker when compared to Meteor 3-5 or any of the NOAA series spacecraft.

Currently the spacecraft is in an orbital plane with early morning and mid to late afternoon passes.

Meteor 3-5
This satellite, launched in 1991, is in a slightly higher orbit than Meteor 2-21. It transmits on 137.300 MHz. Mechanically, it is similar to Meteor 2-21. Which satellite is in operation depends on the sun angles and consequently the seasons. Meteor 3-5 is usually the (Northern Hemisphere) "summer" satellite while 2-21 is in operation for approximately the half-year centered on winter.

Information about the APT operating schedule and frequencies of the Meteor 2-21 and 3-5 satellites can usually be found on postings to the WXSAT-L list. See our links page for more information about subscribing to WXSAT-L.

The Meteor-3M series of satellites is to be an advanced series of polar orbiters with one 1.4 km resolution visible channel and a ten-channel radiometer with 3 km resolution. An APT transmission was planned to only have a reduced resolution (2 km) visible channel data. The first of these, Meteor 3M-N1, was launched in December 2001. The status of any APT capability on this satellite is unclear, but it is thought not to have an APT transmitter. No APT transmissions have been received from this satellite.

Russia also launched the RESURS 01-4 satellite in July 1998. This satellite carries a meteorological package similar to what was planned for the 3M series, and may be a cost saving effort to have these capabilities without launching a separate satellite. APT-like transmissions from RESURS have been heard on 137.40 MHz.

OKEAN 4 and SICH 1 are radar imaging satellites that have been monitored as they dump recorded image data. Both satellites have a transmission frequency of 137.400 MHz. Reception has usually only been within Europe.

More information about these satellites can be found at the "Sputnik" site in Russia. (You will leave the NOAASIS web site and NOAA domain when following this link)

It is also possible to decode visual information from the receiver any time. To do this it is necessary to save the received modulated signal as a WAV sound file on a high quality recorder (we had the best results with SONY Minidisk). If you take holidays in distant countries, it is recommended that you use a portable and easily mounted Quadrifillar Helix antenna, take the receiver described below and a Minidisk. During your trip you can record exotic pictures from any of the meteorological satellites. When you return you can decode the saved WAV sound files in the same manner as during direct reception
 

Description of the receiver RX-134(7)-141MHz

The RX137141MHz receiver has already been produced since 1998 and I have been gradually implementing various improvements of its technical parameters. The most recent modification of the service properties of the receiver consists in a possibility to connect it to a PC via the RS232 interface. This modification was initiated by the interest of the WXtoIMG decoding program http://www.wxtoimg.com/hardware/  users. The WXtoIMG program has outstanding properties for decoding of the APT signals and it enables a completely automated operation and optionally also an automatic sending of the decoded image to the chosen web page. The WXtoIMG program sends a special code into the RX134141MHz receiver shortly before the rise of the satellite to the horizon, which will trigger the setting of the RX134141MHz receiver to one of the following ten frequencies ( 137.20 .. 137.10 .. 137.40 .. 137.50 .. 137.62 .. 137.9125 .. 137.30 .. 137.70 .. 137.80 .. 137.85 MHz ). The process is continuously repeated before each flyover of the satellite in the cycle, which is updated from the database of the current Keplerian elements. The selection of the frequency for the given satellite is substantially more reliable at present than it was in older models of this receiver, with the use of SQL SCAN or PLL 2400 Hz SCAN with the LM567circuit. The impact of interfering signals on the receiver’s input that afterwards caused a process of undesired repeated scanning never occurs now. 

At present I have started the production of a new model of the RX134141MHz receiver with an extended range that was inspired by the needs of users of the converter 1700MHz made in Italy, the output frequencies of which are 134.00 a 137.50 MHz. The new model of the receiver embodies all good properties, which were confirmed during a six-year-long practical use by numerous users in many countries.

The receiver RX-134(7)-141 MHz is designed to receive images of the Earth surface from polar-orbiting satellites in APT (Automatic Picture Transmission) format in 137 MHz band. The receiver RX-134(7)-141 MHz has been designed for high quality reception of signals form polar meteosatellites NOAA, METEOR, OKEAN and others. It is compatible with the converter from 1691.00 MHz to 137.50MHz which is suitable for reception from geostationary satellite METEOSAT 7.

Table: Satellite Status Information ( NOAA Operational Satellites )

Satellite Transmission Frequencies Notes

Not all the satellites given are always active. Some of them are still flying on polar orbits, but their transmitters have been switched off. Some others do not transmit due to a failure, e.g. the modern satellite NOAA 16 only transmits in the mode HRPT at the frequency 1.698 GHz due to a defect. This is the fate of all artificial satellites, when they fail they can only be repaired using very costly methods. Not all the satellites are as important as the Hubble space telescope, which was repaired by the space shuttle that we watched with excitement and admiration. See http://noaasis.noaa.gov/NOAASIS/ml/status.html

NOAA-N  (NOAA-18) Rockets Into Orbit...

Looking at the Table 1, you will find that satellites in polar orbits transmit signals in the range of 137.10-137.62MHz, therefore a very narrow frequency range is sufficient. We have chosen, for practical reasons, a lower frequency of 134.00MHz and an upper frequency of 141MHz. No meteorological satellites transmit at frequencies above 137.85MHz but the frequency of 141MHz will make it possible to use the converter for the METEOSAT 7 satellite. This makes it possible to process information from both channels, the first channel (1691.00MHz) converts to 137.50MHz and the second channel (1694.50MHz) converts to 141.00MHz.