GW7ERI AMATEUR RADIO

TUTSHILL - CHEPSTOW - MONMOUTHSHIRE

UNITED KINGDOM

IO81QP  -  51.38.22N  -  02.39.48W

 

 

 

 

 


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Weather Satellites

 

 

Back in the early 1990's I decided to experiment with receiving images transmitted from space. 

'Low orbiting' weather satellites orbit the earth from North to South poles, continuously transmitting pictures one line at a time, in a similar fashion to sending a document by 'fax.'  

The satellites are in a sun synchronous orbit, and therefore will cross the equator at a similar time each day. As the earth rotates between each successive orbit, a new 'slice' of planet Earth is photographed, and the photographic data is sent back to stations on the ground.

The signals are transmitted on frequencies between 137 to 138 Mhz in 'wefax' format and can be received and decoded using a combination of radio and computer equipment.

The radio (rf) signal can be picked up by a simple scanner type radio - this was how my first attempts were made. Whilst the signals can be detected on a scanner, the satellite transmission has a bandwidth of some 50 Khz, in order to carry the maximum amount of picture information from the satellite camera. Typical scanner receivers have a maximum bandwidth in the order of 15-20 Khz, so it can easily be seen how much picture data is lost when using this type of radio receiver.  

A further problem can occur if using a standard vertical antenna. The signal transmitted from the satellite is low power, about 10 watts, and is circularly polarized.  A vertical antenna can provide a picture, but will result in a lower signal strength at the receiver, and therefore produce lower quality images.

With this in mind, I decided to build a circularly polarized crossed dipole, available from Maplin Elecronics here in the UK at the time. The picture below shows the antenna under test at the time of writing. The wooden mast has been pushed into the ground for testing, but will be permanently mounted in a higher location on a proper bracket.

Unfortunately these antennas are no longer available, but suitable alternatives can be found by following the links at the bottom of this page.

The audio output from the radio was connected to an interface unit and sent to the serial port of a '386 type computer running JVFax decoding software. JVFax decoded the signal and displayed a picture on the screen!

The initial results were encouraging, with some decent quality images received but work and family commitments, forever getting in the way of hobbies, saw the project discontinued about a year or so later. 

The antenna lived in my garden shed until September 2008, when work on this web site gave me the idea of resurrecting the weather satellite station.

As the years have passed improvements have been made in both radio receiver devices and computer hardware/software. I decided to invest in a dedicated weather satellite receiver, and following a bit of searching and reading, settled upon the R2ZX pictured below.

This tiny radio (113x85x31mm) runs from the main 12 volt power supply, but can be powered from the USB port on the computer if needed. The radio scans the 5 pre-programmed channels, and stops when a satellite signal is encountered. A sixth channel exists for the reception of geo-stationary satellite signals via a down converter.

After looking at various software decoding packages, I decided upon WXTOIMG. This software allows the audio output from the R2ZX to be fed to the input of the computer sound card, where it is captured on disk, decoded, displayed on screen, enhanced, and automatically uploaded to the internet.

The images produced by this system can be found here in the weather section of my site, along with a list of 'pass' times for satellites predicted for the Chepstow area.

Further information on orbiting weather satellites can be found at:

http://noaasis.noaa.gov/NOAASIS/ml/genlsatl.html

For information on the R2ZX receiver link to:

http://www.df2fq.de/produkte/Catalog-Engl.pdf

   Information on the WXTOIMG decoding software is at:

http://www.wxtoimg.com/

Antenna and radio products can be found at:

http://www.geo-web.org.uk/