Meteosat

the WEFAX signal

 
The WEFAX (Weather Facsimile) system is based on the Automatic Picture Transmission (APT) scheme developed for use with the early TIROS and successive NOAA US polar orbiting weather satellites. WEFAX is an analogue signal that extends the basic APT system, adding start, stop and phasing signals as well as a digital header. The standards have been defined and adopted internationally and WEFAX is now used by all meteorological satellite operators.

WEFAX was introduced with the first Meteosat in 1977. WEFAX uses a 2.4 kHz carrier signal modulated with a 1.6 kHz video signal. The signal includes frame start and stop tones and horizontal synchronisation bursts for each raster line. The section below shows one raster line with a horizontal sync burst at each end.

Click on one of the icons below to hear the sound of the satellite tranmsission

Sun Audio .au filePC Windows .wav file

The composite audio signal is frequency modulated (FM) onto 1691.0 MHz (Channel A1) and 1694.5 MHz (Channel A2) carriers. These are the signals received by typical secondary data stations in schools and colleges. Each line of an 800 line image takes 250ms to transmit with each complete picture taking about 3 and a half minutes. Meteosat operates a dissemination schedule, transmitting images at regular intervals. Not all of the four minute slots are used for image transmission. Some are used for digitally encrypted data, for relays of other satellite images (GOES/GMS etc).

The original Meteosat images are made up of pixels with a value between 0 and 255. These original raw values are preserved in the high resolution primary data transmissions. However, high resolution images rarely have pixels that span the entire range from 0 to 255. WEFAX images are modified using software at the Darmstadt central processing site to enhance the contrast by extending the range of pixel values used. This modification makes the secondary images much more useful to direct eye interpretation but does mean that all the original quantitative information is lost. No realistic image processing or measurements can be made from secondary WEFAX images.

A complete WEFAX format image contains:

  • a start signal - 3 seconds
  • a phasing signal - 5 seconds
  • a digital header - 2 lines - 0.5 seconds
  • image data - 800 lines - 200 seconds at 4 lines per second
  • a stop signal - 5 seconds

The start signal consists of a 300Hz tone for 3 seconds. This is followed by a phasing signal containing a 12.5 ms period of black level followed by 237.5ms of white level, repeated to cover a total of 5 seconds. The digital header consists of two lines, each of which contain 800 pixels. The black and white boxes are used to represent data that make up the header and are in addition to any headers superimposed on the actual image itself. The standard digital header contains:
    Satellite name (METn)
    Spectral band (VIS, IR or WV)
    Date (YY) year, (MM) month, (DD) day
    Time (HHmm) hour and minute of the end of image acquisition
    Sector (format name)

The image data is made up of 800 lines. Each line begins with a start signal that enables a secondary data station (SDUS) to activate an automatic gain control for each line. All lines begin with a sequence of two white then two black pixels repeated seven times - simulating an 840Hz frequency. Each line contains a 40 pixel line start pattern followed by 800 pixels of image data. Each transmission line takes 250ms so the complete image of 800 lines takes 200 seconds to transmit.

A stop signal of 450Hz for 5 seconds denotes the end of the WEFAX transmission.

Most of the WEFAX transmitted images are originated from Meteosat itself. Most of these images are transmitted on Channel A1. In addition, images from GOES-E and GMS are retransitted on Channel A2 along with whole Earth disk views from Meteosat (CTOT, DTOT and ETOT formats). Administration messages (ADMIN) and Cloud Top Height analyses (CTH) are also broadcast. CTH images depict the altitude of the highest cloud with values being indicated by grey levels. Black means no cloud above 3000m and white means cloud tops above 12000m.

Other Meteosat information

Acknowledgements:
WEFAX information:
ESA/EUMETSAT
Ross Reynolds, Dept. of Meteorology, University of Reading
Steve Marchant, Nottingham University
St. Vincent College, Gosport

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Page update: January 12th 1999
This version: © St. Vincent College