WEFAX image formats


Meteosat images are transmitted in WEFAX format covering the three wavebands detected by the on-board satellite radiometer. The three image bands are coded as:

    C format: 0.4 - 1.1 µm - Visible light band
    D format: 10.5 - 12.5 µm - Thermal infra red band
    E format: 5.7 - 7.1 µm - Infra Red - water vapour absorption band

Details of each image are contained within the image header. A digital header is contained within the image and a header bar is added to the image with plain text superimposed on the image to indicate its origin, date time etc. (as shown).

The standard digital header contains:

    Satellite name METn (currently MET5)
    Day Day of the month
    Month JAN, FEB, MAR etc.
    Year 1996, 1997 etc.
    Time Hour/Minute of the end of original
    image acquisition (0955 etc.)
    Spectral band Spectral channel format letters
    (VIS, IR, WV)
    Segment Image format number
    (C02, C3D, DTOT etc.)

C format

CnD format

These are visible light images from the operational Meteosat and cover the whole Earth disk in 9 boxes labelled C1 to C9 - as in the image above. The C1D, C2D etc. images contain visible data reduced to the sub-satellite 5 km pixel resolution of the infra red images. During the winter months, when some visible images may have little illumination (generally early morning or late afternoon), visible image slots in the C format band may be cancelled. The ADMIN message gives information about cancelled image transmissions.

C format

The C format images are visible formats from Meteosat and cover the Earth using 24 square boxes. However, only C02 and C03 are transmitted. These two cover the North Eastern Atlantic and Western Europe (C02) and Central and Eastern Europe (C03). These image formats are relayed every 30 minutes between 0602 and 1710 UTC.

C02 and C03 images can be transmitted with two resolutions. If both visible light channels on the radiometer have been used (VIS 1+2) then the resolution at sub-satellite point is 2.5 km per pixel. If only one detector was used (VIS 1 or VIS 2) then the resolution is reduced to 5 km. The actual resolution at the northern latitudes is much less than this due to the curvature of the Earth. However, under bright summer illumination, considerable detail may be seen including the Alpine lakes and snow covered mountains, the Isle of Wight etc.

D format

D format images cover the whole Earth disk in the thermal infra red band - in the same 9 boxes as the CnD image segments in the image above. D2 is the only region for which images are transmitted every 30 minutes throughout the day (night-time included). The frequency of other image segments varies - refer to the dissemination schedule.

D format images are inverted by subtracting the value of each pixel from 255. This provides compatibility with the visible light images by showing colder temperatures as white and warm as darker or black shades. The background space appears as white in these formats - background space is colder than the Earth.

E format

The E format images cover the same 9 box areas as C and D formats. Like the thermal D format images, the data is inverted with the result that moist regions appear white and dry regions appear black. This provides some degree of matching with the light grey clouds on the visible and thermal band images. E format images are tranmsitted less frequently than the C and D formats.

CTOT, DTOT and ETOT images

Whole Earth disc images are transmitted on Channel A2 at approximately 3 hourly intervals. These show the whole disc in visible light (CTOT), thermal infra red (DTOT) and water vapour absorption (ETOT). Because the WEFAX image is limited to 800 lines by 800 pixels, the original resolution is impossible to represent. The xTOT images are therefore downgraded to WEFAX standards. However, they are very striking examples of global weather patterns. Downloading and animating a sequence shows the movement of air above and below the equator with fascinating results.

L format

The L format is used for WEFAX image formats which have been received and relayed by CMS Lannion, Brittany, France. LY and LR formats (North and South America respectively) in the thermal band show 8 km resolution at the sub-satellite points. LZ images from the GOES satellite show eastern North America in the visible band with a 1 km sub-satellite resolution.

GMSn format

The Japanese GMS (Geosynchronous Meteorological Satellite) stationed above 140°W relays images of the Far East and Australasia. The images are received in Hawaii and then relayed to CMS Lannion. The images are relayed at regular intervals - divided into two sections, GMS North (GMSN) and GMS South (GMSS). The Northern image shows conditions from India, across Malaysia and the Philippines to Japan. The Southern image shows Papua New Guinea, Australia and New Zealand.

WEFAX information:
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