WEFAX stands for Weather Facsimile and is similar to other types of fax transmissions. There are currently three different countries that transmit WEFAX, these are the US (GOES), European (Meteosat) and Japanese (GMS ) satellites.
WEFAX is a way of getting monochrome analogue picture information through a standard voice audio channel. The signal varies rapidly in
frequency and is sampled from a few hundred times per second to a few thousand times per second depending on the type of WEFAX transmitted.
The varying tones correspond to varying shades of gray that the satellite sees as it scans the earth.
The earth is scanned every half-hour where the raw data is transmitted to a receiver station requiring a 60' dish with sophisticated computing equipment.
The data is reformatted in real time with political boundaries added and transmitted to the satellite where it is retransmitted back to earth at 1691 MHz. A 1691 MHz down converter and a small dish antenna are required to receive WEFAX.
The WEFAX images received are cut into 800 by 800 pixel sections and annotated. The 800 lines of an image each take 250 ms to transmit; hence a whole picture takes about three and a half minutes to receive. A schedule is published detailing what pictures are transmitted at which times and on what channel
Meteorological satellite radiometers measure outgoing radiation through broad spectral intervals called atmospheric windows. Radiation passes here without severe attenuation by the intervening atmosphere.
Weather satellites do not have sensors that cover the visual spectrum to produce true colour images. They have multiple image sensors onboard that covers the red portion of the spectrum (visual) to the Infrared (IR).
Visible images are produced by reflected solar radiation that directly illuminates the earth; this is only available for daytime weather watching.
Visible satellite images provide information about the observed cloud cover.
Areas of white indicate clouds while shades of gray indicate clear skies, this is because thicker clouds have a higher reflectivity and appear brighter on a visible image than thinner clouds
IR imaging is needed, as weather watching is essential day and night. Colour enhanced infrared satellite images are measurements of temperature thus only the differences in temperatures are visible. In an IR image darker is warmer and lighter is colder. Low clouds tend to be warmer than higher clouds. Most satellite images on TV are IR.
Water vapor images are useful for pointing out regions of moist and dry air.
This provides information about the swirling troposphere's wind patterns and jet streams. Darker colours indicate drier air while moisture in the air is seen as white.
5. Geostationary Satellites (APT-WEFAX/PDUS)
The European Geostationary satellite currently transmitting to us is Meteosat 7.
This satellite orbits the Earth at approximately 36,000 kilometres above the Earths surface and appears to be stationary over a specific point.
The Meteosat satellite is at 0° longitude above the equator and sees the same view of the globe as this.
From the point of working with satellite data, data from Geostationary satellites is superior in that it can be received according to a timetable where a new image is transmitted every 4 minutes.
Every half an hour the globe is scanned and the data is split in to smaller blocks and sent down according to a dissemination schedule.
Images covering the European segment are sent down every half an hour.
Because the satellite is stationary it is possible to create an animation of images as they are received and here explore the movement of weather systems.
These images of the entire visible half of the globe are transmitted on Channel 2 These are called Total World images - coded as CTOT, DTOT and ETOT respectively.
The Total World images are cut into 9 segments for transmission at higher resolutions.
The segments are transmitted every 4 minutes on Meteosats Channel 1 frequency
These are coded as C1; C2 ... ; D1; D2 ... ; E1; E2 ... for Visible, Infra Red and Water Vapour images respectively.
In order to receive data from Meteosat you will need to have a fixed dish antenna mounted with a clear facing aspect to 0 deg.
This is connected to a receiver and then to a PC running a suitable APT software package.
Geostationary satellites can see each other and so can transmit data from one to another.
From Meteosat we can also receive retransmitted data from the Russian satellite GOMS, the Japanese satellite GMS and the American satellite GOES East which gives us coverage around the world.
Geostationary satellites provide the kind of continuous monitoring necessary for intensive data analysis. The images produced by these satellites are processed in real time, distributed in digital form to forecasting centres for archival and to the media for weather forecasts.
The majority of the geostationary weather satellites have the primary mission of imaging their hemisphere. The satellites spin on their axes at approximately 100 revolutions per minute (r/min), providing the horizontal scanning, while a motorised mirror with a period of approximately 20 minutes is used to provide the vertical scanning.
This data is relayed back to earth in a very high-density digital format that requires specialised equipment for reception and display. High-
speed computers process this image data on the ground; these provide two different functions. First, the original data is retransmitted in a 'stretched' digital format.
Secondly, they sector the data into individual quadrants, then relay these images in analogue form back through the satellite as part of the Weather Facsimile (WEFAX) program.
The primary instruments on board are the Imager and the Sounder. The Imager senses radiant energy and reflected solar energy from the earth's
surface and atmosphere.The sounder provides data for vertical atmospheric temperature and moisture profiles, surface and cloud top temperature, and ozone distribution.
Imagery obtained by these satellites are used to track cold fronts, to monitor severe convective storms, to identify and track volcanic ash clouds, to derive winds from the observed motion of clouds, to estimate rainfall during thunderstorms and hurricanes for flash flood warnings. They are also used to estimate snowfall accumulations and the overall extent of snow cover.
Of immediate importance to Australia is Japan's current operational satellite, this being Japan's Geostationary Meteorological Satellite (GMS) series. Under a bilateral agreement with Japan, Australia has special supplementary real time access to GMS imagery and products.
Japan runs GMS-5, located at 140 degrees east longitude and covers East Asia, the Western Pacific and Australia.