'WEFAX'
There are two classes of meteorological satellites operated by five different civilian agencies around the world and these operate in both geostationary and polar orbits.
The geostationary satellites are in fixed locations spaced around the equator at an altitude of about 36,000 km. Each is capable of making an image of the full earth disc, in several spectral bands, within a period of 30 minutes or less and with a primary role of providing cloud imagery.
The polar satellites on the other hand circle the earth every 100 minutes at an altitude of about 850 km. The orbits are sun-synchronous and arranged in such a way that the daily rotation of the earth ensures that a successive satellite orbit covers a swathe slightly to the west of the previous track. This means that each satellite can provide images and other data covering the entire earth twice a day. The NOAA satellite system is provided by the USA , with some instrumentation and facilities from Canada, France and the UK. These satellites can provide high quality imagery in five or six spectral bands and with a resolution of about 1 km, used around the world for monitoring clouds, surface temperatures and vegetation cover. Other instruments on board these satellites provide atmospheric temperature profiles and other measurements useful for climate and environmental monitoring.
The geostationary Meteosat weather satellites are the responsibility of Eumetsat in Germany, an inter-govermental organisation involving 17 European States. (Eumetsat's primary objective is to establish, maintain and exploit European systems of operational meteorological satellites). The 'Met' satellites have a main mission to generate 48 cloud images each day , in up to three spectral bands, for use in short period forecasting and the primary satellite, at present Met 7, is situated at zero degrees longitude and is the present one that is in use. The visible channel is used for high resolution cloud images during daylight and the infra-red channels provide cloud images both by day and by night. These are frequently used for moving cloud sequencies shown on television forecasts. The water-vapour channel shows the atmospheric distribution of humidity in cloud free regions.
Images are obtained by an imaging radiometer using mechanical means to scan the earth each half hour. The entire satellite spins at 100 revolutions per minute, enabling the radiometer's telescope to view a narrow east-west strip of the earth's surface. After each revolution the telescope is stepped through a small angle and then scans the next strip, a little further north. It takes 25 minutes to scan the entire earth in this way, then the telescope moves back to its original position and starts a new image. Each line of the image is composed of thousands of individual pixels, or picture elements.
Each image is sent down to the European Space Operations Centre in Darmstadt, Germany, where it is then reprocessed (including the addition of country outlines) and sent back to the satellite in digital form for re-broadcasting. It is these re-broadcasted transmissions which are perhaps useful to the 'amateur forecaster' and which can be received and decoded with a small amount of equipment and without costing a fortune!
The equipment needed to receive these images can either be fairly simple or expensive depending on the degree of resolution and therefore information that is required from the images.
- The equipment I use consists of an offset dish antenna with a home made dipole feed (95K), followed by a commercially made Dartcom 1.6GHz Downconverter which is needed to convert the signal to around 137.5 MHz. This Downconverter is not easy to make oneself in view of the highly accurate RF construction which is needed at this high frequency but ready assembled and tested products are however available from several manufacturers, such as Dartcom. This signal is then fed to a purpose built receiver with suitable bandwidth to accept the type of transmission sent out by the satellites and several kits for these are available at reasonable cost (the RIG 'RX2' for instance). I use a modified Griffin Microsat receiver for this which is crystal controlled and fairly simple, but there are now some very good professionally built 'synthesised' receivers being manufactured for this purpose. The audio output from the receiver is then decoded via a fairly simple home built interface unit such as the RIG 'IF1' which I use. The output of the interface is connected to the parallel port of a standard old 486 PC computer, where a suitable decoding software package can be used to display the images. I use a very good shareware program 'JVFAX' for this and can certainly recommend it, although it is only a DOS program and has been around for some years now. With a modern Computer other packages such as JVCOMM32 and WXSAT can use the SoundCard as an interface and this is much simpler.
- These satellite images are sent out each day on the Amateur Radio Network 80 Metre Band together with NOAA Polar images and also a 'Four-Day' Synoptic Chart.
- A dedicated SSTV program (Pasokon) is used for the radio transmission, rather than JVFAX, and this is just a personal preference. This converts the picture into a series of audio tones which can be heard on the PC's internal loudspeaker. The audio is then taken directly from the loudspeaker and 'cleaned up' via a simple filter circuit. Also any computer DC voltage is removed from it before its final connection to the Data Input circuit of an Icom 756 Pro HF Transceiver and Power Amplifier.
- The Polar Orbiting NOAA 14 and 16 satellite pictures are provided by G3NOX (Jeremy) who uses the Timestep receiver with a Quadrifilar Antenna and WXSAT APT Decoding Software with his own specially calibrated settings (available on the CLIVAN BBS) to receive and decode the images each day. After a small amount of image editing his final picture is transmitted using the WinpixPro SSTV program.
- G3VJF (Paul) is responsible for producing a 'Foursome' each day and this consists of images obtained from the ECMWF Internet Website which are then put together to produce a Synoptic 4-day Forecast. A coloured overlay is added and the final picture uploaded to the BBS where it can be picked off and transmitted by myself, or another amateur if necessary.
(Do remember that shareware authors still have to make a living whilst writing superb programs for us to try out and it is only fair that, if you enjoy using a program, you should register your copy!)
Global Satellite update - May 2001 EUROPE: The status of the three EUMETSAT satellites remains unchanged. Meteosat-7 supports the primary service at 0º Longitude, with Meteosat-6 as the back-up spacecraft at 9º W. Meteosat-5 continues the Indian Ocean Data Coverage service from 63º E. MSG-1 is planned for launch in early 2002 and Metop-1, the first of the EUMETSAT Polar System satellites, for late 2005. USA: GOES-10 (West) is the operational satellite at 135º W. GOES-8 (East) continues to function at 75º W with no significant changes. GOES-11 (launched in May 2000) is stationed at 105º W as back-up. NOAA-15 operates in polar orbit with NOAA-12 as back-up. However, the primary imager of NOAA-15 is experiencing problems and evaluation tests are under way. NOAA-14 continues to function well as the operational afternoon spacecraft but the equator crossing time drift is impacting ozone and imagery products. NOAA-16 was successfully launched on 21st September 2000. Changes are under way to solve problems with its HIRS instrument and APT transmission so that the satellite can replace NOAA-14. NOAA-M is tentatively scheduled for launch in August 2001. RUSSIA: Two satellites of the Meteor-2 and -3 series are operated in circular orbit inclined at approximately 82º. They are operating beyond their lifetimes and with reduced capabilities. The first of the new Meteor-3M generation of polar satellite, Meteor-3M-N1, is being prepared for launch into a morning sun-synchronous orbit in the second quarter of 2001. Meteor-3M-N2 is planned for launch in 2003. The second Russian geostationary satellite, GOMS-Electro-N2, is planned for launch in 2003 and will be positioned at 76º E. CHINA: The FY-28 satellite (launched in June 2000) is located at 105º E. Good VIS, IR and WV images have been received. The FY-1C polar satellite (launched in May 1999) is operating in a morning sun-synchronous orbit. The next in the series, FY-1D, is scheduled for launch at the end of 2001. JAPAN: GMS-5, the current operational geostationary meteorological satellite, will continue to operate at 140º E until the next generation. MTSAT-1R is planned to be launched in early 2003 and MTSAT-2 in 2004. INDIA: INSAT-1D operates at 74º E and INSAT-2E at 83º E. INSAT-2B now has an inclined orbit and is the back-up satellite. INSAT-3A and METSAT (with an imager similar to the INSAT-2E VHRR) are scheduled to be launched by the end of 2001 and INSAT-3D in 2004.
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