Articles
Geosynchronous Weather Satellites
The satellites
There are three main agencies providing geosynchronous satellite services. The National Oceanographic and Atmospheric Administration (NOAA) administer GOES-8 (at 75 deg. West) and GOES-10 (at 135 deg. W) giving coverage of both North and South America. Japan has GMS (at 140 deg. East) which covers the western part of the Pacific including Japan and Australia. Eumetsat, which is part of the European Space Agency has Meteosat-7 which is located over the equator on the Greenwich Meridian (0 0N, 0 0W). This satellite gives full coverage of Europe, Africa and parts of South America, the Middle East and the extreme western part of the Indian Ocean. There is also INSAT/Meteosat-5 located over the Indian subcontinent for coverage of Asia and Russia.
We will concentate in this article on the Meteosat-7 as I am located in the United Kingdom and make use of this satellite. Images from some of the other geosynchronous satellites are rebroadcast over Meteosat and GOES so that it is possible to view weather conditions outside these satellite's normal coverage area.
The services
There are two types of signal broadcast by Meteosat-7. There is an encrypted digital service, the primary data user service (PDUS) and an analogue WEFAX service which can be accessed free of charge by all. The PDUS service is due to replace the analogue service, but WEFAX will continue to be available until at least the end of 2003.
PDUS gives 2.5 km resolution images of Europe every 30 minutes, the rest of the world is transmitted frequently in lower resolution. PDUS data is transmitted from the satellite on 1694.5 MHz and requires specialist equipment using a 1.8m dish.
The secondary data user service (SDUS) or WEFAX service is in fact a rebroadcast of selected material from the PDUS service, but at lower resolution. The primary data is downlinked to Darmstatd in Germany where it is processed so that the data is presented in any easy to view way and coastal outlines are included before it is re-broadcast from the satellite. Two radio channels are used for the re-broadcasts: channel 1 on 1691.0 MHz and channel 2 on 1694.5 MHz. Channel 1 is used for the Darmstadt WEFAX images whereas channel 2 is used to re-broadcast GOES images (which are uplinked from Lannion in France) and GMS (which are forwarded from Japan via land-line). The RF modulation used is FM and requires a receiver bandwidth of about 30 - 50 kHz for optimum signal to noise ratio without distortion. The actual signal transmitted on the FM channel is very similar to automatic picture transmission (APT) used in polar orbiting satellites so that it is possible to use much the same equipment for both services. However, because of the bandwidth requirement poor reception is often obtained using a conventional scanner receiver (which tends to have a choice of 180 kHz or 15 kHz FM bandwidth). Some models, such as the AOR AR5000, has a suitable bandwidth filter and gives very good results. Normally, a 90cm dish is used as the antenna but other types (such as yagis) are also often used instead.
The imaging
The source data, giving coverage of the whole Earth's disc as seen from the satellite is divided up into nine segments as follows:
Nine Segments used for WEFAX
Re-broadcast
There are three types of sensor: thermal infra red (type D), water vapour IR (type E) and visible (type C). These sensor types are combined with the segmentation given above to create a repeating 24hr transmission schedule for the WEFAX re-broadcasts. You can tell what data is being transmitted according to the schedule from the format designator which combines the sensor type with the segment number. For example, D1 is the format designator for a thermal infra red image of the eastern part of the Atlantic.
The schedule repeats updates for each segment at regular intervals, but since segment 2 is the most useful (it covers all of Western Europe) this one is updated at half hour intervals using thermal infra red sensor (D2).
Weather Satellites | Polar Satellites | Geosynchronous Satellites
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Copyright © 2000 Simon Collings