Eyes Above the Sky
by Arthur Andrews
These satellites which orbit at a height of 850 km above the earth transmit real time images of what they see as they pass over, enabling significant weather features to be studied and tracked. Whilst the 5 spectrum channel high resolution images are beyond most pockets to receive, the good news is that anyone with a 386/486 type PC can receive the low 4 km resolution images (APT) direct from the satellites as they pass over, for about $700. This has proved to be an exciting hobby of many people in all walks of life and a rewarding one for the serious weather watchers and farmers etc.
There are two types of Low Earth Orbiting (LEO) weather satellites, the American NOAA series and the Russian Meteor series. The NOAA's transmit two pictures side by side. One is from a near visible light sensor (VIS) and the other is from an infrared (IR) sensor. At night there are no VIS pictures so two IR images are transmitted instead. The Russian Meteor satellites orbit slightly higher than the NOAA's and instead of two images side by side they transmit one large VIS image during daylight hours. These tend to be more spectacular than the NOAA images especially when the sun is high in the sky and features such as the salt lakes in South Australia are clearly visible. At night the Meteor satellites sometimes switch to an IR channel, but this is not a common occurrence. The NOAA satellites are "sun synchronous", which means the satellite passes any given point about the same solar time each day. The Russian satellites are not sun synchronous. An interesting point to note is that the satellites appear to reverse direction between one set of passes and the next set 12 hours later. For instance, the NOAA series during their morning passes rise in the north and set in the south. During the evening passes the position is reversed and they rise in the south and set in the north. This is due to the earth having turned through 180 degrees during the 12 hour period between passes.
Not all the satellites are turned on at the same time but normally there are at least three operational. At the moment the active satellites are as follows, NOAA 12, NOAA 14, Meteor 2/21 and Meteor 3/5. A further NOAA will be launched sometime this year.
Beside allowing you to see the weather situation at a glance, a lot more information can be gained from the images received, especially in the IR mode. Land and sea temperatures can be measured, sea currents identified and river systems traced using some of the more sophisticated software available.
Besides the orbiting weather satellites, Australia is also serviced with the Japanese "Himawari" (Sunflower) series of geostationary satellites. The new GMS 5 satellite is currently in service. Every hour GMS transmits two images of Japan and surrounding areas, one in VIS and one in IR. Every third hour, besides transmitting the above images, it transmits a full earth globe in four separate sections, two of the Northern Hemisphere and two of the South. The latter two are of most interest to us as they cover Australia, New Zealand and the Pacific and Indian Oceans. The images are in IR format allowing the weather systems to be studied both by day and night. By animating the images over a period of time the movement of the weather systems can be studied. The cost of a basic GMS receiving station would be around $750 plus the cost of a parabolic dish.
The technical details of setting up a receiving station are beyond the scope of this small article but if anyone is interested you are welcome to write to me for further information, but please enclose a SAE. We have developed weather satellite receiving systems for schools and would welcome any enquiries from interested teachers.
Updated: 6th April, 2004
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