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Weather Satellites

Tiros satellite

Weather satellites have been our eyes in the sky for over thirty years, ever since the April, 1960 launch of Tiros I. Today, satellite images showing the advance of weather fronts are regular elements of the evening news. This meteorological information is also available to anyone with a personal computer and relatively inexpensive hardware and software. A network of American, European, Japanese, and Russian satellites orbits the earth in various configurations to provide "real-time" monitoring of our environment. Many of these satellites transmit signals directly to ground stations in schools, including the Frank H. Harrison Middle School in Yarmouth, Maine, and Wiscasset Primary School in Wiscasset, Maine. Of course, highly-trained technicians, like Georgie Thompson's third grade students, operate the controls of such a station. They are able to predict when the satellites will be overhead, when they can expect to receive an image, and they can loop together several images of cloud conditions and movements from different passes of the satellites to make reliable weather predictions. Any school can establish such a ground station at a surprisingly low cost.

Both polar orbiting and geostationary weather satellites are available to the educational community.

A low earth orbit satellite
Polar Orbiting Satellites

TIROS Polar Orbiting Satellites (NOAA-class), launched and operated by the United States, are the principal sources of environmental data for the 80% of the globe that is not covered by conventional monitoring equipment. These satellites make measurements of temperature and humidity in the Earth's atmosphere, record surface ground and surface sea water temperatures, and monitor cloud cover and water/ice boundaries. They have the capability to receive, measure, process, and retransmit data from balloons, buoys, and remote automatic stations distributed around the globe. These satellites also carry Search and Rescue (SAR) transponders, which help locate downed airplanes or ships in distress. Polar orbiting satellites send back pictures to Earth via Automatic Picture Transmission (APT) or High Resolution Picture Transmission (HRPT).

NOAA (National Oceanic Atmospheric Administration) class satellites (and Russian Meteor class satellites) have orbits that cross very close to the poles on each revolution of the Earth. At an altitude of 860 km. (500 miles), the sensors scan the entire surface of the Earth over a 24-hour period. The sensors are sensitive to both visible light and infrared (IR) radiation. As each NOAA polar-orbiting satellite orbits the Earth, it constantly sends a stream of data.

Instruments on board the satellite scan the Earth's surface from side to side (perpendicular to the ground track), with each scan covering an area about 2 km. high and 3000 km. wide. Typically, APT imagery is transmitted at 2 lines/second, or 120 lines per minute. In a pass lasting 12 minutes, this translates into an image approximately 5800 km. long and 3000 km. wide. As an example, the entire east coast of the United States would be visible at one time, from southern Florida north to Hudson Bay, and from the Atlantic Ocean to west of the Great Lakes.

The information transmitted in the APT format provides users with imagery at 4 km. resolution; that is, anything 4 km. (2.5 miles) in size should be visible on the screen. Sebago Lake appears as a dot on the image. Other large lakes, such as Lake Winnipesaukee in New Hampshire and Moosehead Lake in Maine are also visible.

During the day, this data stream consists of one visible and one infrared image. At night, both channels are infrared. Imagery in both the visible and Infrared formats is transmitted simultaneously. Students are familiar with the visible image because it is like a conventional camera. Understanding what the infrared imagery represents is sometimes harder to grasp. Various land and water bod ies absorb heat differentially, so they reflect different levels of heat energy. The Gulf Stream is an excellent example. On an infrared image, the warmer temperatures of the Gulf Stream are clearly delineated as the darker portions of the image, while the cooler temperatures of the surrounding Atlantic are lighter in color. With some computer software, students can use a mouse to place a cursor anywhere on the image and accurately measure the surface water temperature to within +/- 2 degrees F.

The NOAA-class polar-orbiting satellites may also transmit High Resolution Picture Transmission imagery, which provides much greater detail than the APT imagery. HRPT imagery has a resolution of 1.1 km., so it can depict small lakes and other land features that are only 1/2 mile across. Increased resolution means a much more sophisticated ground station. Further, HRPT makes available two visible and three infrared channels.

With the appropriate equipment, schools can download both APT and HRPT imagery in real time.

Geostationary (GOES) Satellites GOES 9

In late 1966, ATS-1 was launched into a geostationary orbit over the equator, south of Hawaii. For the first time, meteorologists could monitor the weather on a continuous basis during daylight hours. It provided images of nearly one-third of the earth's surface every 23 minutes with 4 km. resolution.

In May of 1974, the first of a new series of GOES satellites was launched. Both visible and infrared images were acquired simultaneously by the Visible and Infrared Spin Scan Radiometer (VISSR) onboard the spacecraft. The visible channel has ground resolution of .8 km. for sections of the full earth view and 6.2 km. resolution in the infrared spectra. The greatest advantage of having both visible and infrared capability is that weather systems can be monitored both day and night (at 30 minute intervals). Thus, we are able to track destructive hurricanes around the clock. Most satellite images seen on our local evening news and the Weather Channel are produced by GOES satellites. Usually, the infrared images are "loop animated" to show the progression and movement of storms.

While the United States maintains and operates its GOES satellites, the European community is served by its European Space Agency (ESA) Meteosat satellite, and Japan with its GMS satellite. This network provides complete global coverage of all but the extreme north and south polar regions. GOES satellites make day and night observations of weather in the coverage area and transmit real-time VISSR data, monitor cataclysmic weather events such as hurricanes, relay meteorological observation data from surface collection points, and perform facsimile transmission of processed graphic and imaged weather data. This rebroadcast function is known as WEFAX, which stands for Weather Facsimile.

Orbits of several satellites

The primary function of our GOES Satellites to the education community is to provide imagery of varying resolution and time frames. VISSR is the most stunning example, although it requires a much more sophisticated ground station to receive and process the signal (than APT or WEFAX). From Hawaii to Maine, land features can be examined to 0.8 km. resolution. The snow-capped Rocky Mountains stand out nicely, as do larger lakes and reservoirs.

WEFAX, on the other hand, is easily received with relatively simple equipment. Much of the imagery transmitted via WEFAX is considered low resolution, usually 4 km. Along with satellite imagery, weather charts and information are also transmitted on a regular basis.

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