Gps : The Gps Satellite

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4.2.2 The GPS Satellite System

The 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are travelling at speeds roughly 7,000 miles per hour.

GPS satellites are powered by solar energy. They have backup batteries onboard to keep them running in the event of a solar eclipse, when there’s no solar power. Small rocket boosters on each satellite help them keep flying in the correct path.

Here are some other interesting facts about the GPS satellites (also called NAVSTAR, the official U.S. Department of Defense name for GPS):
• The first GPS satellite was launched in 1978.
• A full constellation of 24 satellites was achieved in 1994.
• Each satellite is built to last about 10 years. Replacements are constantly being built and launched into the orbit.
• A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.
• Transmitter power is only 50 watts or less.

Fig.4.2.2 GPS Nominal Constellation (24 satellite in 6 orbits)
4.3 Remote Sensing

Space borne remote sensing of the Earth began in 1972 with the successful launch of the Earth Resources Technology Satellites ERTS-1 (later renamed as LANDSAT) by the United States of America. Since then, remote sensing has made rapid advances in technology as well as in application domains. Remote sensing data provides large area coverage
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