The Global Positioning System (GPS) is comprises a group of 24 satellites that orbit Earth at 10, 907 nautical miles (20,200 km) above Earth. GPS satellites provide precise location information anywhere on Earth. Full accuracy only available to the military, which own and operate the satellites, but there are numerous commercial applications for GPS. By the 1990s, GPS receivers became small enough to be hand held, increasing public interest in the technology.
At a cost of $3 billion, GPS was developed by the United States military beginning in the 1970s, first in the Navy, and later by the Air Force. Their initial goal was to guide missiles and tanks. The Department of Defense launched the first GPS satellites in the 1980s, and all 24 were in orbit by 1996. GPS was first used by the military in the Persian Gulf war in 1991. Each GPS satellite has four atomic clocks, and continually broadcasts data concerning time, its path, and its location. A GPS receiver picks up the signal from three satellites, and compares it with its own clock, then calculates the distance to ascertain its location, within 52 ft (16 m). There are also four permanent receiver stations on Earth. When information from a mobile receiver is combined with that from a permanent, a location reading can be calculated within inches. Initially, the receivers were big, unwieldy, expensive boxes, costing at least $50,000. By 1998, receivers were the size of credit cards, and the cheapest could be obtained for a few hundred dollars.
Commercial applications of GPS abound. Trucking companies use GPS to monitor the location of their fleet. Airlines use GPS to avoid mid-air collisions and land planes in fog. GPS can also help provide better routes for airplanes. GPS can monitor changes on Earth, perhaps forewarning of earthquakes. GPS can steer boaters, help locate hikers, such as when someone is lost or injured. When GPS is combined with other technologies, such as certain computer software, the applications of GPS are endless. Computer maps in cars can direct drivers in unfamiliar cities. GPS satellites also have nuclear detection payload capabilities to ensure nuclear test ban treaty compliance.
There are, however, several potential problems with GPS. One concerns the Department of DefenseÕs operation of the satellites. The military deliberately degrades the satellites' signals, making the clock incorrect by nanoseconds, so most commercial receivers are off by 109 yd (100 m). This is done in the name of national defense, though completely accurate receivers are used by the military. Because of this inaccuracy, some commercial entities have been forced to develop their own technology to correct this problem at great cost. Still, GPS is not immune to terrorist obstruction, or even solar interference from the sun. At the peak of the solar sunspot cycle, which happens about every 11 years including the year 2000, certain high-energy solar particles and ionospheric scintillation could interfere with the operation of the GPS satellites.
Technologies similar to GPS have been developed by the Russians. The Glosnass (Global'naya Navigatsionnya Sputnikovaya Sistema) also employs 24 satellites which orbit at 11,842 mi (19,100 km) above Earth. However, Glosnass operates on a different frequency.
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