The deployment portion of the GPS history began in 1973 with the decision to develop a satellite navigation system based on existing technology of the U.S. Air Force and the U.S. Navy. The system went through extensive testing during the next three years, and in 1977 the first transmitters were installed on the surface of the Earth and tested.
Between 1978 and 1985, 11 satellites were launched into space and put into position. In 1979, the decision was made to increase the number of satellites to 18. In 1980, the first Block I satellite was launched. This satellite had sensors specifically designed to detect atomic explosions, and was placed in orbit as a means of monitoring the Soviet Union’s compliance with the 1963 agreement with the United States to refrain from nuclear testing.
Also in 1980, the onboard atomic clocks, the most accurate timepieces in the world, were activated. Developed by physicists, atomic clocks measure time by the change in energy levels of electrons. These clocks are stable, continuous, and accurate to a nanosecond, or one-billionth of a second.
GPS GOES PUBLIC
After Korean Air Lines Flight 007, carrying 269 people, was shot down in 1983 after straying into the USSR’s prohibited airspace, President Ronald Reagan issued a directive making GPS freely available for civilian use. The first satellite was launched in 1989, and the 24th satellite was launched in 1994.
In 1986, the Challenger space shuttle tragedy halted space shuttle launches, and thereby delayed the GPS system, because shuttles were supposed to transport the new Block II satellites. Eventually, Delta rockets, the original transport, were put back into use to launch the satellites, and in 1988, the decision was made to increase the number of satellites to 24, because functionality with 18 satellites was still limited.
Initially, the highest quality signal was reserved for military use, and the signal available for civilian use was intentionally degraded. This changed when President Bill Clinton ordered “Selective Availability” to be turned off at midnight on May 1, 2000. This improved the precision of civilian GPS from 300 feet to 65 feet. The executive order signed in 1996 to turn off Selective Availability in 2000 was proposed by the US Secretary of Defense, William Perry, because of the widespread growth of differential GPS services to improve civilian. Moreover, the US military was actively developing technologies to deny GPS service to potential adversaries on a regional basis.
Though the GPS system was available for public use, the first Gulf War in 1990 saw temporary deactivation of use by the public, because the military needed more receivers. Public use returned in 1993 along with the decision that the GPS system would be available free of charge to the entire world
. “Full Operational Capacity” was achieved in July of 1995 with the placement and activation of the last of the 24 satellites.
CONTINUED IMPROVEMENTS
Since 1995, GPS history has seen considerable technological advancement. More satellites have been put into orbit, increasing both availability and accuracy. That accuracy improved to finding subjects within 33 to 49 feet, and at present can be measured within inches in some cases.
Availability improved from utilization in military and the position-required industries of aviation, nautical navigation, and land surveying, to include many personal applications. With the use of a fairly inexpensive receiver, employers use GPS to track their fleet of vehicles, parents use the system to keep track of small children and of disobedient teenagers, and pet lovers use it to keep track of their dogs. The most popular use, of course, is for driving directions in vehicles.
HOW IT WORKS
A GPS receiver calculates its position by precisely timing the signals sent by GPS satellites high above the Earth. The receiver uses the messages it receives to determine the transit time of each message and computes the distance to each satellite. These distances along with the satellites’ locations are used with the possible aid of trilateration to compute the position of the receiver. This position is then displayed, perhaps with a moving map display or latitude and longitude; elevation information may be included. Many GPS units show derived information such as direction and speed, calculated from position changes.
Receivers use four or more satellites to determine the receiver’s location and time. The very accurately computed time is effectively hidden by most GPS applications, which use only the location. A few specialized GPS applications do however use the time; these include time transfer, traffic signal timing, and synchronization of cell phone base stations.
Editor Phil Robertson is an award-wining journalist and graphic designer. With a degree from the University of Florida’s School of Journalism, his career in journalism and publishing spans over 30 years, and includes positions as editor and publisher for several newspapers and magazines. During his career he has received a first-place award for investigative journalism from the Society of Newspaper Editors, and five ADDY awards for advertising design.
