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Navstar: GPS Satellite Network

gps network
The orbits of GPS satellites are inclined to the Earth’s equator by about 55 degrees. The system is designed to ensure that at least four satellites are visible at least 15 degrees above the horizon at any given time anywhere in the world.
Credit: NOAA

Navstar is a network of U.S. satellites that provide global positioning system (GPS) services. They are used for navigation by both the military and civilians.

These GPS satellites orbit Earth every 12 hours, sending a synchronized signal from each individual satellite. Because the satellites are moving in different directions, a user on the ground receives the signals at slightly different times. When at least four satellites get in touch with the receiver, the receiver can calculate where the user is – often to a precision of just a few meters.

GPS signals used to be "degraded" for civilian use, meaning that they were only really precise in military applications. In 2000, however, President Bill Clinton authorized the switch-off of this "selective availability."

These days, many people take Global Positioning System devices for granted.

Keeping submarines in the right direction

The U.S. Navy developed the first operational satellite navigation system – called Transit – in the 1960s. These "spin-stabilized" spacecraft – meaning that their spin kept the spacecraft pointing in the same direction – were first used for navigation in 1964 by Polaris submarines.

Even in those early days, however, the people who built the system could see scientific uses for GPS.

"Both of us have been enormously pleased at some of the past and present uses of satellite navigation, including the tracking of migrating birds and animals and effective search and rescue techniques that can pinpoint trouble in remote areas worldwide," wrote George Weiffenbach and William Guier, who both contributed to the build of Transit, in a 1998 article for Johns Hopkins University.

"Of course, we underestimated progress in electronics. In particular, we did not predict the incredible extent to which size and cost would be reduced for everyday applications for the mass market, e.g., navigation systems for our automobiles and pleasure boats, and even handheld units for hikers."

There were other early systems as well, such as the Navy's Timation satellite that tested the use of accurate clocks in space. This was an important predecessor to Navstar, as that GPS system relies on timing to keep the satellites synchronized.

navstar
Navstar-2F satellite of the Global Positioning System (GPS)
Credit: U.S. Air Force

Generations of satellites

Meanwhile, other branches of the U.S. military were working on their own navigation systems. With so many systems trying to keep users on track, eventually the military realized it might be worth it to develop a system that all of the branches could use.

In 1973, the Pentagon proposed a global positioning satellite system that today is known as Navstar.

"The main reasons for GPS development were the need to deliver weapons precisely on target, and to reverse the proliferation of navigation systems in the U.S. military," wrote Rick Sturdevant in a 2007 NASA book concerning space in civilian applications.

"From the beginning, however, the Department of Defense (DOD) recognized the usefulness of GPS to the worldwide civilian community."

Navstar is designed to operate with at least 24 satellites in orbit. The first generation of Navstar satellites began launching in 1978. Save for the failure of Navstar 7, in 1981, which failed to reach orbit, about 10 satellites built by Lockheed Martin made it into space.

Starting in 1989, according to Gunter's Space Page, the first full-fledged GPS satellites were launched. They were designed to operate 14 days without the need for intervention for the ground. Nine of these satellites launched in quick succession between 1989 and 1990.

There have been several generations of Navstar satellites since then, with today's versions boasting an estimated lifetime of 12 years as well as faster processing power. There have been three launches of the Navstar 2-F system, with the latest one taking place in 2012.

There are other Navstar 2-F satellites in storage, ready to launch, and some still under production, noted Boeing in a system update in June of that year.

The next generation of GPS, called GPS III, is supposed to be three times more accurate than what is currently available, according to maker Lockheed Martin. The satellites also will be equipped with an "anti-jam" system for security.

Lockheed's first batch of GPS III satellites will launch in the 2014-15 timeframe, according to a statement from the company in 2012. Once they make it to orbit, they're expected to last at least 15 years. The Air Force could order as many as 32 of these satellites from Lockheed.

Satellite Quiz: How Well Do You Know What's Orbitin...
The space age dawned with the launch of Sputnik 1, Earth's first artificial satellite, in 1957. Thousands of additional spacecraft have followed in Sputnik's footsteps, serving humanity in a variety of ways. How well do you know Earth's satellites?
A Soviet technician works on Sputnik 1 before the satellite's Oct. 4, 1957 launch.
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Satellite Quiz: How Well Do You Know What's Orbitin...
The space age dawned with the launch of Sputnik 1, Earth's first artificial satellite, in 1957. Thousands of additional spacecraft have followed in Sputnik's footsteps, serving humanity in a variety of ways. How well do you know Earth's satellites?
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A Soviet technician works on Sputnik 1 before the satellite's Oct. 4, 1957 launch.
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Selective availability

GPS for civilian users took a leap forward in 2000 when Clinton authorized the end of "selective availability." Previously, civilians received a much "coarser" view of their position because the military manipulated factors such as orbital data or the frequency of the satellite's clock.

It wasn't long, however, before civilians feared the option could be turned on again. The terrorist attacks of Sept. 11, 2001, on the World Trade Center and Pentagon prompted a raft of new security measures. Six days after the attacks, the Interagency GPS Executive Board stated there was no change to the United States' policy: to not use selective availability again.

In 2007, President George W. Bush accepted a recommendation to go a step further. On advice from the Department of Defense, he directed that the Navstar III generation would not have the ability to do selective availability at all. With GPS now a multi-billion dollar industry, it appeared commercial considerations had a strong voice in Washington.

"While this action will not materially improve the performance of the system," read a statement from the Department of Defense, "it does reflect the United States’ strong commitment to users by reinforcing that this global utility can be counted on to support peaceful civil applications around the globe."

Unlike the early days, Navstar faces competition from other GPS-like systems built by other countries for civilian and military use. The Russians have a system called GLONASS, while the Europeans have another one called Galileo that is in the early stages of deployment.

Navstar's managers are working to compete with them by improving the accuracy of the system's positioning. Today's civilian GPS systems are accurate to within 40 feet, according to the Federal Aviation Administration, and military systems are precise to within just a few inches.

— Elizabeth Howell, SPACE.com Contributor

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