Pinpointing our world: Global positioning systems

From weapons systems to directional mapping gadgets in our vehicles, global positioning systems have changed the way we live, do business and keep time.

The first GPS was conceived by Roger L. Easton, a physicist with the U.S. Naval Research Laboratory who was working on the Navy Space Surveillance System.

In an April 1955 paper, “A Scientific Satellite Program,” Easton wrote, “By observing the satellite it would also be possible to determine the absolute longitudes and latitudes by observation of the satellite. Such observations would also yield the height of the observer above the center of the earth.”

One of Easton’s patents, filed in 1963, provided the basis for GPS, which utilized synchronization of a navigator’s clock with the satellite clock. By tracking time and an object’s longitude and latitude, three-dimensional data could be achieved to pinpoint the object’s location.

It’s wasn’t long before the military took off on the concept.

In April 1973, Deputy Defense Secretary William P. Clements Jr. wrote a memo, “Defense Navigation Satellite Development Program,” encouraging the Air Force to build and test a GPS system. He also formed the GPS Joint Program Office. That same year, the Air Force built the “Navigation Technology Satellites,” called NTS-1 and NTS-2, and transmitted the first navigational signal from space.

There are three segments to GPS: the space segment, the control segment and the user segment.

In the space segment, the Air Force maintains a “constellation” of 31 satellites, which orbit approximately 12,550 miles above the Earth. Additionally, the Air Force maintains three to four additional decommissioned satellites that can be reactivated as part of the constellation in case one of them fails. The military’s goal is to have 24 satellites operational 95 percent of the time.

The satellites are arranged in six equally spaced orbits, and each has four “slots” occupied by baseline satellites. This arrangement allows at least four satellites in view from virtually any point on the Earth at any given time.

The control segment is a worldwide monitoring system on the ground. It controls and maintains the satellites in proper orbits. The operators of the system can make adjustments to the satellites’ clocks, upload navigational data and monitor the status of the constellation. The U.S. Coast Guard maintains a website that reports the status of the constellation, important announcements and alerts on any outages.

The user segment comprises GPS users who have receiver equipment. This can be a GPS in a vehicle, a cell phone or other receiving device, which transmits the navigational coordinates of the receiver and calculates its location and time.

There are many industries that use GPS besides the military, including agricultural, environmental, supply-chain management, aviation/air traffic control, mobile phones, marine, disaster recovery, public safety, rail, traffic and roadways, surveying and mapping, banking, power grid management, space, recreational and, of course, time tracking.

And the technology continues to evolve.

On May 15, 2008, the Air Force Space and Missile Systems Center awarded Lockheed Martin a contract for the next generation of GPS satellites known as GPS III. This is part of a modernization program to keep the system up to date.

According to the Lockheed Martin website:

“GPS III will improve position, navigation and timing services and provide advanced anti-jam capabilities yielding superior system security, accuracy and reliability.

“The first GPS III satellites will deliver signals three times more accurate than current GPS spacecraft and provide three times more power for military users, while also enhancing the spacecraft’s design life and adding a new civil signal designed to be interoperable with international global navigation satellite systems.”


[youtube_sc url=”” height=”315″ autohide=”1″]

Photo/Video Credits:
Roger L. Easton – U.S. Patent & Trade Office

Satellite Constellation Graphic: National Coordination Office for Space-Based Positioning, Navigation, and Timing

GPS III Graphic and Lockheed Martin Ad: Used with permission –Lockheed Martin Information Systems & Global Solutions


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