The Navstar Global Positioning System, commonly called GPS, is a radio-navigation system developed in April 1973 for military purposes by the Department of Defense. Today, it serves a dual-purpose, as President Ronald Reagan opened the yet-to-be-completed GPS technology to civilian use in 1983, after the downing of Korean Airline Flight 007 over then-Soviet airspace. It took a dozen more years before Navstar was fully operational in both the civilian and military sectors.
The Clinton administration took the Reagan directive one step further by establishing an executive board to manage Navstar as a national asset. It also upgraded civilian signals. The Bush administration replaced that executive board with a committee called the National Space-Based Positioning, Navigation and Timing Executive Committee, in 2004, fully cementing GPS as a part of the global civilian and military landscape. User performance was modernized in 2005 with the launch of the L2C satellite; the most recent satellite launched in 2008. Modernization is a continual process as system architecture evolves.
Man has always used fixed points of reference to determine location. Global positioning systems draw on at least four satellites nestled in a constellation of 31 broadcast-active satellites to calculate location by using four measurements to solve for the three dimensional variables of space as well as for very specific measurements of time.
Before late 2007, there were just 24 uniformly orbiting satellites. Today’s extra seven satellites provide redundant data, increasing the likelihood of locational success should any of the 24 fail to transmit. Instead of the satellites being fixed in space, they are now non-uniformly arranged with at least six satellites always orbiting in 12,600 mile orbits, inclined at 55 degrees and always within line-of-sight from any Earth location. The oldest satellite still in use was launched in 1991.
GPS works by satellite to ground triangulation; that is, measuring radio signal distance over time. The satellites operate in high orbits over the earth, which are monitored constantly. Location can be triangulated by measuring distance, satellite position, and correcting for error differentiation as the signals pass from space through our atmosphere to the ground receiver.
The U.S. Air Force and the National Geospatial-Intelligence Agency track and control all GPS satellites from monitoring stations located in Hawaii, Colorado, the Marshall Islands, the South Atlantic, and the British territory of Diego Garcia in the Indian Ocean’s Chagos Archipelago.
The forerunners of current GPS technology were the world’s first operational satellite system, Transit, who’s algorithms played a large role in today’s satellite prediction algorithms. The other was the space-based Timation, launched in 1967, which significantly contributed to the three-dimensional technology used in today’s global positioning systems.
Prior to those satellite systems, American scientists monitoring Soviet radio after the launch of Sputnik in 1957, discovered that satellites could be pinpointed along their orbits using the “Doppler effect” or signal frequency distortion and a fixed point on Earth. This principle was a cornerstone in the advancement of GPS science throughout the last half century.
Despite the highly accurate measurements of today’s GPS systems, natural barriers such as the atmosphere, specifically the ionosphere and the troposphere, create inconsistent conditions affecting microwave signals passing through to the receivers on Earth–challenging scientists to use mathematical modeling to compensate for error.
Signal delays may also be due to multipath issues, changes in receiver altitude, data and clock error. The current cost of maintaining the GPS constellation is over three quarters of a billion dollars each year to replace aging satellites and to continue research and development of newer technology and systems architecture.
Civilian and military use of global positioning system technology includes navigation, search and rescue, weapons guidance, and map-making. The most common colloquial usage of the technology is synonymous with vehicle and cellular map locators. Just say GPS, and for most of the public, that is the vision that comes to mind.
For more information on the history, mathematics behind GPS technology and its applications, check out these links:
Rand GPS History: This 34-page monologue details the history leading up to today’s innovative location technology.
GPS Principles Tutorial: This tutorial gives you a basic understanding of why GPS technology is needed and the logic behind how it works. Users will find the tutorial manageable as it is presented in bite-size sections from a rather less technical point of view.
Classroom GPS: Forty minutes is all it takes to cover the applications and scientific collaborations of the global positioning system, how it affects the Internet and email, and an introduction to the Global Mapping Experiment. Students also fill in a worksheet about GPS applications.
GPS Magazine: Online magazine offering up-to-the-day news, resources, consumer and professional business and technological updates. Includes modernization, transportation, defense, wireless and agricultural GPS-related news.
GPS Navigational Policy: Modernization policy from the Federal Aviation Administration site details navigation future, modernization, global operations and benefits of new satellite constellation frequencies, as well as mitigating satellite interference.
Space-Based Positioning, Navigation and Timing: Established by presidential directive, this executive committee advises and coordinates federal matters regarding GPS and other navigational systems.
U.S. Global Positioning: The official U.S. government site regarding GPS system information, applications, user support, technical and policy management.
GPS Air Force Wing: The GPSW is the Department of Defense acquisition wing directed by the U.S. Air Force and managed by the Space and Missile Systems Center in Los Angeles. This is a fact sheet.
GPS Navigation Center: Operated by the U.S. Department of Homeland Security and the U.S. Coast Guard, the Navigation Center details GPS and other navigational points of interest.
GPS Applications Exchange: NASA’s online informational resource about global positioning applications. This site is searchable by both application and by country.
GPS Glossary: Definitions of GPS terms from absolute positioning to zero baseline testing.
GPS Circle Math: Get a great visual of three satellites and its receiver location.
GPS Receivers: Find out here how they work by two and three-dimensional trilateration and its calculations. See a video about how GPS works.
GPS Facts: Get the launch facts about the satellites in orbit from 1978 to 1996.
GPS Relativity: How Einstein’s Theory of Relativity relates to global positioning.
How GPS Signals Work: A description of GPS signal structure criteria in determining the parameters of passive positioning, Doppler effect, navigational transmission, simultaneous signal transmission and signal error.
PPT: Signal Structure: GPS code schematics, characteristics and correlations discussed in this PowerPoint presentation.
GPS Links: GPS overview, maps, space, user, and control segments, precise and standard positioning, satellite signals and data in an easy to navigate site from Peter H. Dana’s Geographer’s Craft Project from the University of Colorado at Boulder.
