The Global Positioning System (GPS) is a massive and costly engineering project, regarded as the third-largest space project following the Apollo moon landing and the Space Shuttle. During the Gulf War, although the GPS system was not yet fully completed, its initial use demonstrated remarkable capabilities. With the full establishment of the GPS satellite network in space in 1993, its application areas continued to expand. In 1994, the United States announced that it would provide free access to the GPS system worldwide for ten years, prompting countries around the world to compete in utilizing the system. On February 29, 1996, the U.S. government officially declared GPS as a dual-use system for both military and civilian purposes, but still implemented the SA (Selective Availability) policy, deliberately degrading positioning accuracy and thus limiting civilian applications. It was not until May 1, 2000, that the U.S. President announced setting SA to zero, which undoubtedly greatly promoted the development and popularization of civilian GPS applications.
A popular saying in the GPS industry goes, "The only limit to GPS applications is human imagination." Currently, civilian uses of GPS include land transportation, maritime transport, civil aviation, communications, surveying and mapping, construction, mining, agriculture, power systems, medical applications, scientific research, household appliances, entertainment, and more. Today, GPS positioning accuracy has improved from the 10-meter level to the millimeter level.
GPS is a global positioning, navigation, and timing system consisting of 24 satellites launched by the U.S. Department of Defense. GPS can provide positioning and navigation because every GPS receiver, at any time and anywhere on Earth, can simultaneously receive signals containing orbital information from at least four GPS satellites. By calculating the positioning information from each satellite, the receiver can determine its own location, thereby providing highly accurate three-dimensional (longitude, latitude, altitude) positioning, navigation, and timing information.
Overall, GPS mainly consists of three parts: the space segment, the control segment, and the user segment. The space segment is made up of a satellite constellation, the control segment is managed by ground-based satellite control centers, and the user segment is developed, sold, and serviced by military and civilian manufacturers. The space and control segments are currently both under the control of the U.S. Department of Defense.
According to statistics from the U.S. Department of Commerce, the global annual sales of GPS user equipment in 2000 amounted to 8–9 billion U.S. dollars. By 2003, civilian GPS products were expected to exceed 16 billion U.S. dollars. It is no exaggeration to say that by 2010 and beyond, GPS application products will gradually become an indispensable part of daily life and serve as a foundational platform for the development of various products.
Whether from the perspective of military competition or civilian demand, GPS has drawn the attention of decision-makers around the world. They all realize that whoever controls a global GPS satellite network and GPS control system will significantly enhance national strength and gain access to a vast commercial market. The former Soviet Union began building its own global satellite positioning system (GLONASS) as early as 1982, and Russia later continued this system project. Despite economic difficulties in previous years, it still launched nine global navigation satellites belonging to the GLONASS system. Meanwhile, Europe has also accelerated efforts to establish its own global navigation satellite system — the Galileo system.
The Development of GPS:
Before the advent of satellite positioning systems, long-distance navigation and positioning primarily relied on radio navigation systems.
Radio Navigation Systems
● LORAN-C: Operates at 100 KHz, consists of three ground-based navigation stations, covers a navigation area of 2,000 KM, with general accuracy of 200–300 M.
● Omega: Operates at tens of kilohertz. Consists of eight ground-based navigation stations, capable of global coverage. Accuracy is a few miles.
● Doppler System: Utilizes the Doppler shift principle; by measuring frequency shifts, it calculates parameters of a moving object (ground speed and drift angle) to deduce the position of an aircraft. It is a self-contained dead reckoning system. Errors accumulate with increasing travel distance.
Disadvantages: Limited coverage area; radio wave propagation is affected by the atmosphere; positioning accuracy is not high.
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