Лекция: WHAT IS GPS?
GPS, short for Global Positioning System, is a space-based positioning system that uses satellites and computers to measure positions anywhere on the land, air, or sea at any time. This system is distinguished from other positioning systems by its 24-hour, seven-day per week availability and its pinpoint accuracy.
The system was born from communication problems U.S. troops experienced in the Vietnam conflict. During that time a localized system, LORAN, was used that was prone to problems inherent in radio communication such as poor reception during ominous weather or during the night. The U.S. Department of Defense responded to this problem by investing into a system known as the Navy Navigational Satellite System, TRANSIT, which most recently evolved into GPS.
GPS is a second generation, satellite-based, positioning system that is available anywhere and anytime and is capable of measuring land, air and sea positions with millimeter accuracy. GPS is referred to as a system because it is an assemblage of three distinct components or segments: Space, Control, and User.
The Space Segment refers to the constellation of satellites and the navigational data they provide. The Control Segment refers to monitoring and updating of the satellites’ clocks and navigational messages by a master control station that uses five regional monitoring stations distributed around the world. Lastly, the User Segment refers to the GPS receivers that calculate the time required for a radio signal to travel from the visible satellites to the receiver in order to measure its position using a technique called triangulation.
GPS has a civilian and military user community. Although GPS is funded by the U.S. DoD, civilians worldwide can use GPS’ Standard Positioning Service (SPS) provided a proper receiver is used. SPS provides positional accuracy of 100 meters in 2-Dspace with 95% confidence. The U.S. Military and its allies use a more highly accurate service called Precise Positioning Service (PPS) that is capable of accuracy within ten meters in 3-D space 95% confidence.