The Global Positioning System (GPS) is a space-based radio-navigation system, owned by the U.S. Government and operated by the United States Air Force (USAF). It can pinpoint a three dimensional position to meter-level accuracy and time to the 10-nanosecond level, worldwide and 24/7.

GPS is comprised of three different parts:

• Space Segment: A constellation of at least 24 US government satellites distributed in six orbital planes inclined 55° from the equator in a Medium Earth Orbit (MEO) at about 20,200 kilometers (12,550 miles) and circling the Earth every 12 hours.
• Control Segment: Stations on Earth monitoring and maintaining the GPS satellites.
• User Segment: Receivers that process the navigation signals from the GPS satellites and calculate position and time.

History of GPS

GPS has its origins in the Sputnik era when scientists were able to track the satellite with shifts in its radio signal, known as the “Doppler Effect,” which became the foundational idea for modern GPS. Today the GPS satellite constellation (the space segment) consists of over 30 operational satellites, each equipped with redundant atomic clocks and tracked by a ground control network (the control segment). Each satellite transmits its position and time at regular intervals and those signals are intercepted by GPS receivers (the user segment). The receiver is able to determine its position by calculating how long it took for the signals to reach it.

In the early 1970’s, the Department of Defense (DoD) wanted to ensure a robust, stable satellite navigation system would be available. Embracing previous ideas from Navy scientists, the DoD decided to use satellites to support their proposed navigation system. DoD then followed through and launched its first Navigation System with Timing and Ranging (NAVSTAR) satellite in 1978. The 24 satellite system became fully operational in 1993.

Today, GPS is a multi-use, space-based radio-navigation system owned by the US Government and operated by the United States Air Force to meet national defense, homeland security, civil, commercial, and scientific needs. GPS currently provides two levels of service: Standard Positioning Service (SPS) which uses the coarse acquisition (C/A) code on the L1 frequency, and Precise Positioning Service (PPS) which uses the P(Y) code on both the L1 and L2 frequencies. Access to the PPS is restricted to US Armed Forces, US Federal agencies, and selected allied armed forces and governments. The SPS is available to all users on a continuous, worldwide basis, free of any direct user charges. The specific capabilities provided by SPS are published in the Global Positioning System Performance Standards and Specifications.

For more details visit,

GPS