Why GPS is in your future

Origins, changes and current state of the GPS System

Origins & Changes
The origins of the present-day NAVSTAR [Navigation Signal Timing and Ranging] GPS system go back to the launch of the Sputnik satellite in 1957. By studying the orbit of the tiny satellite, scientists found that it could be tracked by its radio signal. Derived from this was the opportunity for a person on earth to determine his or her position on the globe by reading the signal from the satellite if the precise orbit of the satellite was known. (Taylor, 2000) The MINITRACK system, which was developed in the late 1950sfor the U. S. Navy’s Naval Research Laboratory’s [NRL] Vanguard Satellite Program, used the signals emitted by Sputnik and later satellites to determine their positions and orbits. This evolved into tracking non-radiating satellites by signals reflected off of them. (U.S. Navy NRL, 2000)

The U. S. Navy’s NRL Naval Center for Space Technology (NCST) created the TIMATION (TIMe/navigATION) program in 1964. This program was designed to provide the basis for a navigation system with three-dimensional coverage (longitude, latitude, and altitude) throughout the world. In 1967 the Navy launched the TIMATION-I satellite to lay the foundation for the navigation system. (U.S. Navy NRL, 2000) The presence of an onboard crystal oscillator clock would allow the user to determine the length of time it took each signal to travel from the satellite to the receiver. The user then knew the distance to the satellite. The TIMATION I was followed in 1969 by the TIMATION-II satellite launch. (Beadles, 1998)

In 1973 the TIMATION program was merged with the Air Force's 621B program to form the NAVSTAR GPS program. The NRL’s TIMATION III satellite was re-designated as the Navigation Technology Satellite 1 (NTS-1) and was launched in the summer of 1974 in connection with the NAVSTAR effort. NTS-1 had onboard two atomic clocks, while the earlier TIMATION satellites had carried crystal oscillator clocks to test the NRL theory. (U.S. Navy NRL, 2000)

Another factor spurring the evolution of the satellite-based navigation system is found in the communication problems U.S. military personnel experienced during the Vietnam conflict. During that period, a system known as LORAN was utilized for locational information. The LORAN system 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. (Oguneye, 2000)

Although the TRANSIT system was a significant leap forward in GPS technology, it suffered from poor accuracy and was often inefficient. In the early 70's, the United States military began a program that would later be known as the NAVSTAR GPS program. (Taylor, 2000)

Research and development work on the NAVSTAR system began in 1973. In April, the U.S. Navy TIMATION system and Air Force System 621B 3-d navigation system were combined in a joint effort to develop a Defense Navigation Satellite System (DNSS). This combined system would later become NAVSTAR. (Beadles, 1998) Unlike earlier systems that had one satellite, the NAVSTAR system would offer several satellites for navigation. Availability of positioning data would be increased, and moving vehicles would not have to wait for extended periods of time to get the necessary readings. Originally named the Navigation Technology Program [NTP], the first four satellites of the NAVSTAR constellation were launched in 1978. (Taylor, 2000)

On July 17, 1995, the U.S. Air Force issued a press release announcing that the Global Positioning System satellite constellation had met all requirements for Full Operational Capability.

Current State The NAVSTAR GPS system today consists of 24 satellites in six orbits. The satellites are spaced in orbit so that at any time a minimum of 6 satellites will be in view to users anywhere in the world. (US Coast Guard, FAQ page, 2000)

There are currently four generations of GPS satellites: Block I, Block II/IIA, Block IIR and Block IIF. The Block I satellites were used for testing the principles of the system.

Block II and IIA satellites make up the current constellation. The third generation Block IIR satellites are currently being deployed as the Block II/IIA satellites reach their end-of- life and are retired. Block IIF satellites will be the fourth generation of satellites and will be used for operations and maintenance replacement. (Andrews Space & Technology, 2000).