Global Positioning Systems

The global positioning system (GPS) is a satellite-based navigation system designed and operated by the U.S. Department of Defense (DoD). GPS can provide three-dimensional (3D) position and guidance in any weather and at any time of the day over the entire surface of the Earth, in the air, and in low space orbits. GPS consists of a control segment run by DoD, a space segment consisting of 24 or more satellites, and a user segment that includes military and civilian receivers.

GPS evolved from earlier regional and global radio navigation systems such as the Navy Transit System, Omega, and Loran-C. It was first described in the mid 1970s, and by 1985, there were enough satellites to allow development and testing of receivers for land, sea, and air navigation and guidance as well as for time and frequency dissemination and for both geodetic and plane surveying. The system was declared fully operational in 1995.

There is a wide range of GPS services and techniques with different capabilities, limitations, and [Page 1343]costs. There are two primary services that are provided by GPS and controlled by DoD. The Precise Positioning Service (PPS) is for use by the U.S. military, approved allied armed forces, and some agencies of the U.S. government. The PPS provides for encryption of the PPS bit stream (the P-Code) that is transmitted by the GPS satellites, mitigating the threat of spoofing, or tricking, a military receiver into tracking GPS-like signals transmitted by an adversary. Decrypting the PPS signals requires authorization and access to secure cryptographic keys.

The Standard Positioning Service (SPS) is now available, without restriction or charge, to everyone. For a time, between 1990 and 2000, the SPS bit stream (the C/A code) was intentionally degraded by Selective Availability to deny high-accuracy positioning to non-DoD users. The SPS, sometimes known as civilian GPS, is used throughout the world by a wide range of users in a wide variety of applications. Recreational hikers and boaters, users of automobile navigation systems, general aviation pilots, and data attribute collectors use inexpensive receivers to track the SPS signals.

The 24 or more GPS satellites orbit the Earth every 12 hrs. (hours). Earth rotates beneath the constellation, so the ground track of the satellites repeats in just less than 24 hrs. Monitored, adjusted, and provided with orbital and clock information by the control system, the satellites send their positions, their atomic clock errors, and complete system information on a bit stream called the Navigation Message. Relative ranges to satellites are determined within the receiver by lining up bit streams (P-Code and C/A code) sent by the satellites with identical codes produced in the receiver. The Navigation Message is sent from the satellites to the receiver over microwave frequency carrier signals that “spread” the SPS and PPS codes to resemble noise. When the receiver aligns its version of the codes with the noiselike GPS signals, the carrier frequencies are “de-spread,” providing the receiver with the contents of the Navigation Message and a code-phase relative arrival time for each satellite signal. Using at least four satellite messages and code-phase arrival times, a GPS receiver can produce a full 3D position solution and a correction for the receiver's inexpensive clock.

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