        Understanding the GPS System

GPS navigation is based on measuring the distance from the user position to the precise locations of the GPS satellites as they orbit. By measuring the distance to four GPS satellites, it is possible to establish the three coordinates of a user's position (latitude, longitude, and altitude), as well as GPS time.

Each GPS satellite transmits a unique code that is detected by the ground receiver. Each code is broadcast with precise timing as determined by the onboard clock. The code also identifies the satellite's exact location. The receiver estimates the signal travel time by calculating the time difference indicated by the satellite and the receiver, multiplied by the speed of light. This calculation is called the estimated range. If the clock in the receiver was as precise as the atomic clock onboard the satellite, this range could be considered accurate. However, today's receivers rely on inexpensive quartz clocks, and the first calculation is actually a pseudo-range.

Each position fix relies on four separate measurements. The pseudo-range measurements produce four separate equations and four unknowns-three user coordinates and the clock error. The GPS receiver uses the fourth satellite signal to mathematically eliminate the clock error, providing a true measurement of the receiver's range to the satellites and synchronizing the receiver's clock to GPS system time. If the receiver is moving, its velocity is calculated similarly, using the pseudo-range rate estimated by measuring the Doppler shift of the broadcast frequency to produce four separate equations. The position calculation is performed using an Earth-centered, Earth-fixed coordinate system, which is defined by the World Geodetic System 1984 (WGS 84). WGS 84 is a precise model that provides a common grid system for describing positions on the Earth. The principles behind the Global Positioning System are simple. The system is based on measuring the distance from satellites in known orbit positions.

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