What is GPS?

GPS is a satellite-based navigation system originally developed for military purposes and is maintained and controlled by the United States Department of Defense. GPS permits land, sea, and airborne users to determine their three-dimensional position, velocity, and time. It can be used by anyone with a receiver anywhere on the planet, at any time of day or night, in any type of weather. This is an amazing capability!

There are two GPS systems: NAVSTAR - United State's system, and GLONASS - the Russian version. The NAVSTAR system is often referred to as the GPS (at least in the U.S.) since it was generally available first. Many GPS receivers can use data from both NAVSTAR and GLONASS; this report focuses on the NAVSTAR system.


GPS uses radio transmissions. The satellites transmit timing information and satellite location information. The system can be separated into three parts:

Space Segment

Control Segment

User Segment

This page includes several figures to help describe the system. The following figure illustrates how the three segments fit together (Figure 1)

System Segments (Fig 1)
Figure 1 - GPS Segments


Space Segment

The space segment consists of the satellites themselves. According to the United States Naval Observatory, there are currently 27 operational GPS satellites about 11,000 miles up in space. This constellation (see Figure 2 below) provides between five and eight GPS satellites visible from any point on the earth. The next scheduled launch is May 10, 2000.

Figure 2 - The Space Segment

It takes each satellite about twelve hours to orbit the earth. There are six orbital planes with at least four satellites in each plane.



Control Segment

The control segment is a group of ground stations that monitor and operate the GPS satellites. There are monitoring stations spaced around the globe and one Master Control Station located in Colorado Springs, Colorado (see Figure 3 below). Each station sends information to the Control Station which then updates and corrects the navigational message of the satellites. There are actually five major monitoring systems, the figure below does not include the Hawaiian station.

Control Segment
Figure 3 - The Control Segment



User Segment

The user requires a GPS receiver in order to receive the transmissions from the satellites. The GPS receiver calculates the location based on signals from the satellites. The user does not transmit anything to the satellites and therefore the satellites don't know the user is there. The only data the satellites receive is from the Master Control Station in Colorado. The users consist of both the military and civilians.




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