Although this is a text on GIS—and hence primarily concerned with positional issues—it would not be complete without mentioning what may, for the average person, be the most important facet of GPS: providing human beings with a universal, amazingly precise, and accurate time source. In fact, GPS probably should be called a GPTS (Global Positioning and Timing System). Allowing any person or piece of equipment to know the exact time has tremendous implications for things we depend on every day (like getting information across the Internet, like synchronizing the electric power grid and the telephone network). Further, human knowledge is enhanced by research projects that depend on knowing the exact time in different parts of the world. For example, it is now possible to track seismic waves created by earthquakes, on one side of the Earth, through its center, to the other side since the exact time4 may be known worldwide.5
1 These accuracies pertain to “mapping grade” data collection. By spending more money and much more time one can shift to “survey grade” GPS, with accuracies down to a centimeter.
2 Officially, the GPS system is divided up into a space segment, a control segment, and a user segment. We will look at it a little differently. One of the many places to see official terminology, at the time this book went to press, is http://tycho.usno.navy.mil/gpsinfo.html#seg.
3 An almanac is a description of the predicted positions of heavenly bodies.
4 Well, okay, ...