Global Positioning System (GPS)

          Global Positioning System (GPS) is a space based radio navigational system that was designed and implemented by the United States Department of Defense to be a military force enhancement system 1. The GPS network is managed jointly by the Department of Defense and the Department of Transportation 2. The network has, however, become more accurate, effective, and popular for civilian use.

          GPS is easiest understood when divided into three major segments:

          1) space, 2) control, and 3) user 3.



          The first GPS segment is the space segment, which consists of 24 satellite vehicles that orbit the Earth in six orbital planes (see right)4. There are 4 equally spaced satellites per orbital plane, and the vehicles complete an orbit once every 12 hours5. This means that there are always between 5 and 8 satellites visible anywhere on Earth at any given time6. The satellite vehicles broadcast radio signals that can be detected by GPS users 12,000 miles away on Earth7. The GPS receiving units measure these digital transmissions that arrive from the satellites to figure how far away each satellite is, and then apply trigonometry and the concept of triangulation to pinpoint its location on Earth8. In order to get an accurate measurement, the receiver must gather signals from at least 4 different satellites9.

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          The second GPS segment, the control segment, consists of a worldwide network of GPS monitoring stations and a Master Control Station (MCS) near Colorado Springs, CO11. This network performs the crucial task needed to make precise positioning measurements by ensuring the accuracy of satellite positions and their clocks12. There are five monitoring stations that continuously track all satellites in view13. The information that these stations gather is then sent to the MCS, which determines the state of each satellite’s position and the accuracy of each satellite’s atomic clock14. The ultimate goal of the MCS, and the control segment, is to upload up to date and corrected navigational data to the satellites15.


          The third GPS segment is the user segment, which consists of the GPS receiver and the entire user community16. The GPS receivers can be used to receive and compute solutions for a variety of uses including navigation, positioning, velocity, precise timing, and other forms of research. The figure at right shows how a GPS unit receives signals from four different satellites to get an accurate measurement17.


          GPS technology was used in the Stephanie Way area (near Carson City, Nevada) to map the Ground Penetrating Radar (GPR) lines (below left), and to map certain characteristics of the landscape, such as a recent flow channel cut into an alluvial fan (below right).


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          GPS measurements were taken continuously over the highway throughout the week of the Western Nevada Field Studies course.  The picture below-left shows the GPS receiver that was mounted on top of a vehicle and held in place with a magnet.   The GPS data that was gathered from Reno, Nevada, to Pyramid Lake, Nevada, is shown in the picture to the lower right.


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1 Herring, T. A. 1996. The global positioning system. Scientific American, 274:1-7.
2 Arnold, J.A., 1998. Surface transportation and global positioning system Improvements: L5 and DGPS. Public Roads,   61:1-10.
3 Dana, P., 1998. Global positioning system overview.
4 Dana, P., 1998.
5 Arnold, J.A., 1998.
6 Dana, P., 1998.
7 Dana, P., 1998.
8 Hurn, J. 1989. GPS: A guide to the next utility. Sunnyvale, California: Trimble Navigation Ltd., 38-56.
9 Hurn, J. 1989.
10 Dana, P., 1998.
11 Arnold, J.A., 1998.
12 Arnold, J.A., 1998.
13 Arnold, J.A., 1998.
14 Arnold, J.A., 1998.
15 Arnold, J.A., 1998.
16 Dana, P., 1998.
17 Dana, P., 1998.