Introduction to Earth Systems: Lesson 7 The Global Positioning System Reading Assignment: Internet link:  How GPS works     and printed version:     Jeff Hurn for Trimble Navigation           GPS, A Guide to the Next Utility     NAVIGATION Reading Key Concepts Discussion More Links Exercises
Side Notes	Key Concepts	Links
System of satellites	These are the key concepts in this lesson. You should be able to define or describe them after you have read this lesson, finished the assigned reading and explored the links. You should also be able to do the exercises and answer the questions at the end of the lesson. GPS GPS satellite system GPS accuracy Applications of GPS The Global Positioning System The Global Positioning System (GPS), developed for military purposes, has evolved to include a role as a primary tool for many non-military applications. It's primitive purpose is to enable the determination, from any place on Earth, of coordinates that name that place. From this come applications now increasing rapidly in variety and volume. The system depends on an array of satellites, distributed in orbit 12,500 miles above Earth. At any given time and location, barring local barriers, several satellites are scattered around the sky and are in line of sight. They are equipped with transmitters that are in continuous operation, and that have very precise clocks.	GPS overview
Accuracy               Navigation       Map-making	Use of the system requires an electronic receiver at the place of interest. Assuming favorable conditions for reception, the receiver uses signals from satellites, preferably at least four, to calculate approximations for Earth coordinates and elevation. All receivers can report readings in the latitude-longitude system; some can also report in UTM and other systems. The receivers vary in quality and function, but all have the capability to use the satellite signals for determining location. Available accuracy varies from approximately 100 meters to less than a centimeter depending on quality of equipment, signal conditions and presence of obstacles. Real-time determination of location at affordable prices has stimulated new products for navigation and data-gathering. Several hand-held models of GPS receivers ranging in price from $100 to $300 are available through suppliers of GPS hardware, outdoor recreation equipment and electronic hardware. Much consumer interest in such low-priced GPS equipment is stimulated by interest in navigating along walking and bicycle trails, and in recreational boating. Commercial applications for the navigation and real-time locating capabilities of GPS include some from traditional settings, i.e. for navigation in air and sea travel. More recent commercial applications include the tracking of vehicles in transportation systems, generating real-time driving instructions toward a preset destination, and keeping track of fleets of vehicles while they are working. Beyond the uses described above, GPS provides extensive capabilities for gathering of data. Portable receivers are used to store latitude-longitude data, thereby recording, for entry into a graphical data base, data that can be illustrated as maps. As was the case prior to the availability of GPS technology, maps can represent a wide variety of attributes associated with location. Some examples are identification of cities, types of land use, topographical features, political sub-divisions. These maps can illustrate continuous objects such as trails, building perimeters, etc. or discrete objects such as trees, phone booths, etc. Thus the data is useful for making maps of parks, infrastructures, transportation routes, archeological digs, and many other applications involving location. All of the applications described above involve the collection and use of data that has several components, one of which is location. For example, one data point in a data base might be associated with latitude, longitude, elevation, terrain, location within a political subdivision, etc. As before, such data, in which location is an attribute of every data point, is called "spatial data". Besides its other capabilities, GPS provides a means for easy, rapid collection of such data.	Real time display of satellite orbits, including GPS satellites
	Links GPS links. NAVSTAR GPS GPS Novice Net     TOP   Discussion Exercises For each of the following areas of application, describe a situation for which real-time knowledge of location or easy collection of location-based data would be useful. A highway towing service A corporate farming operation, comprised of several dozen plots of land and managed through use of computer data bases. A visual record of all trees in a large park that meet certain specifications, e.g. size, health, species.   Describe three examples, other than those cited in Exercise 2., of potential applications for GPS. These will be applications using spatial data. Describe the attributes that are associated with location. Go to WWW site Real time display of satellite orbits, including GPS satellites, and determine the orbits of some of the GPS satellites. You will see that these satellites are not geo-synchronous, but their plane of orbit makes a large angle with the equatorial plane. Assuming that this angle is the same for all satellites, examine the illustration of orbits at the site and estimate the angle.