Introduction to Earth Systems: Lesson 8 Collection of GPS Data   Reading Assignment: David Greenhood, Mapping, Chapters 1 and 2 NAVIGATION (To return, use back button) Reading Key Concepts Discussion More Links Exercises
Side Notes	Key Concepts	Links
Orbits of Satellites           Signal quality	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. orbits of satellites signal quality role of data in GPS Collection of GPS data The GPS satellites are distributed in orbits around Earth at approximately 12,500 miles altitude and among six orbital planes. There are four working satellites in each plane and, in some, backup satellites. Each plane is inclined to the plane of the Equator at an angle whose measure is 55°. The lines that are intersections of the orbital planes with the plane of the equator also intersect Earth's axis and are distributed uniformly around the axis. At any time of any day from any location on Earth, there are likely to be at least four satellites in view. Subject to variations in arrangement of the satellites, a GPS receiver processes several signals simultaneously to determine the current location of the receiver. Quality of results also depends on signal quality. Optimum results require favorable distribution of satellites at the time that data is collected. Since a receiver uses several satellites, results are generally best when the satellites are in substantially different directions from the receiver and are not at extremely high or extremely low angles of elevation. From satellites at low angles of elevation, signals tend to be degraded by long paths through the atmosphere. From high angles of elevation, geometry is unfavorable for the determination of altitude of the receiver. Factors affecting signal quality also include atmospheric conditions and selective availability, the latter a deliberate degradation of signal. The degree of degradation caused by selective availability varies with need as determined for reasons of national security.
Data in navigation	Processing of data Navigation: The immediate use of data collected is, of course, for reporting coordinate names of points, usually locations of the receiver. So, for example, a GPS unit in a moving car can report coordinates of the car's position as it changes location. And a hiker can use a hand-held GPS unit to determine his/her location. As an extension of this capability, one can store coordinates of locations in a GPS receiver and then navigate toward the locations. Thus a route can be defined by a sequence of locations and then, with the aid of a GPS receiver containing the coordinates of the locations, traveled according to that definition. In addition to storing locations for navigation, some GPS receivers can display a map of a route and show the location of the receiver on the map.	GPS navigation.
Data in map-making	GIS applications: Increasing use is made of GIS for generation of maps and for analysis using methods of GIS (geographic information systems). In those cases, data points are identified, usually during data collection, according to a collection of categories called a "data dictionary." These categories provide a basis for annotation on maps and for illustration of themes, i.e. collections of data with common attributes other than the fact that the data points have a location component. Examples include city maps showing specific attributes or parts of infrastructure, e.g. trees, large buildings or communication lines. In some products, the receiver is capable of storing one or more data dictionaries. Data collected is then immediately identified in terms of the categories in the data dictionary. From there, in most cases, the data is eventually uploaded to a computer for analysis, categorization, map-making, etc.	GPS and GIS.
	Links How GPS works. Principles and Practices of GPS Surveying Precision farming Sources of GPS receiver data     TOP   Discussion Exercises Describe the system of satellites that comprises the Global Positioning System. Include the number of satellites, descriptions of orbits, etc.   With respect to a particular location on Earth, describe optimum conditions, including distribution of satellites, for high quality reception at a GPS receiver.   Describe two examples illustrating the use of GPS data for navigation in a commercial application, e.g. a public transportation system, package delivery system, etc. Include description of some advantages provided by GPS technology in these settings.   Apply the instructions in Exercise 3 to two commercial uses of GPS data for map-making.