Introduction to Earth Systems: Lesson 9 GPS Accuracy   Reading Assignment: Trimble Navigation, Introduction to Differential GPS NAVIGATION (To return, use back button) Reading Key Concepts Discussion More Links Exercises
Side Notes	Key Concepts	Links
Varying quality in estimates of location                 Sources of error	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. causes of error in GPS data collection signal degradation satellite geometry obstacles in path of signal range of accuracy differential correction   Errors Through use of the precise clocks on the GPS satellites, and of encoded signals transmitted by the satellites, a receiver on Earth can determine, with some error, its distance from the satellite. With only that data and the locations of the satellites in terms of Earth coordinates, it is known to be on a sphere whose center is the satellite and whose radius is the distance from the satellite to the receiver. Use of communication with four such satellites places the receiver on the intersection of four spheres, i.e. theoretically at a point. At all times, the location of a satellite, in terms of Earth coordinates, is known to the satellite. The accumulation of errors in the estimates of distances, and estimates of the locations of the satellites, induces errors in calculation of coordinates of the GPS receiver. Nevertheless, GPS methods can be used to calculate useful approximations of location of the receiver in terms of Earth coordinates. Depending on quality of the data and of capabilities of the equipment, errors produced vary from more than 100 meters to less than one centimeter. The primary reason for errors in the estimates of distance from the satellites is degradation of the signal sent from the satellite. Causes of the degradation include atmospheric conditions, effect of satellite location on the length of the signal path through the atmosphere, obstacles in the signal path, and selective availability. To minimize degradation due to atmospheric conditions, one can wait for clear, calm weather or some degree of that, and choose satellites well above the horizon, e.g. at angles of elevation more than 15 degrees. And satellite location can be determined in advance in various ways, e.g. from data easily available on the Internet. Obstacles can be worked around through remote data collecting with the aid of measuring devices, e.g. compass, tape, and/or electronic measuring tools. Beyond its effect on signal quality, location of the satellites also affects accuracy through the geometry of their locations with respect to each other. Best estimates are obtained when the satellites used by the receiver are scattered asymmetrically in widely different directions about the sky and, at different angles of elevation ranging from approximately 15 degrees -- this depends on atmospheric conditions -- to approximately 70 degrees. Selective availability is deliberate degradation of signal imposed for reasons of national security. The degree of degradation varies with the threat, perceived by those in control of the GPS system, posed by unmodified GPS communication, e.g. as a source of information for enemies of the United States. Removal of selective availability is now being considered.	Introduction to Differential GPS by Trimble Navigation
Improving accuracy	Processing of data Error Correction: Without access to correction processes other than those available in the simpler hand-held receivers, some improvement of accuracy can be effected through patience and calculation. Determining estimates for a single location is generally done by accumulation of several "hits" from the satellites, from which the receiver calculates an average. Assuming favorable signal conditions, absence of moving obstacles, etc. accuracy improves with increases in the number of hits. It is possible to improve accuracy by spending more than minimum time at a location in order to collect more data. Differential correction is a commonly used method for refining estimates of location and is generally more accurate than the method described above. It requires use of a base station, i.e. a receiver that does not move and whose location coordinates are known with a high degree of accuracy. After data is collected by the GPS receiver, the data is compared with that collected by the base station for the same time interval. Differential correction is the mathematical method used to refine the estimates from the GPS receiver based on data from the base station. With medium-grade receivers, the estimates can be refined from errors measured in tens of meters to, in many cases, errors of less than two meters. With some such receivers, differential correction is managed by processing data on a computer after it is uploaded from the GPS receiver. In some higher grade receivers, differential correction is done in real time, i.e. at very nearly the time of collection. Available accuracy varies to less than a centimeter.	GPS and GPS accuracy                   Real time differential correction
Applications using GPS data	Application A primary use of corrected GPS data is in applications of GIS (geographical information systems). Such data is used for the creation of maps, i.e. images of spatially-based data. Such images can be maps of parks; of infrastructure such as roads and communication lines; of land use, e.g. for applications in farming; of forest management; of urban land use; of archeological digs; many other examples. With the aid of image-processing software designed for manipulation of GPS data, the data can be used to generate images that overlay onto existing maps. Thus, for example, land use maps can be updated by overlaying images of recent data onto existing images. In conjunction with processing of data for accurate representation, the use of a data dictionary makes possible the creation of accurate, annotated maps from GPS data. With the availability of high accuracy in real time, i.e. so that coordinates of a location are immediately available, very precise navigation is possible. Thus, for example, one can navigate accurately to the location of a specific plant, archeological artifact, street signal, etc.
	Links Center for Advanced Spatial Technologies Time service department Overview of GPS How GPS works Real time map of locations of GPS satellites     TOP   Discussion Exercises Usefulness of GPS applications varies with their tolerance for error in their computation of coordinates. For each of the following error magnitudes, describe a GPS application for which the given error provides accuracy that is necessary and sufficient for the application. 100 meters 10 meters 1 meter 1 centimeter   Describe the role of a data dictionary in the collection and use of GPS data.   Describe some causes of degradation of signals from GPS satellites.