A variety of factors, including the clocks in satellites and receivers, the atmosphere, satellite orbits, and reflective surfaces near the receiver, degrade the quality of GPS coordinates. The arrangement of satellites in the sky can make matters worse (a condition called dilution of precision). A variety of techniques have been developed to filter out positioning errors. Random errors can be partially overcome by simply averaging repeated fixes at the same location, although this is often not a very efficient solution. Systematic errors can be compensated for by modeling the phenomenon that causes the error and predicting the amount of offset. Some errors, like multipath errors caused when GPS signals are reflected from roads, buildings, and trees, vary in magnitude and direction from place to place. Other factors, including clocks, the atmosphere, and orbit eccentricities, tend to produce similar errors over large areas of the Earth's surface at the same time. Errors of this kind can be corrected using a collection of techniques called differential correction.
In this section you will learn to:
- explain the concept of differential correction and other methods used to improve the accuracy of GPS positioning; and
- perform differential correction using data and services of the U.S. National Geodetic Survey.