Chapter 5: Mobile Mapping Fundamentals

Nicolas MalloyAuthor: Nicolas R. Malloy


5.21 GPS Uncertainty and Error


Collecting geospatial data using GPS units requires an understanding of the sources of uncertainty and error and the limitations of the global positioning system. The United States Department of Defense (DOD) originally developed NAVSTAR for military applications. As a defensive measure, they included intentional degradation and error in the system called selective availability (SA). At any time, the DOD could introduce this error into the system when necessary for military defense.  In the early years of GPS, the DOD had SA “turned on” continuously.  In May 2000, Pres. Bill Clinton directed the U.S. government to discontinue the use of selective availability. This was done to make the use of GPS more reliable for research and commercial applications. Currently in development are a new series of satellites without the selective availability feature called GPS III.
With selective availability no longer an issue, that leaves us with a series of errors called user equivalent range errors (UERE). UERE are sources of error in satellite ranging that contribute to the total error budget, the cumulative sum of range errors from each source. These sources of GPS error include:

  • Satellite clock
  • Orbital
  • Atmospheric
  • Receiver clock
  • Signal Masking
  • Multipath

GPS Receiver Sources of Error – SixtySec by ExploreGate