Global Positioning System is the now first choice for most navigation applications. With the fully operational satellite constellation, it is common to have overdetermined solutions using more than four GPS satellites. These overdetermined solutions have become a key technology in the more demanding GPS applications. The surplus measurements should be consistent; if not, one or more of the measurements is probably incorrect. Consistency measures provide detection of measurement failure, and possible removal of measurement failure, for timing and navigation applications where the accuracy of the solution is critical. The same consistency measures have become a necessary element in the development of real-time kinematic systems, where carrier phase interferometry leads to solutions with centimeter-level accuracy.