Editing Global Positioning System (section)

=== Receiver in continuous operation ===
The description above is representative of a receiver start-up situation. Most receivers have a [[track algorithm]], sometimes called a ''tracker'', that combines sets of satellite measurements collected at different times—in effect, taking advantage of the fact that successive receiver positions are usually close to each other. After a set of measurements are processed, the tracker predicts the receiver location corresponding to the next set of satellite measurements. When the new measurements are collected, the receiver uses a weighting scheme to combine the new measurements with the tracker prediction. In general, a tracker can (a) improve receiver position and time accuracy, (b) reject bad measurements, and (c) estimate receiver speed and direction.

The disadvantage of a tracker is that changes in speed or direction can be computed only with a delay, and that derived direction becomes inaccurate when the distance traveled between two position measurements drops below or near the [[random error]] of position measurement. GPS units can use measurements of the [[Doppler shift]] of the signals received to compute velocity accurately.<ref>{{cite book |title=Global Positioning Systems, Inertial Navigation, and Integration |edition=2nd |first1=Mohinder S. |last1=Grewal |first2=Lawrence R. |last2=Weill |first3=Angus P. |last3=Andrews |publisher=John Wiley & Sons |year=2007 |isbn=978-0-470-09971-1 |pages=[{{google books|plainurl=y|id=6P7UNphJ1z8C|page=92 |title=Extract of pages 92–93}} 92–93] |url={{google books|plainurl=y|id=6P7UNphJ1z8C}}}}</ref> More advanced navigation systems use additional sensors like a [[compass]] or an [[inertial navigation system]] to complement GPS.