11. Repeatability
Repeatability is related to the spread of a measure, also referred to as precision. It refers to how close a position solution is to the mean of all the obtained solutions, in a static location scenario.
The difference between accuracy and precision is shown below:
Although the two words precision and accuracy can be synonymous in colloquial use, they are deliberately contrasted in the context of the scientific method. Source: Wikipedia.
Most common precision metrics are defined below:
Measure  Formula  Confidence region probability 

2D 2DRMS  95 %  
2D DRMS  65 %  
2D CEP  , if  50 % 
3D 99 % SAS  99 %  
3D 90 % SAS  90 %  
3D MRSE  61 %  
3D SEP  50 % 
which are the same expressions as those defined for accuracy, but now the standard deviations are not referred to a reference value but to the mean of the obtained results:
where is the mean of all the coordinates of the obtained positioning solutions, are the East coordinates of the obtained positioning solutions, and is the number of available position fixes. Similar expressions can be defined for the North and Up coordinates:
where , and
where .
Example:
2D position scatter plot and the circles containing 50 %, 65 % and 95% of position fixes (corresponding to the CEP, DRMS and 2DRMS precision errors, respectively)^{1}.
Indicators of Repeatability
It follows a list of possible repeatability indicators for a softwaredefined GNSS receiver:
 Standalone receiver’s static positioning precision.
 Differential GNSS static positioning precision.
 Average convergence times to submetric precision.
References

C. FernándezPrades, J. Arribas and P. Closas, Turning a Television into a GNSS Receiver, in Proc. of the 26th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+ 2013), Nashville, TN, Sep. 2013, pp. 1492  1507. ↩
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