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.
The most common precision metrics are defined below:
Measure  Formula  Confidence region probability 

2D 2DRMS  \(2\sqrt{\sigma_E^2+\sigma_N^2}\)  95 % 
2D DRMS  \(\sqrt{\sigma_E^2+\sigma_N^2}\)  65 % 
2D CEP  \(0.62\sigma_N+0.56\sigma_E\), if \(\frac{\sigma_N}{\sigma_E}>0.3\)  50 % 
3D 99 % SAS  \(1.122 \left(\sigma_E^2+\sigma_N^2+\sigma_U^2\right)\)  99 % 
3D 90 % SAS  \(0.833 \left(\sigma_E^2+\sigma_N^2+\sigma_U^2\right)\)  90 % 
3D MRSE  \(\sqrt{\sigma_E^2+\sigma_N^2+\sigma_U^2}\)  61 % 
3D SEP  \(0.51 \left(\sigma_E^2+\sigma_N^2+\sigma_U^2\right)\)  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:
\[\begin{equation} \sigma_{E}^{(precision)} = \sqrt{\frac{1}{L1}\sum_{l=1}^L \left(E[l]  \hat{E}\right)^2}~, \end{equation}\]where \(\hat{E}=\frac{1}{L}\sum_{l=1}^{L}E[l]\) is the mean of all the \(E\) coordinates of the obtained positioning solutions, \(E[l]\) are the East coordinates of the obtained positioning solutions, and \(L\) is the number of available position fixes. Similar expressions can be defined for the North and Up coordinates:
\[\begin{equation} \sigma_{N}^{(precision)} = \sqrt{\frac{1}{L1}\sum_{l=1}^L \left(N[l]  \hat{N}\right)^2}~, \end{equation}\]where \(\hat{N}=\frac{1}{L}\sum_{l=1}^{L}N[l]\), and
\[\begin{equation} \sigma_{U}^{(precision)} = \sqrt{\frac{1}{L1}\sum_{l=1}^L \left(U[l]  \hat{U}\right)^2}~, \end{equation}\]where \(\hat{U}=\frac{1}{L}\sum_{l=1}^{L}U[l]\).
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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