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 than 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 .
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 software-defined GNSS receiver:
- Stand-alone receiver’s static positioning precision.
- Differential GNSS static positioning precision.
- Average convergence times to sub-metric precision.