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Accuracy
| Fundamentals | |
|---|---|
| Title | Accuracy |
| Author(s) | GMV |
| Level | Basic |
| Year of Publication | 2011 |
Accuracy[footnotes 1] is the degree of conformance between the estimated or measured position and/or velocity of a platform at a given time and its true position or velocity
Measuring Accuracy
Although being very easily understood from a conceptual point of view, the way that accuracy is measured and what is measured is not always obvious. The accuracy concept is generally used to measure the accuracy of positioning but can be also be used to measure the accuracy of velocity and even the accuracy of timming. For positioning there are 3 variants depending on the number of dimensions being considered: unidimensional accuracy (used for vertical accuracy), bidimensional accuracy (used for horizontal accuracy) and tridimensional accuracy (combining horizontal and vertical accuracy)
In literature and in system/product specifications it can be found measurements of accuracy such as CEP, rms, Percentile 67%, Percentile 95%, 1 sigma, 2 sigma. Some of these accuracy measures are averages while others are counts of distribution[2]:
- x% Percentile (x%): Means that x% of the positions calculated have an error lower or equal to the accuracy value obtained. Typical used values are 50%, 67%, 75% and 95%. Having an accuracy of 5m (95%) means that in 95% of the time the positioning error will be equal or below 5m.
- Circular Error Probable (CEP): Percentile 50%. Means that 50% of the positions returned calculated have an error lower or equal to the accuracy value obtained.
- Root Mean Square Error (rms): The square root of the average of the squared error. This measurement is an average but assuming that the error follows guassian distribuition (which is close but not exactly true) it will correspond to the percentile 68% in unidimensional distribuitions (e.g. vertical error or timming error) and percentile 63% for bidimensional distributions (e.g. horizontal error). For the horizontal error this measurement is also refered as drms and can have variants such as 2rms or 2drms (2 times rms)
- x sigma: One sigma corresponds to one standard deviation. Assuming guassian distribuitions this corresponds to Percentile 68% in unidimentional distribuitions and Percentile 40% for bidimentional distribuitions. See below the the correspondence between sigma and percentiles that can be used to correlate statistically between the different measurements used.
Assuming normal deviations these measurements can be converted between themselves
Notes
- ^ This accuracy definition has been taken from the 2008 US Federal Radionavigation Plan[1]
References
- ^ Federal Radionavigation Plan, DOT-VNTSC-RITA-08-02/DoD-4650.5, 2008
- ^ GNSS Accuracy: Lies, Damn Lies, and Statistics, GPS World, Frank van Diggelen, January 2007