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For exemple on the GNS430 satellite status page you can read, in addition to the GPS receiver status :

  • EPE (Estimated Position Error)
  • DOP (Dilution of Precision)
  • HUL (Horizontal Uncertainty level)

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I found here explanations about DOP computation, and GNS430 manual states that:

"EPE uses DOP and other factors to calculate a horizontal position error, in feet or meters".

About HUL, the same manual explains that

"the Horizontal Uncertainty Level displays a 99% confidence level that the aircraft position is within a circle with a radius of the value displayed in the HUL field"

It seems to me that EPE and HUL are exactly the same thing and differ only by the confidence level. If it's the case, what is the confidence level of the EPE ? If not, what is the difference between EPE and HUL ?

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EPE is the 1-sigma error level and thus represents the position error circle with a 68% confidence level. Reference: NPS

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