in Pilot Notes by John

What is EPU – Estimated Position Uncertainty? A320

Estimated Position Uncertainty is a measure of the accuracy or uncertainty associated with the estimated position of an aircraft. For an Airbus A320, EPU typically refers to the uncertainty in the aircraft’s calculated position. This uncertainty can be influenced by various factors including the accuracy of the navigation sensors, environmental conditions, and the quality of the position estimation algorithms used by the aircraft’s systems.

It is important to understand here that Flight Management Guidance System (FMGS) employs the following prioritized positions for navigation, listed in descending order of importance. Each position is associated with specific parameters and factors used to calculate the Estimated Position Uncertainty (EPU) value, tailored to the current navigation mode among four available options:

  • IRS-GPS (GPIRS position) – this is the condition when GPS Primary is displayed on MCDU
  • IRS-DME/DME (Mix IRS and Radio position)
  • IRS-VOR/DME (Mix IRS and Radio position)
  • IRS only (Mix IRS position)

EPU calculation with IRS-GPS (GPIRS) as current mode of navigation

In GPIRS mode, EPU is calculated with the help of figure of merit. Figure of Merit (FoM) in the context of GPS (Global Positioning System) refers to a simplified measure of the accuracy of the position solution provided by the GPS receiver. FoM is typically expressed as a numerical value or a scale, where a lower value indicates a higher quality position solution. If the value of calculated EPU is above 0.28 NM FMS rejects the GPS position and switch to IRS-DME/DME, IRS-VOR/DME or IRS only for navigation as per priority mentioned above.

EPU calculation with IRS-DME/DME (Mix IRS and Radio position) or IRS-VOR/DME as current mode of navigation

This is the condition when EPU of GPIRS position is more than 0.28 NM or GPS is not available. EPU here is dependent on the distance between the aircraft and the VOR/DME.

  • 0.1 NM + 0.05 X DME DIST
  • minimum 0.28 NM

EPU calculation with IRS only (Mix IRS position) as current mode of navigation

When aircraft is flying with IRS only (i.e. without the availability of GPS or radio navigation aids), FMGS has no source to update/recheck the position. In this scenario EPU increases continuously.

  • +8 nm/h for the first 30 min
  • 0 nm/h for the following 60 min
  • +4 nm/h for the following 30 min
  • 0 nm/h for the following 60 min
  • +2 nm/h after

Because of this increased position uncertainty, certain limitations (time restriction) are applicable to IRS only navigation mode. These limitations are given in PRO-SPO-PBN section of A320 FCOM.

Good to know points – EPU

  • After an IRS alignment or at takeoff, EPU is set to 0.2 NM
  • FMS displays EPU on progress page and compares it with RNP. If EPU is less than RNP value, accuracy is HIGH else LOW
  • Displayed RNP can be value entered by flight crew, navigation database procedure value or system’s default value
  • GPS PRIMARY is displayed as long as EPU is HIGH and system has capability to detect as failure and provide appropriate warning of it.
  • When GPS PRIMARY mode is lost, EPU must be constantly checked manually (it can be related to basic RNAV navigation mode)

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