How is aircraft position calculated in A320?

There are two FMGC (Flight Management Guidance Computers) on board. Each FMGC computes its own aircraft position (called the “FM position”). To grasp the method of position calculation in the A320, it’s crucial to first comprehend three key terms individually. Once these concepts are clear, connecting them becomes much simpler. In this article, we aim to simplify this understanding for you. The three terms in question are: MIX IRS position, Radio position, and GPS position.

Mix IRS position

Each FMGC receives position info from IRS 1, 2, and 3. This info is utilized by each FMGC independently to calculate a mean-weighted average position. This average position is called MIX IRS position. This approach helps improve the accuracy and reliability of the position calculation by combining data from multiple sources. So, both FMGC have their own MIX IRS position.

Radio Position

Each FMGC utilizes onside navaids to compute its own radio position. The available navaids include DME/DME, VOR/DME, LOC, DME/DME-LOC, and VOR/DME-LOC. If one or more navaids fail, each FMGC can use offside navaids to compute the VOR/DME, or the DME/DME radio position.
The radio navaid selection is displayed on the DATA “SELECTED NAVAIDS” page.

FMGS automatically tunes the radio navaids for computation of radio position. Manual selection of a VOR or VOR/DME may prevent FMGS from automatically tuning a VOR/DME to compute position. In this case, the related MCDU displays “TUNE xxx yyy.yy”. (ident and frequency)

GPS position

Each IRS computes a mixed IRS/GPS position known as the GPIRS position by integrating GPS data. Each IRS can independently select its GPS source to maximize GPS data availability. Now each FMGC receives three GPIRS positions and FMGS selects one GPIRS position based on figure of merit and priority hierarchy:

  1. Onside GPIRS position
  2. GPIRS 3
  3. Opposite GPIRS position

Computation of position – A320

Now coming back to where we started, there are two FMGC (Flight Management Guidance Computers) on board. Each FMGC computes its own aircraft position (called the “FM position”). FM position calculation can be attributed to two methods:

Method 1 – This includes MIX IRS position and computed radio position
Method 2 – GPS position (this GPS position itself is GPIRS position)

FM uses following positions to navigate in decreasing order of priority:

  • 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)

During approach flown with ILS/MLS/GLS/SLS the system performs a temporary lateral update, via IRS-GPS/LOC, IRS-DME/DME-LOC.

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