ADR data

IR data
IR data
-Autonomous architecture for aircraft equipped with ADIRU 10MCU not able to process GPS position. TAWS uses the GPS position directly from MMR or GPS-SU. It applies on former AIRBUS aircraft (A300/A310 and former A320).

MMRs or GPS-SU
GPS

+
ADR data
IR data

GPS

+
IR data

MMRs or GPS-SU

ADR data IR data
IR data

4.1.4.1. EGPWS GEOMETRIC ALTITUDE – T2CAS CPA ALTITUDE
When operating with extreme local temperature variations, in non-standard altitude conditions (i.e. QNH or QFE), or when the altimeter is not set correctly, the barometric altitude may significantly deviate from the current altitude.
To provide efficient alerts with appropriate altitude clearances regardless of temperature/pressure variations, QNH/QFE or manual error settings, new generation TAWS use the Geometric Altitude (also known as Alternate Vertical Position based on GPS for T2CAS). The Geometric Altitude takes into account the GPS altitude, an improved barometric altitude calculation, the radio altitude, and the terrain/runway elevations. The Geometric Altitude provides a more reliable altitude indication to TAWS. Indeed, temperature/pressure variations, altimeter settings (QNH/QFE or manual) do not affect the GPS altitude.
4.1.4.2. USE OF GPS FOR LATERAL POSITIONING
In-service experience has shown that improper IR alignments or erroneous navaid signals may affect FM and ADIRU data. Consequently, the TAWS position (FM position or IR data) may significantly deviate from the current aircraft position. This deviation from the current aircraft position is known as Map Shift.
Note : The SIL 22-043 describes the root causes of a Map Shift (See AIRBUS
References) and proposes some solutions. The following summarizes the
root causes:
-On ground:
o Incorrect PPOS at IR initialization,

o Take-off update: incorrect FM position at take-off power setting,

o ADIRU auto-calibration malfunction.

-In flight:
o Incorrect Navaids coordinates in FM NAV database,

o Excessive IR drift.

-During approach:

o LOC update: incorrect LOC data in FM NAV database,

o Incorrect information provided by Navaids.

The TAWS displays the terrain background on ND according to its terrain database and the FM position (with former TAWS architecture). The deviation of the FM position from the current aircraft position induces a shift of the terrain display on ND. The Map Shift may cause some spurious alerts or inhibit real alerts. Figure 4-16 illustrates a Map Shift that leads to spurious alerts.

The Use of GPS for Lateral Positioning (also known as Alternate Lateral Position based on GPS for T2CAS) significantly reduces errors in the calculation of the aircraft position. Consequently, it reduces occurrences of spurious errors.
　
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