The assessment of the vertical position of the aircraft, in relation to the glide path angle (GPA) during theapproach is based on barometric data.

The calculated true air speed (TAS) is based on the standard atmosphere and any significant differences willlead to an approach sneed which could be too slow or too fast.

The difference between true height and published decision height has to be limited to a maximum value.

Static vents can be blocked due to icing when approaching in IMC, hence leading to a loss of verticalquidance.