Errors in the pressure and Blanchard altitudes for a cross country flight

Abstract

The pressure altitude obtained from a barometric altimeter is commonly used by an Inertial Navigation System (INS) as an altitude reference for the vertical channel. Pressure altitude is computed from the pressure based on a stationary atmospheric model for the standard day. Significant error (up to 10%) exists in the pressure altitude due to deviation of the actual atmosphere from the standard day. An alternate algorithm that possibly computes a more accurate altitude is the Blanchard algorithm.