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

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. The Blanchard algorithm computes altitude based on a dynamic atmosphere model using measured air pressure, temperature, and crosswind. This paper computes both the pressure and Blanchard altitudes versus GPS for a cross country flight from Eglin to PAX. On the ground before takeoff of the aircraft, the pressure altitude differs from GPS altitude by -304 ft. The atmospheric pressures used to compute the pressure altitude for the entire flight are adjusted for this pressure altitude error so that the corrected pressure altitude equals the GPS altitude when the aircraft is still on the ground before takeoff. The computed Blanchard altitude is initialized to agree with the GPS altitude on the ground before aircraft takeoff. The mean and Root Mean Square (RMS) values of the pressure and Blanchard altitude errors are used for comparing the performance of the two algorithms. In the accuracy comparison, the pressure altitude outperforms the Blanchard altitude for this particular flight. Both errors are analyzed by least-square-error curve fit. The residual for fitting the Blanchard altitude error is much smaller than fitting pressure altitude error (44.7 ft vs. 185.1 ft RMS). Contributions to altitude error from the following error sources are assessed: scale factor, bias, pressure, pressure time delay, vertical velocity, and head wind. The model improves the static Blanchard altitude RMS error by a ratio of 20 to 1 for this particular flight.