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Pressure altitude is commonly utilized as an altitude reference for an inertial navigation system (INS) to damp the error growth in the inherently unstable vertical channel. A precise altitude reference for use in the INS vertical channel can be obtained using the Blanchard algorithm, which computes altitude from atmospheric pressure, temperature, aircraft ground velocity, and wind velocity data. This paper computes both the pressure and Blanchard altitudes for an entire test flight of an F-18 aircraft from the atmospheric data measured during the flight. The flight repeats 4 cycles of a climb, level-off, dive, level-off trajectory. The altitude computed from GPS during flight is considered to be the truth altitude. The errors in the pressure and Blanchard altitudes are computed and compared. In addition both altitude errors are analyzed in order to determine the scale factor, bias offset, and time delay utilizing the least square error fit method. The Blanchard altitude is a much more precise altitude reference than pressure altitude during actual flight of an F-18 aircraft.