Analysis of a Transition from Baro Altitude to Global Navigation Satellite System Altitude

Classic vertical air navigation is based on air pressure. The flight level or the height in flight is measured by the baro altimeter, and for surveillance purposes, the transponder encodes the static pressure to be transmitted to the secondary radar. At the same time, with the latitude and longitude, the onboard Global Navigation Satellite System (GNSS) receiver measures the height of the flight with respect to the reference ellipsoid (WGS-84). This information is not provided to the pilots on the Primary Flight Display, but it is included in the Enhanced Surveillance mode of the Automated Dependent Surveillance–Broadcast. The GNSS height is considered more accurate than the baro height because it does not depend on the local weather. This encouraged the adepts of the transition from baro altitude to true altitude based on the GNSS receiver, with a preference for more accurate measurement. This research is based on a model for the difference between the two, validated by experiments, and an analysis of the impact of such a transition on both the individual aircraft’s vertical navigation and air traffic management. As a conclusion of this research, the transition from baro to GNSS altitude is not recommended.