ICAO GNSS is currently being implemented, so in couple of years civil aviation will have worldwide primary navigation source by radio signals from the space, i.e., SBAS or satellite-based augmentation system. While the current standard defines navigation performance of SBAS only in horizontal mean for oceanic airspace and enroute, essentially SBAS has capability of providing vertical guidance to numerous aircraft within its coverage. This means GNSS altimeter can easily be implemented once SBAS becomes operational.

Traditional barometric altimeter is the simple instrument to measure altitude of aircraft. It, however, requires frequent adjustments by pilot, which would be the potential source of human error. Furthermore, it is always affected by meteorological condition and never indicates the accurate geometric height of the aircraft.

In the paper we describe potential advantages of GNSS altimeter the barometric altimeter. Characteristics of both altimeters would be explained with emphasis on the potential error sources. The adjustment method for barometric altitude currently used in the lower altitude airspace will be described, and an additional temperature correction method follows. The evaluation of them based on the meteorological observation data collected in Japan will show the temperature correction works well. Finally, the results from our flight experiment measuring altitudes by both altimeters will augment the theoretical considerations.