An Analysis of Geometric Altitude Data in ADS-B Messages

Nur Asheila Taib and Busyairah Syd Ali

Peer Reviewed

Abstract:	Altitude data has long been deployed for aircraft navigation (pilot) and separation (Air Traffic Control (ATC)). Two types of altitude data are down-linked by radar; actual flight level (Mode C) and selected altitude (level). Selected altitude presents intent, and therefore cannot be used for separation purposes. Mode C indicates aircraft pressure altitude (barometric altitude) used by ATC for aircraft vertical separation. Emergence of satellite technology; Global Satellite Navigation System (GNSS) has introduced geometric altitude in the cockpit via GNSS receiver and to the ATC via Automatic Dependent Surveillance Broadcast (ADS-B) system broadcast. Literatures to date have identified many advantages of geometric altitude over barometric altitude. However, till today, the barometric altitude is still the only altitude data used for aircraft navigation and separation. This paper analyzes characteristics of geometric altitude data in the ADS-B messages. It then measures deviation of the geometric altitude from the barometric altitude data. Finally it identifies and discusses potential factors that may influence the variations. Findings showed obvious variation between the altitudes during different phases of flight. The barometric altitude displayed higher readings than geometric height while the aircraft is cruising. The discrepancies between the two altitudes were bigger during the climbing phase. It is also found that the absolute difference between geometric height and barometric altitude ranges from 25 feet – 1325 feet with an average of 569 feet. Various statistical methods are used to analyze the sample data collected from ADS-B ground stations and aircraft avionics and make model information from airline.
Published in:	Proceedings of the 2016 International Technical Meeting of The Institute of Navigation January 25 - 28, 2016 Hyatt Regency Monterey Monterey, California
Pages:	697 - 704
Cite this article:	Taib, Nur Asheila, Ali, Busyairah Syd, "An Analysis of Geometric Altitude Data in ADS-B Messages," Proceedings of the 2016 International Technical Meeting of The Institute of Navigation, Monterey, California, January 2016, pp. 697-704. https://doi.org/10.33012/2016.13392
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