Global Positioning System (GPS) Altitude for Obstacle Clearance

Today's increasingly sophisticated cockpits provide pilots unprecedented levels of situational awareness. With the integration of Digital Terrain Elevation Data (DTED) displayed as Height Above Terrain (HAT) banding onto digital maps comes a natural tendency and an associated risk that pilots may use GPS HAT banding as a "poor man's" terrain avoidance system. Consequently, it becomes imperative that all pilots understand the accuracy of their GPS-guidance systems and any associated shortfalls. This paper is a long answer to a short question of whether GPS as an altitude source is accurate enough for terrain avoidance. Research into the current sources of barometric and GPS altitude error suggests that GPS is an extremely reliable and a more consistent source of altitude information than is barometric in isolated area operations. Even with GPS, however, pilots must account for differences in the geodetic datums in certain parts of the world. To that end, this study explores the global reference frames, quantifies both barometric and GPS altitude accuracy, and juxtaposes those altitude sources in the current operational environment. Further, the paper examines how HAT banding generated from common onboard systems can assist a pilot with situational awareness and terrain avoidance and, more importantly, how it can lead a pilot into trouble.