Baro-Inertial Loop For The USAF Standard RLG INU

J. Stanley Ausman

Abstract:	The vertical channel of an inertial system is inherently unstable because the gravity compensation from its computed altitude creates positive feedback. To overcome this instability one typically compares inertially generated altitude with barometric altitude and feeds back this error signal to appropriate nodes in the inertial channel. The best feedback gains to use are a function of barometric altitude errors which in turn are different in different flight regimes of the aircraft. The whole objective of a bare-inertial loop is to obtain vertical velocity, and not altitude. The barometric altimeter determines the altitude, and over the long term the inertial altitude must track the barometric altitude. For this reason, the altitude output from a bare-inertial loop is not appropriate for weapon delivery systems where accurate changes in altitude are required. For this purpose one should simply integrate the vertical velocity externally to the baro-inertial loop. The bare-inertial loop mechanized in Litton’s LN-93/94 ring laser gyro (RLG) systems is the culmination of 20 years of progressive improvements in the way bare-altitude is combined with inertial data to form an accurate, stable vertical velocity. This paper describes the mechanization, explains the functions of its various components, and narrates some of the historical circumstances which led to this particular formulation. Finally, the paper shows simulated performance in response to certain maneuvers and error sources.
Published in:	Proceedings of the 1991 National Technical Meeting of The Institute of Navigation January 22 - 24, 1991 Sheraton San Marcos Hotel Phoenix, AZ
Pages:	303 - 308
Cite this article:	Updated citation: Published in NAVIGATION: Journal of the Institute of Navigation
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