Similarities Between Classical Celestial Navigation and Electrostatic Gyro Navigation

Abstract

In the absence of torque, a gyroscope's spin axis will maintain a fixed angular orientation in space. This angular orientation is analogous to the fixed line of sight to a star. In principle then, one should be able to perform celestial navigation using gyroscopes as self-contained stars. Unfortunately, most current inertial auto-navigators apply torques to the gyroscopes. Ideally these torques are of just the right magnitude to cause the stable platform on which the gyros are mounted to stay locally level. In the case of strapdown inertial auto-navigators, the gyroscopes are torqued to follow the host vehicle angular rates. This results in the gyroscope becoming just an angular rate sensor, and the only measure of true angular orientation rests within the system computer. These mechanizations lead to concepts unfamiliar to many navigators familiar with celestial navigation. Electrostatic gyroscope (ESG) inertial navigation systems are unique in that they are normally operated untorqued. As such, the orientation of their gyro spin axes remain fixed in inertial space. This makes it possible to consider their operation in terms of classical celestial navigation concepts. This paper will develop this similarity to the point where one familiar with celestial navigation will be able to understand the alinement problem and the fundamental error sources of ESG inertial systems. A specific type of ESG employed at Rockwell International in both gimbaled and strap-down systems will be discussed. This will provide the basis for relating angle readout errors and drift rate to their counterparts in celestial navigation. The buzz words used in inertial navigation, such as Schuler tuning and Kalman filtering, will be related to concepts familiar to celestial navigators. The paper will be based on straightforward physical principles and the concepts presented without resorting to mathematical development.