Medium accuracy inertial systems of the 1 nautical mile per hour class, enjoyed significant acceptance in the market for land survey systems in the early 1970's - prior to the introduction of GPS. This occurred because such systems could be implemented with software that capitalized on "Zero Velocity Updates" (ZUPTS) which enabled the resultant system to achieve real-time navigation accuracies of a few meters over several hours of operation. "ZUPTS" are realized by simply bringing a land vehicle to a stop or, in the case of a helicopter, by hovering across a fixed point on the,ground. Those initial inertial survey systems were large, heavy, expensive and required substantial power. Since the technology for realizing medium and high accuracy inertial systems has changed so dramatically over the last few decades, these systems have become highly suitable as the core of the navigation and control suite for small submersibles. The paper will discuss and illustrate the trend in inertial technology from that of gimbaled spinning wheel gyro based systems initially employed for surveying; to the ring laser gyro based systems available today and the next generation of systems employing fiber optic gyros. Performance obtainable with these medium grade inertial systems will also be presented as a function of the quality of zero velocity updates. Realization of zero velocity updates by different means will also be a topic of discussion.
