Recent Progress in Inertial Guidance

Abstract

N is traditionally the art of steering a vehicle from an origin to a destination over a mapped course; inertial navigation is a mechanization of this art. In inertial navigation, the force in the vehicle is measured with accelerometers in a gyro referred coordinate system. Thus, during certain intervals, navigation is carried on without reference to the environment of the vehicle. From the measured force and a foreknowledge of the gravitational field through which the vehicle moves, the acceleration of the vehicle through the field is deduced. Part of this acceleration may be due to the rotation of the Earth, which must then be accounted for; thus sidereal time is also an input to the navigation system. The acceleration on course is doubly integrated to indicate the displacement along the course, or position; these data are then used for steering. Inertial navigation's beginnings, involving partial inertial guidance systems, go back to the first practical marine gyrocompass by Anschutz of Germany early in the present century (l). The next development pertinent to inertial navigation was the artificial horizon for air navigation, a long period pendulous gyroscope. As this device was improved it became a nonpendulous gyro coupled, through a pneumatic control system, to separate single axis pendulums. Thus, the general principles of today's inertial navigation devices emerged: The measurement of force with single axis accelerometers in coordinates determined by gyros. This type of system was the basis of the guidance of the German V-2 rocket during World War II (2). After the war, reports from abroad (3), plus the technological improvements developed during the war, stimulated fresh approaches, and the method was investigated for various airborne, marine and missile applications in this country. Most of the present system principles were in use in the designs of the mid-1950's (4) and successful tests were recorded. In 1953, for example, the MIT Instrumentation Laboratory was responsible for the first fully inertially guided coast to coast airplane flight. During this period, several other groups were also working on this problem: Among the earliest work may be mentioned that of the Autonetics Division of North American Aviation, Inc., and also that of the Army Ballistic Missile Agency. Engineers in the USSR also appear to have been interested in the navigation system problem (5) and particularly in the design of gyros (6). The distinctions between airborne, marine and missile applications of inertial navigation are drawn in terms of two -circumstances: The force environment to which the navigation system is subjected, and the expected time of operation as a purely inertial device. With respect to the force environment, the burden is, first, on the accelerometers, in that they must have the desired precision (this includes precision of analog to digital data conversion, if digital computation is