Inertial Navigation

Abstract

The birth date of inertial guidance is a matter of interpretation, because it was neither the result of any single event nor a discovery attributable to any single individual (1). Sir Isaac Newton is credited with the law of inertia, but the clue was given earlier by Galileo, who wrote in his Two New Sciences: any velocity once imparted to a moving body will be rigidly maintained as long as the external causes of acceleration or retardation are removed, a condition which is found only on horizontal planes; for in the case of planes which slope downward, there is already present a cause of acceleration; while on the planes sloping upward, there is retardation; from this it follows that motion along a horizontal plane is perpetual; for, if the velocity be uniform, it cannot be diminished or slackened, much less destroyed. In 1896, the use of gyroscopes was proposed for marine sextants for those cases where the navigator could see the stars but could not see the horizon. This invention, by Admiral Fleurias (2) of the French Navy, consisted of a small top mounted in a vacuum box attached to the sextant; lines ruled on the glass lenses were used to observe the vertical as the top precessed around the vertical. The shipboard gyrocompass, invented in Germany in 1908 by Hermann Anschütz-Kaempfe, elevated the spinning rotor from the role of a toy or mathematical novelty to that of useful instrumentation. In 1911, Elmer A. Sperry obtained his first U. S. patent on a gyrocompass. On July 15, 1924, U. S. patent 1501886 was issued to C. G. Abbot for a system with a three-axis gyro and a gravity pendulum, the first system, as far as we know, with some inertial capability. An inertial guidance system operates in a coordinate system that is not rotating or accelerating with respect to the “fixed stars.” An inertial or Newtonian coordinate system is one in which Newton's laws are valid. This “inside-out”-type of definition bothered Einstein (3), who asked; “Can we formulate physical laws so that they are valid for all coordinate systems, not only those moving uniformly, but also those moving quite arbitrarily, relative to each other? If this can be done, our difficulties will be over. We shall then be able to apply the laws of nature to any coordinate system.” He solved the problem with his general relativity theory, which, when applied to inertial coordinate systems, gives the special relativity theory as a special case.