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Inertial Navigation Technology: Cold War-Era Tool Evolves
ISS Fleet Support Team, Boeing
ISS Chief System Engineer, Boeing
Starting in the early 1950s, with nuclear deterrence as the top military priority, the United States set out on a path that produced remarkable technological advances, including nuclear power for propulsion, solid fuels for rocket motors, and accurate inertial navigation equipment for missile alignment and ship positioning.
An achievement sometimes overlooked is the development of a machine to navigate without the need for external references, such as star readings, land sightings, radar and radio beacons. The concept seemed simple: maintain a steady platform through use of gyroscopes and conduct acceleration measurements on that platform using mathematics and knowledge of the Earth's dynamics to compute the position, velocity and attitude of the vehicle. Initially, these were very crude devices, but through a continuous series of technological breakthroughs in instruments, computers and mechanization improvements, these machines, known as inertial systems, achieved extraordinary levels of accuracy. Those used for guiding missiles were soon called inertial guidance systems, and those used to navigate over the surface of the earth became known as inertial navigation systems (INSs).
In 1954, the USS Nautilus was launched using nuclear propulsion, allowing it to remain submerged for a seemingly endless period. While the USS Nautilus was breaking records and traveling to locations previously beyond the limits of submarines, Russia launched Sputnik 1 in October 1957, and the space race was on, with the U.S. far behind. The American need to show the world that the U.S. was just as technologically advanced was answered in one way when the USS Nautilus made the first under-ice transit of the North Pole.
The ability to navigate under the ice with no access to stars or other references at extreme latitudes was enabled by the N6A-1 inertial navigation system, a naval adaptation of the N6A Navaho supersonic intercontinental cruise missile.
Following the success of the N6A-1, the MK 2 Mod 0 ship inertial navigation system (SINS) was developed using a gas-bearing, single-degree-of-freedom gyro and pendulous velocity meter. The first U.S. nuclear ballistic missile submarine, the USS George Washington, departed for its first 60-day operational mission on November 15, 1960 with SINS, which allowed the submarine to remain submerged and hidden for long periods, creating a truly credible nuclear deterrent.
As the Cold War escalated in the 1970s, the need for more stealth and accuracy became apparent. Industry responded by developing more technology, such as the electrostatically supported gyro navigation (ESGN) system. ESGN was based on the dynamics of a spinning mass; if one could isolate this spinning object from all environmental effects, its accuracy would be unlimited. This complex machine proved to be a strikingly accurate inertial navigator and is the primary inertial navigation system for the entire U.S. Trident fleet today.
An effort to find a new gyro technology that could eventually replace the ESGN turned out to be a difficult task. No other technologies in the commercial or military complex could meet the accuracy needed. Eventually, in the late 1990s, it was recognized that a fiber-optic rate-sensing gyro, with a mechanization that exploited the benefits of fiber-optic technology while suppressing its weaknesses, had the potential to satisfy the Navy's need for precision and endurance.
Today, new breakthroughs are being sought to address future weapons systems requirements and meet the need for reduced lifecycle cost through modularity, automation and usage of open architecture. To meet those needs, industry is exploring advancements in fiber-optic INS, ground-reference velocity, signals of opportunity, gravity matching for bounding of position errors and measuring ship shaft speed as a back-up source for velocity references. Inertial navigation technology will continue to evolve to customer needs and mission requirements.