Item History: The very large ring laser gyroscopes (RLGs) shown in these pictures were manufactured in about 1980 by the Honeywell Corporation. Designated the GG1389, they are about 8.9 inches on a side. These gyroscopes were proposed as part of a Navy exploratory development program aimed at providing a high accuracy strapdown inertial navigator for attack class submarines. At the time, this gyroscope represented the evolution of 19 years of Honeywell experience in manufacturing ring laser gyros. As in their previous series, the GG 1389 had solid block construction, visible (0.633 μm) neon transition operation, triangular cavity configuration, inertial mechanical dithering, optically contacted-multi-layer dielectric mirrors, and a symmetrical. Ring laser gyroscopes measure inertial angular motion using a principle called the Sagnac effect. Two counter rotating laser beams of light are established within the 68 centimeter cavity length. Any rotation about the input axis perpendicular to the plane of the path effectively causes one of the laser beams to travel a longer distance than the other one. This results in a phase difference between the beams proportional to the angular motion. This phase difference is then measured precisely using optical interferometric techniques.
The large size of this gyroscope was principally dictated by a fundamental limit of optical gyroscopes that causes drift white noise due to the spontaneous emission of photons. The magnitude of this drift white noise is inversely proportional to the cavity path length, and therefore to the size of the gyroscope. With a cavity path length of 68 centimeters, the GG 1389 theoretically had a quantum limit of white noise drift of about 200 microdegrees per root-hour. Assuming the white noise drift was the only error source than the 200 microdegrees per root-hour drift would result in an angular error of 2 millidegrees in 100 hours.This was considered more than adequate for the system performance levels.
Another factor, bias drift stability, particularly for the envisioned strapdown configuration, was considered more challenging. Bias drift stability encompasses many issues, including gas flow, aperture effects, mirror effects, mechanical block stability, transducer mirror tilt and electronics. These factors are generally exacerbated by the strapdown configuration envisioned for the submarine.
The 1389 RLGs were tested in 1982 at the inertial facility of the Naval Air Development Center which is now the home of Pennsylvania State University’s Navigation Research and Development Center. The GG 1389 never did transition to the fleet and it is not believed that an RLG of this size is employed anywhere. Instead, an earlier generation, smaller RLG, the Honeywell 1342 was employed in an indexing gimbaled configuration. This system, now designated the WSN-7a, has been installed on all Navy attack submarines.