Navy and industry investigate new super-accurate optical gyros for possible use on ballistic missile submarines
By Ed Walsh
WASHINGTON — U.S. Navy researchers are eying a new fiber optic gyro (FOG) inertial navigation system for their fleet of Ohio-class (SSBN 726) ballistic missile submarines that would replace the near-infallible system now in service aboard these nuclear submarines within the next 10 years.
The Navy's Strategic Systems Programs office, seeking to preserve the pinpoint accuracy of the fleet's Trident submarines, is moving deliberately towards a decision, now expected in 2003, to start developing a new FOG-based navigation system.
Engineers in the former Rockwell Autonetics business unit in Anaheim, Calif., built the current fielded Trident system, referred to as an electrostatically supported gyro navigator — otherwise known as ESGN. The Boeing Co. recently acquired Rockwell Autonetics and renamed the Anaheim unit as Boeing Battle Management/Command, Control & Communications (BMC3). Boeing BMC3 is the systems integrator for a potential replacement called a fiber-optic gyro navigator (FOGN) that would incorporate interferometric fiber-optic gyroscopes or IFOGs from Honeywell.
Navy and company officials stress that Navy Strategic Systems officials have made no firm commitment to shifting to the FOGN/IFOG technology, and could opt to continue supporting the electrostatic gyros. No other alternative navigation technology is under consideration for the SSBNs, however.
The Boeing FOGN/IFOG effort, in progress since the mid-1990s, receives money from Navy Strategic Systems, from the Defense Advanced Research Projects Agency (DARPA) in Arlington, Va., and from the Pentagon's Technology for the Sustainment of Strategic Systems (TS3) program in Arlington, Va., which supports Air Force and Navy strategic programs. Boeing and Honeywell have invested in internal research and development in the effort.
Company officials say they also hope to persuade the Navy's surface-warfare community to adopt the FOGN new system for Los Angeles- (SSN 688), Seawolf- (SSN 21), and Virginia-class (SSN 774) fast-attack submarines, as well as for surface combatants such as aircraft carriers and amphibious-assault ships.
The Navy currently is fielding a ring-laser-gyro (RLG)-based system from the Northrop Grumman's Sperry business unit in Charlottesville, Va. These gyros are designated the WSN-7 for surface ships, and WSN-7A variant for attack submarines. The WSN-7 replaces old spinning-mass gyros aboard the surface ships, while the WSN-7A replaces a Boeing electrostatic gyro system called the WSN-3 aboard 688-class attack boats.
Boeing and Navy Strategic Systems officials say that the electrostatic gyro, first developed in the 1970s, is the world's most accurate inertial navigation system; it meets the extremely stringent standard required for launching nuclear-tipped ballistic missiles after being submerged for long periods without position updates from offboard sources. The WSN-7 and WSN-7A RLG-based systems require updates from the global positioning system (GPS) every 14 days.
Boeing and Navy Strategic Systems engineers say electrostatic gyros are so inherently stable that their "angle random walk" (ARW) — a measurement of error — is considered to be zero. Ralph B. Morrow and Dwayne Heckman of Boeing, in a technical paper on the program, point out that the electrostatic gyro "exceeds the Strategic Weapons Systems performance, reliability, and maintainability requirements."
They add, though, electrostatic gyros are expensive to repair, and spare parts are scarce. The move to an IFOG-based system that incorporates solid-state electronics would reduce lifecycle costs while maintaining the electrostatic gyro level of navigation performance, Navy and Boeing officials say.
In 1996 Boeing engineers started to develop a proof-of-concept IFOG, consisting of Honeywell high-accuracy fiber-optic gyros. Company tests determined that these IFOGs were comparable in performance to electrostatic gyros, but were too large for deployment. Boeing systems integrators installed the Honeywell technology into a smaller advanced development model (ADM-1). Boeing and Honeywell then developed a second IFOG ADM, or ADM-2, to demonstrate performance superior to that of an RLG-based system and meeting the ARW requirements for strategic-weapons launch from a Navy SSBN.
Boeing officials say their latest FOGN navigation system consists of three Honeywell-built ADM-2 IFOG gyros, accelerometers, a stabilized gimbaled platform, and ancillary solid-state electronics. The companies have built nine ADM-2 IFOG units.
Engineers at the Boeing commercial aircraft unit in Seattle introduced low-precision IFOGs for the company's 777 jumbo jetliners as backups to an RLG-based navigation system. The military services also have incorporated relatively low-accuracy IFOGs, developed with DARPA funding, for some missile and aircraft programs.
High-accuracy IFOGs, capable of the precision similar to that required for the SSBNs, also are being developed for classified defense space-pointing applications.
Boeing and Navy Strategic Systems engineers say the IFOG gyro is a rotation-rate sensor that monitors rotation rate by forming an interference pattern between two nearly coherent light beams. The IFOG being developed by Honeywell, they say, has achieved navigation values "about an order of magnitude" more precise than those of an RLG, partly through the reduction of random system noise.
They point out also that an RLG experiences "lockup" between its two counter-rotating laser beams. To eliminate the lockup, the RLG system uses a mechanical method called "dithering," a technique that Boeing and Navy Strategic Systems engineers say amounts to shaking the device, which increases error and adds structure-borne acoustic noise in the range of 300 Hz to 900 Hz.
The IFOG, they say, requires no interaction between its two light beams, and uses a solid-state structure that produces no acoustic signature.
The IFOG technology, Boeing and Honeywell officials say, is based on the sensor's ability to detect and measure tiny shifts in the interference pattern produced by rotation, a phenomenon called the Sagnac effect. An IFOG combines counter-rotating light beams; the rotation rate shifts light phase at the detector component. The detector and the IFOG signal processor determine the gyro's rotation direction and rate.
Boeing points out that error-causing system noise comes from detector electronics and light source, analog-digital converter, and other components. The IFOG minimizes such noise with a high-power broadband super-florescent light source and customized signal processing electronics. Engineers say a polarization-maintaining fiber-sensing coil provides the sensitivity and "bias stability" necessary to keep some types of system noise to a minimum.
Bruce Pope, Boeing's chief engineer for naval electronics and navigation, says the length of fiber in a fiber-optic gyroscope is inversely proportional to the error rate: the more fiber, the less error — or the greater accuracy. The Boeing-Honeywell IFOG, he says, is based on a design that "packs as much fiber into the space available"
Jim Davey, manager of navigation systems for the Detection, Navigation, and Processing branch of the Naval Sea Systems Command in Arlington, Va., points out that IFOGs have experienced problems with temperature. Davy oversees navigation programs for the surface ships and attack submarines, but not for the Trident ballistic missile submarines.
Pope says Boeing engineers have designed system features such as special winding techniques and packaging to protect the system against environmental extremes in temperature, humidity, barometric pressure, and vibration. Boeing engineers employ Kalman filtering techniques at the FOGN system level for calibration and alignment to project and compensate for predictable system errors.
For the current-generation electrostatic gyro and the FOGN system, Boeing engineers use auto-compensation techniques to reduce "bias drift," but in significantly different ways from what they have used in the past.
The electrostatic gyro, based on the use of four gyros, four torque motors, and other components, employs a technique called "flip/dwell" auto-compensation that company experts say can reduce drift error by a factor of 3,000. The IFOG-based system approach for auto-compensation requires only three IFOGs, which mount orthogonally on a platform. The platform rotates continuously, similar to a "rate-biased" ring-laser gyro-based system.
Boeing engineers, as FOGN system integrators, say they hope to set up an at-sea test for the FOGN system in mid-2002. Funding is scarce, however, officials say, and currently will not be adequate to support follow-up testing or test analysis.
Company officials now say they're "looking for a requirement" that would support the IFOG technology as an alternative to RLG-based for attack submarines and surface ships. The potential opportunity to build systems for a broader market than the 18-ship Trident fleet, they say, would free up additional IR&D funds to push forward with the FOGN work.
No such requirement currently exists, however.
NAVSEA officials say that the RLG-based WSN-7 and WSN-7A systems exceed current surface fleet and attack submarine requirements. Boeing counters that RLG technology is some 20 years old and, is vulnerable to loss of the GPS signal to jamming or combat activity. Officials at Northrop Grumman Sperry the WSN-7 and WSN-7A builder, say moreover that they are also looking at potential navigation technologies that eventually will replace the RLGs.
