Part of a three-part series on the development of the laser gyroscope and its military use. Back to Part One
Enter the Laser
Despite its name, a laser “gyroscope” does not use a rotating mass to measure angular momentum. Instead, it takes advantage of the Sagnac effect, the fact that two beams of light sent in opposite directions along a rotating circuit will travel slightly different distances and thus end up slightly out of phase. Measuring the phase difference lets you determine the rate of rotation. Using a laser made the effect measurable for practical applications. The first practical demonstration of the technology was made by a team at Sperry Gyroscope in January 1963, just under three years after the Hughes Research Laboratories demonstrated the first functioning laser. Other instrument designers such as Kearfott, Honeywell, Autonetics, and Draper experimented with the technique too – Draper considered it a distraction from perfecting his floated gyros, which had far better accuracy.
Like strapdown INSs in general, the laser gyroscope’s lack of moving parts seemed to offer better reliability. Its use of laser beams also had the promise of potential performance beyond the physical limits of conventional gyroscopes. Still, in the early 1970s the technology was by no means mature. Despite its future promise, the laser gyroscope remained less accurate and less reliable than more mature gyroscope designs.
Though several companies continued to develop laser gyros, the most successful of them was Honeywell, where there was no major existing INS business that had to protect its turf. With some funding from the US Naval Weapons Center Honeywell’s Florida division built the Advanced Tactical Inertia Guidance System (ATIGS) in 1974. Mounted in a pod on a Navy A-7 attack aircraft, the Honeywell system showed drift rates in the range of 5–10 nautical miles per hour, not nearly as good as current production systems. Honeywell’s Minneapolis division was already designing a strapdown inertial system for the aviation market using a conventional floated gyro, and it now switched to the laser gyro. Before flight testing at Holloman Air Force Base, the Minneapolis team spent $30,000 to install the prototype in a GMC RV. Having demonstrated a circular error probability of 0.9 nautical miles per hour in flight tests, Honeywell took the laser gyro INS on the road, still in the van, to demonstrate it to potential customers. The Department of Defense gave Honeywell $8.5 million to develop a prototype that would meet both the 1 nautical mile per hour drift requirement and the form-fit-function (or F3) requirements for fitting into military aircraft. What did not happen after the Holloman AFB tests was a flood of military orders for production systems. Nor, in fact, were there any orders for laser gyroscopes at all. The technology remained both unproven and less precise than other systems.
Salvation for laser gyro came from an unexpected direction: Boeing’s commercial aviation division, which was beginning to design its next generation of airliners, the 757 and 767. Despite its various disadvantages, the laser gyro had several features that made it particularly attractive in this situation. The laser gyro’s output was digital rather than analog, which made it a good fit with the new, increasingly computerized avionics Boeing planned to use. Its high reliability due to the lack of moving parts would offer cost savings in maintenance and replacement, something Boeing was keen on. Lastly, as MacKenzie explains it, “the sheer glamour of the laser gyro was appropriate to the ‘high-tech’ image of the 757 and 767 that Boeing was cultivating.” Relative imprecision was an acceptable price to pay under the circumstances. Getting laser gyroscopes into Boeing airliners required the engineers involved not only to sell their senior management on the still unproven gyro, but also ensure that the multi-company committee that set specifications for commercial avionics would approve specs that accommodate physically larger inertial navigators, since laser gyros had not yet been miniaturized to match the latest generation of mechanical gyros. Boeing signed its contract with Honeywell in November 1976, delivered its first prototype unit in 1979, and the first production units in 1981. Once Boeing made the Honeywell gyro standard on the forthcoming 757 and 767, Airbus quickly followed, putting a Litton laser gyro on the Airbus A310.
Despite the commercial breakthrough, success in the military market was slow coming. Though Honeywell offered a laser gyro-based INS in the competition to provide an INS for the AV-8B, the US Marine Corps chose a conventional mechanical system in 1983. By then, Honeywell’s commercial systems were racking up 10,000 hours of flight time every week. The first US military contract was for Litton’s LTN-90, to be on the Boeing E-6A, a Navy strategic communications airframe. Even then, the laser gyros were only to be used as inertial reference sensors, not in an integrated navigation system. The floodgates only opened in 1985, when Honeywell got an $8.8 million production contract for laser gyro INS to be used in F-15s, F-111s, RF-4Cs, C-130s, and HH-60s. Litton and Kearfott got contracts to build laser gyros for the US Navy, and Litton also received an $11.8 million contract to supply the same gyro used on the E-6A for F-15E. Soon enough, most military aircraft were flying with an INS that relied at its core on a laser gyroscope.
To Part Three: Meaning and Legacy