Honeywell, the provider of the laser inertial navigation system (INS), was a typical case of a co-investing F-20 subcontractor. They had a certain management / engineering team who had come to Honeywell from the INS market leader, Litton. They were preaching the gospel of the laser gyroscope, something they were unable to get Litton to promote vigorously enough.
Conventional INS systems used rotating mechanical gyroscopes that went back to the platforms developed by the Germans for the V-2 in world war II. This cage of four nested rings would be spun up, and then its resistance to rotational movement measured and translated into changes in the angle of the aircraft compared to that when it was powered up. Accelerometers mounted in three axes in the INS measured accelerations in each axis. Summing up these acceleration measurements, combined with the information as to the orientation of the aircraft, allowed the computer in the INS to calculate the aircraft's position and orientation at all times, relative to its original position. Naturally these mechanical devices were not completely accurate, and small errors accumulated over time. The typical conventional INS by the late 1970's drifted an average of 1 nautical mile per hour of flight.
These mechanical gyroscopes were precision devices, with high-precision bearings and special lubricants. They took a few minutes to warm up and then spin up after power-on. The pilot would have to wait for this period of time before the aircraft could start rolling. This meant that aircraft on runway alert, awaiting a scramble order, had to have their engines running, their electrical power on, and their INS already aligned and operating, in order to make an instant start.
The laser gyroscopic had no moving parts and used the phenomenon of wave interference familiar to high school physics students. In the Honeywell laser gyro, a triangular cavity was machined into a block of super-high-quality glass, which was resistant to deformation in heat or cold. Each corner of the triangle had a mirror, and the cavity was filled with a lasing gas. Once turned on, two exciters began laser beams cascading in opposite directions within the cavity. These counter-rotating beams were combined in a prism and reflected to a photocell pick-up. The two beams of the same frequency produced different interference patterns depending on whether the glass block was rotating or not. The diffraction bands would march at a certain rate up the photocell if it was rotating in one direction, downward for rotation in the other direction, with no motion if there was no rotation in that axis. Therefore the output from the photocell could be used to determine the direction and magnitude of rotation. A set of three gyros, one in each axis, provided a gyroscope package that would replace the conventional mechanical gyroscope.
The laser gyro's accuracy depending mainly on its physical size. Honeywell was developing a whole family of these gyros for different application. Those for navigation by ship or nuclear submarine were enormous, over a foot in diameter, and the errors would be less, for something that might spend days before getting a positional update near the surface. Tiny laser gyros were made for precision artillery shells, which had a time of flight measured in seconds. For a fighter aircraft, with a flight time of an hour or tow, gyros about nine inches in diameter were used.
Honeywell's laser gyros used a triangular cavity, while those built by Litton were rectangular. But the boys that had left Litton for Honeywell oversold the ease with which the new technology could be implemented. Finding the right glass, the right fabrication methods, the right hardware and software engineering solutions to using the signals from the gyros, proved daunting. Honeywell had already sold laser gyro systems to the airlines, which had a benign flight environment and could receive constant positional updates from radio navigation aids. But for the F-20, the INS would have to retain high accuracy throughout the flight without radio updates, in order to allow precision bomb delivery and navigation to targets in hostile territory. And it would have to retain this accuracy during violent maneuvers, the vibrations of gun firing, and while exposed to the thin air and low temperatures, or thick air and high temperatures, in an external equipment bay.
The development was sold to Honeywell management as being easily accomplished, taking about 12 months, and costing $1.5 million. In fact the problems multiplied, and it took 18 months and cost nearly $7 million. Meanwhile LItton had filed a lawsuit against Honeywell and the two former Litton employees for theft of trade secrets. The sales manager who had oversold the whole thing to Honeywell was eventually fired; the suit was settled out of court; but Honeywell persevered and ended up with a world-beating product, dominating the laser gyro market for years before LItton finally entered it.
The inertial navigation system had an extra circuit board that performed the role of backup bus controller. In the event of failure of the mission computer, this Honeywell-programmed board provided a minimum set of mux bus traffic control commands and functions to allow the aircraft to get home.
The laser gyros, cast and machined out of milky blocks of super high quality glass, were calibrated in the sub-basement of the Honeywell facility in Minneapolis. Here, in a controlled environment, the gyros were mounted on cement blocks which were in turn cast into the glacier-scrubbed granite bedrock of the North American continent. This room of gyros, glowing red as they slowly sensed the rotation of the earth, was a truly eerie sight.
An example of an undetected software bug in the F-20 INS occurred during the two-aircraft world tour. After GG1001 crashed in Korea, GI1001 had to make the lonely trip home, alone, across the north Pacific from Hawaii to Alaska. The two aircraft had traveled around the world easterly, flying from California, to Farnborough in England, then across Europe, North Africa, and Asia. As the aircraft crossed the international date line for the first time, the inertial navigation system went haywire. The pilot had to use the compass, and once within range of Alaska, radio homing to navigate. After landing, shutdown, and restart in Alaska, the system worked fine. The problem was found to be a multiplier with the wrong sign deep in the software code. The problem would never emerge until the moment the aircraft first crossed the international dateline, going from west to east.
Honeywell observed that Northrop required the F-20 navigation system to pass a much more severe environmental qualification test than Honeywell's Space Shuttle Main Engine controllers.
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© Mark Wade, 1997 - 2006 except where otherwise noted.
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