How Things Work: Ring Laser Gyros
- By Linda Shiner
- Air & Space magazine, September 2002
CHARLES DRAPER, WHOSE EPONYMOUS LABORATORY developed the guidance systems for Apollo spacecraft, referred to inertial navigation as “astronomy in a closet.” It is based on a celestial reference system but is at the same time a self-contained process.
Inertial guidance systems determine location and orientation not from sightings of the stars or landmarks, nor from signals from the ground, but solely from instruments carried aboard a moving craft.
The Apollo astronauts were able to navigate to the moon because they had four sets of information. First, they knew where they were when they started. Inertial systems measure change from a single starting point and within a frame of reference (which indicates true vertical and two axes at right angles to the vertical and each other). Instrument measurements provide the three remaining sets of information: changes in acceleration, time, and rotation. Accelerometers enable pilots (or their computers) to calculate speed and, by integrating speed and time, distance. Changes in rotation, including heading, are the province of gyroscopes, mechanical devices consisting of a spinning wheel mounted in one or more gimbals.
One of the simplest of such devices, the turn indicator on a light aircraft, operates on the same principle as do the massive, complex gyroscopes that help maintain the orientation of the International Space Station. That principle is gyroscopic inertia, the tendency of rotating objects to maintain a fixed orientation, resisting forces trying to tilt them. Gun makers take advantage of gyroscopic inertia by cutting grooves in the interior of a gun barrel to impart spin to the exiting bullet and so keep its path straight.
The gyro wheel on an airplane’s turn indicator, driven by a stream of air to spin as fast as 10,000 revolutions per minute, imparts a stabilized reference line, or axis, against which rotations can be measured. When a rotational force, or torque, is applied to the gyro assembly, the assembly reacts in a predictable way: It resists the torque and rotates instead about its gimbal axis. The predictability of this action enabled engineers to design the device that measures an airplane’s rate of turn. When an airplane turns right, the gyroscope turns over left; the top of the wheel moves over in the opposite direction.
In the 1980s, laser gyroscopes began to take over the work of their mechanical, and later, electronic, forebears, without the slightest resemblance in principle or operation to the earlier devices. The idea behind the ring laser gyroscope actually dates back to 1913, when a French physicist, Georges Sagnac, experimented with rays of light moving in opposite directions around a circular cavity on a turntable. Sagnac showed that when he rotated the turntable, the light traveling with the rotation arrived at a target slightly after the light traveling against the rotation. He believed he had proven the existence of ether in space. In fact, he was demonstrating a property of light that came to be understood much better with the invention of the laser in the 1950s.
A laser (light amplification by stimulated emission of radiation) operates by exciting atoms in a plasma to release electromagnetic energy, or photons, in a cavity. Each end of the cavity reflects the energy back and forth, and it forms a standing wave pattern. The wave frequency—its pattern of peaks and troughs—is determined in part by the length of the cavity.
“If you had a linear laser and the light bounced back and forth between two mirrors at either end, and if you [increased] the spacing between those two mirrors slightly, you would actually stretch the wavelength of the light in the cavity,” explains James Koper, the manager of ring laser gyro components for Kearfott Navigation and Guidance Systems, which manufactures the laser gyroscopes used in the B-2 bomber, the Global Hawk reconnaissance craft, and the Joint Stand-off Weapon, a glide bomb.
“What causes the light to stretch? The fact that it had to go farther. Because when it comes back, it has to come back exactly the same way it left,” says Koper. “It has to resonate.”
Sagnac’s counter-rotating beams of light are analogous to beams in a linear cavity. If the turntable rotates clockwise, the beam traveling clockwise has farther to go to catch its starting point; the path of the counterclockwise beam is shorter.
In a given medium, “light travels at a constant velocity,” Koper says. “Einstein says you can’t change that. We definitely know that the beam going clockwise takes longer to get there than the beam going counterclockwise.”
In a ring laser gyroscope, the two counter-rotating beams are channeled to a photo detector. If the vehicle is not rotating, the beams remain in phase. If rotation is occurring, one beam continuously changes phase with respect to the other. A diode translates that moving interference pattern into digital pulses, each pulse representing an angle of rotation (typically .0005 degree per pulse, according to Koper). The rate at which the pulses are produced is also a measure of the rate of rotation.
In the JSOW glide bomb guidance package, Koper’s company also includes GPS receivers to update the ring laser gyros, which are arranged to measure yaw, pitch, and roll. Though the gyros are necessary for the constant feedback required for flight controls, the GPS system corrects any errors that inevitably build up in inertial systems, making them dependent, if only temporarily, on something outside the instruments in the closet.