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.