Inertial Reference System — Page 262, Lesson 306

Let’s pick up right where the laser gyro’s light path leaves off, because that’s the heart of how an Inertial Reference System senses rotation. I want to walk you through the rest of this passage, and then we’ll close with the accuracy story. So, we have this triangular device. The two beams travel in opposite directions at the same speed — the speed of light. When the device is perfectly still, the beams cancel each other out. But the moment you induce movement — that is, rotation — one beam takes longer to complete its triangular path, and the opposing beam takes a measurably shorter time. That whole timing difference is measured by devices called gain elements, and from that timing difference the rate of rotation can be calculated. That’s the core principle. Now, in an IRS, the laser gyro is used specifically as an Angular Rate Sensor — it senses how fast the aircraft is rotating about its axis. Here’s the key term: the change in frequency, caused by the change in path length due to rotation of the gyro, is called the SAGNAC effect. So when the gyro rotates, the effective path length for one beam changes, which changes its frequency — that frequency shift is the Sagnac effect, and it’s what the system measures. Now let’s look at the hardware inside. There are three mirrors, and they are not identical. One mirror makes micro adjustments to keep the physical light path accurately aligned. Another mirror is partially transparent — that’s deliberate, because it allows the laser light to escape and be detected on the photo cell detectors. Included with that second mirror is a prism. The prism flips or redirects the light beam around, causing it to meet and interfere with the light beam that is aimed directly at the photo cell. When those two beams meet, they alternately cancel and reinforce each other — that’s called interference — and this generates what’s known as a fringe pattern. Think of it as a pattern of bright and dark bands created by the beams combining. The photoelectric cell then detects the direction and speed at which that fringe pattern moves. The direction the pattern moves — one way or the other — depends on which way the laser gyro is being rotated. And the faster the rotation, the faster the fringe pattern moves across the photoelectric cell. That movement is then converted into signals used within the aircraft systems. Now, the limitations and accuracy. The principal source of error with this device — just like with the conventional gyro-stabilized platform INS device — is associated with random drift. In a conventional gyro, that drift is caused by imperfections of the gyro bearings and mass imbalances. But with the laser system, the cause is noise, and that noise is derived almost entirely from imperfections in the mirrors and their coatings. So the takeaway: the laser gyro replaces mechanical bearings with light, but it still isn’t perfect — the mirrors themselves introduce the noise that becomes drift. That’s the accuracy story for this device. Take a moment to let that settle — the Sagnac effect, the fringe pattern, and the mirror-induced noise. When you’re ready, we’ll move on.