Inertial Reference System — Page 262, Lesson 306

Let’s start at the very beginning of the Inertial Reference System, because this is where the magic happens — the laser gyro. I want you to picture a triangular path, like a closed loop with three sides. Inside that loop, we fire two beams of laser light in opposite directions. Both beams travel at the same speed — the speed of light — and they travel along that same triangular path. Now, here’s the key idea. If the device is sitting perfectly still, those two beams travel identical distances and arrive back at their starting point at the same time. They cancel each other out. But the moment we rotate the device, something changes. One beam has to travel a slightly longer path, and the beam going in the opposite direction gets a measurably shorter journey. That difference in travel time is what we measure. We measure it with devices called gain elements. From that measurement, we can calculate the rate of rotation. So the laser gyro, in an IRS — an Inertial Reference System — is used as an Angular Rate Sensor. That’s its job: it senses how fast the aircraft is rotating. Now, the change in frequency that happens because of that change in path length due to rotation — that has a proper name. It’s called the SAGNAC effect. Remember that term, because it’s the physical principle the whole laser gyro is built on. Let’s look at the hardware. The triangular path uses three mirrors. And here’s a detail that surprises people: the three mirrors 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 lets some of the laser light escape so it can be detected by the photo cell detectors. Now, attached to that second mirror — the partially transparent one — there’s a prism. The prism flips and redirects the light beam around, causing it to meet and interfere with the light beam that’s aimed directly at the photo cell. When those two beams meet, they alternately cancel and reinforce each other. That’s called interference, and it generates what we call a fringe pattern — a pattern of light and dark bands. The photoelectric cell then detects two things: the direction and the speed at which that fringe pattern moves. The direction of movement tells us which way the laser gyro is being rotated. The speed tells us how fast. The faster the rotation, the faster the fringe pattern moves across the photoelectric cell. And that movement is converted into electrical signals that are used within the aircraft systems. Now let’s talk about limitations and accuracy, because nothing is perfect. The principal source of error with this device is drift. And here’s the interesting contrast: in a conventional gyro-stabilized platform INS — that’s an Inertial Navigation System — drift is caused by imperfections in the gyro bearings and mass imbalances. But with the laser system, drift is caused by noise, and that noise comes almost entirely from imperfections in the mirrors and their coatings. So the mirrors are the weak link — the source of the error. Let me show you the diagram so you can see the triangular path and the two beams. So to tie it all together: the laser gyro senses rotation by measuring the time difference between two counter-propagating light beams — that’s the Sagnac effect. The fringe pattern it produces tells us direction and rate of rotation, and the only real error source is drift from mirror imperfections. That’s the heart of the IRS.