Inertial Reference System — Page 262, Lesson 309

We're starting the Inertial Reference System, the IRS, and I want to begin with the accuracy of the laser system, because that's the heart of why this technology works the way it does. The accuracy of the laser system is directly influenced by the length of its optical path. The optical path is the distance the light beams travel inside the laser ring. The longer the path available, the greater the accuracy. And here's the key relationship: a small percentage increase in that length leads to a substantial increase in accuracy. So it's not a linear trade-off — a little more path buys you a lot more precision. That's why the physical size of the ring matters so much in design. Now, the most significant potential problem with this system is something called lock in, also known as laser lock. It occurs at very low rotation rates. Let me explain what happens. At very low rotation rates, the output frequency can drop to zero. This happens as a result of back scattering between the two beams. Back scattering means light from one beam gets scattered back into the other beam. When that happens, the two beams synchronize — they lock together. And once they're synchronized, they no longer indicate the rotation correctly. In fact, they introduce undesirable errors. So at the very moment you need the sensor to be most sensitive — when the aircraft is barely rotating — the system goes blind. This phenomenon is overcome by the introduction of a vibration device known as a piezo electric dither motor. Piezo electric means it converts electrical energy into mechanical vibration. The motor breaks the lock in. It's mounted in such a way that it vibrates the laser ring about its input axis, through the lock in region. So it physically shakes the ring back and forth, forcing the beams out of their synchronized state. That unlocks the beams and enables the optical sensor to detect the smaller movement of the fringe pattern. The fringe pattern is the interference pattern created by the two beams — the thing the sensor reads to measure rotation. And importantly, the motions caused by the dither motor are decoupled from the output of the ring laser gyro, or rate sensor. Decoupled means the vibration is filtered out so it doesn't contaminate the rotation measurement. The dither shakes the ring, but the output you read is pure rotation, not the shaking. Now let's move to the platform versus strap down principles, because this is the big architectural difference. The INS — the Inertial Navigation System — uses a platform set in gimbals. Gimbals are the mechanical rings that allow the platform to stay level while the aircraft moves around it. That platform system requires three rate integrating gyros and accelerometers to achieve an output we can use. And this process is started by ensuring that the platform is horizontal at the correct latitude. So before you even begin, you have to physically align the platform. The IRS, on the other hand, is strap down. Strap down means the three laser gyro rate sensors and accelerometers are attached directly to the aircraft chassis. There's no gimbal mechanism. The sensors move with the aircraft. So how do you get a stable reference? High speed microprocessors achieve a stable platform mathematically rather than mechanically, as per the INS. The computer does in software what the gimbals did in hardware. And this results in greatly improved accuracy and reliability, because you've removed the moving mechanical parts. The integration principles are used as per the older INS system. Integration is the mathematical process of combining the acceleration measurements over time to get velocity, and velocity over time to get position. So the IRS keeps that same mathematical core. And finally, gravity. The microprocessor — and the excerpt cuts off there, but the point is that gravity is a factor the microprocessor must account for. The accelerometers measure specific force, which includes gravity, so the system has to separate the aircraft's true acceleration from the gravitational component. That's the gravity correction the microprocessor handles. So to tie it together: the laser gyro's accuracy depends on optical path length, lock in is its Achilles heel at low rotation rates, solved by the piezo electric dither motor. And the IRS straps the sensors to the airframe, letting microprocessors create the stable platform mathematically, with integration and gravity compensation carried over from the older INS.