Inertial Reference System — Page 262, Lesson 309

Let's start with the accuracy of the laser system, because that's the foundation of everything we're about to discuss. The accuracy of the laser system is directly influenced by the length of its optical path. What does that mean? The optical path is the distance the laser beams travel inside the ring. The longer that path is, the greater the accuracy. And here's the key relationship: a small percentage increase in length leads to a substantial increase in accuracy. So it's not a linear trade-off — a little bit more path length buys you a lot more precision. That's why manufacturers design these rings as large as they can within the physical constraints of the aircraft. Now, the most significant potential problem with this system is something called lock in, also known as laser lock. This occurs at very low rotation rates. Let me explain what happens. At very low rotation rates, the output frequency can drop to zero. Why? Because of back scattering between the two beams. Back scattering means the beams interfere with each other, and this causes the beams to synchronize. When they synchronize, they no longer indicate the rotation correctly, and worse, they introduce undesirable errors. So instead of giving you a clean signal, you get a false or zero reading right when you need sensitivity the most. How do we overcome this? We introduce a vibration device called a piezo electric dither motor. The name tells you a lot: piezo electric means it uses piezoelectric crystals that change shape when voltage is applied, and dither means it vibrates rapidly. This 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. The input axis is the axis about which the gyro measures rotation. By vibrating through that lock in region, it unlocks the beams. That enables the optical sensor to detect the smaller movement of the fringe pattern — the fringe pattern being the interference pattern created by the two beams. 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 doesn't contaminate the actual rotation measurement. The dither is there to break the lock, but it's filtered out of the final signal. Now let's move to platform versus strap down principles, because this is the heart of the Inertial Reference System. There are two approaches. The first is the platform system, which is the older INS — Inertial Navigation System. The platform is set in gimbals. Gimbals are the mechanical rings that allow the platform to stay level regardless of aircraft attitude. This platform system requires three rate integrating gyros and accelerometers to achieve an output we can use. The process starts by ensuring the platform is horizontal at the correct latitude. So before anything else, you physically level that platform and align it to your position on Earth. The second approach is strap down, which is what the IRS — Inertial Reference System — uses. Instead of gimbals, the IRS attaches the three laser gyro rate sensors and accelerometers directly to the aircraft chassis. The chassis is the airframe itself. So the sensors move with the aircraft. That sounds like a problem, right? How do you get a stable reference if everything is bolted to a moving aircraft? The answer is high speed microprocessors. They achieve a stable platform mathematically rather than mechanically, as in the INS. The computer calculates what the platform would be doing and corrects for it in real time. This results in greatly improved accuracy and reliability, because you've removed the mechanical complexity and wear of gimbals. Now, integration principles are used as per the older INS system. Integration is the mathematical process of combining the accelerometer outputs over time to get velocity, and then integrating velocity to get position. So even though the hardware is different — strap down versus gimbals — the underlying math of how you turn acceleration into navigation data is the same as the older system. Finally, we have gravity. The excerpt cuts off here, but I want to set the stage. Gravity is a critical factor in an inertial system because accelerometers measure specific force, not pure acceleration. That means they sense the reaction to gravity as well as actual motion. A stationary accelerometer on the ground reads 1 g upward, because it's resisting gravity. So the microprocessor has to model gravity accurately and subtract it out, otherwise your navigation solution drifts. That's why the platform system had to be horizontal at the correct latitude — to align the gravity vector properly. The strap down system does this mathematically with its gravity model. Let me bring this together. You have three laser gyros and three accelerometers strapped to the airframe. The gyros measure rotation rate, the accelerometers measure specific force. The microprocessors integrate those signals, apply the gravity correction, and produce a mathematically stable platform. The accuracy depends on the optical path length of the lasers, and the lock in problem is solved by the piezo electric dither motor. That's the complete picture of how the IRS works at its core.