Inertial Reference System — Page 254, Lesson 302

Let’s start with the big picture, because this chapter is a real turning point in how we think about navigation. I want to walk you through the Inertial Reference System, or IRS, and the heart of it is something called the laser gyro. The laser gyro has caused a technological revolution in the design of inertial reference and navigation systems. It’s a solid-state, high-precision, angular rate sensor. Let me unpack that. “Solid-state” means there are no moving mechanical parts inside — no spinning wheels, no bearings. “Angular rate sensor” means it measures how fast the aircraft is rotating — its rate of turn about an axis. And it’s high-precision, so it’s extremely accurate. Because it’s solid-state, it’s ideally suited for what we call a strap-down system configuration. Let me explain that term carefully, because it’s central to this whole chapter. In a conventional inertial navigation system, the sensors are mounted on a mechanically stabilized platform — a set of gimbals and bearings and torque motors that keep the platform level and aligned with the Earth regardless of how the aircraft moves. That’s a lot of moving hardware. The laser gyro eliminates the need for gimbals, bearings, torque motors, and other moving parts. Instead, the sensors are literally “strapped down” — fixed rigidly to the aircraft structure. And that changes the system operation considerably from conventional inertial navigation systems. Now, let’s define what inertial navigation actually means. Inertial Navigation means the determination of a vehicle’s location without the aid of external references. No radio beacons, no GPS, no ground stations — the system figures out where it is purely from its own internal measurements. Strap-down inertial navigation goes a step further by enabling navigation without the use of a mechanically stabilized platform. So we’ve removed both the external references and the mechanical platform. How is that possible? Through the advent of laser gyros and rate sensors, and powerful, high-speed microprocessors. Here’s the key idea: the laser gyros allow a microprocessor to maintain a stable platform mathematically, rather than mechanically. Instead of physical gimbals holding the sensors level, the computer uses the gyro measurements to calculate, in software, what the platform orientation would be — and then corrects the accelerometer readings mathematically. That’s the revolution. Let me show you what this hardware looks like. — this is the GEC-Marconi FIN3060, a commercial aircraft inertial reference unit. That’s the physical box we’re talking about. Now, the Inertial Reference Unit, or IRU, is the heart of the Inertial Reference System, the IRS. It provides all required inertial reference outputs for the aircraft’s avionics. So it’s not just one instrument — it’s a central source that feeds many systems. Let me list the outputs, because you need to know exactly what this box produces. Primary attitude — that’s pitch and roll. Heading — and here’s an important distinction — it provides both True heading and Magnetic heading. Accelerations — and note the three axes: Lateral, Longitudinal, and Normal. Angular rates — Pitch, Roll, and Yaw rates. Inertial velocity — and this is a rich output: North/South velocity, East/West velocity, Ground Speed (GS), True Airspeed (TA), and Vertical rate. Position — Latitude, Longitude, and inertial altitude. Wind data — Wind speed, wind angle, and drift angle. And finally, calculated data — and this is a long list: Flight path angle and acceleration, Along and across track acceleration, Inertial pitch and roll rate, Vertical acceleration, and Potential vertical speed. So you can see, the IRU is a very rich source of information. Now, who uses this information? Let me give you the list of consumers, because it shows you how central this unit is. The Flight Management Computer, the Flight Control Computer, the Thrust Management Computer, the Stability Augmentation System, the Weather Radar, the Anti-skid and Auto Brake systems, the Attitude Direction Indicator, the Horizontal Situation Indicator, the Vertical Speed Indicator, the Radio Direction Magnetic Indicator, and the Flight Data Recorder. That’s a huge spread — from navigation computers down to the basic flight instruments and even the data recorder. Now, here’s a critical question: where does the IRU get its information? The primary sources of information for the IRU are its own internal sensors — three laser gyros and three inertial accelerometers. Three of each, one per axis. But it can’t work alone. The only other inputs required are initial position, barometric altitude, and True Airspeed, or TAS. Let me explain why each of those is needed, because this is where the logic becomes clear. Initial position is required because present position is calculated from the distance and direction travelled from the initial start position entered. In other words, the IRU doesn’t know where it is until you tell it. It integrates — it adds up — all the accelerations and rotations from that starting point to figure out where it has moved to. So you must enter the starting coordinates. Barometric altitude stabilizes the vertical navigation, and thereby stabilizes the vertical velocity and inertial altitude outputs. The inertial vertical channel drifts over time — it’s not inherently stable. So we feed in barometric altitude from the air data system to keep the vertical velocity and inertial altitude outputs accurate and stable. And the TAS input allows the IRU to calculate wind speed and wind direction. Here’s the logic: the IRU measures the aircraft’s motion over the ground — its inertial velocity. But the aircraft is moving through an air mass. True Airspeed is the aircraft’s speed through the air. The difference between the ground motion and the air motion is the wind. So by comparing its own inertial velocity with the TAS input, the IRU can compute the wind speed and wind direction. So let me pull this together. The laser gyro is the enabling technology — solid-state, no moving parts, high precision angular rate sensing. It allows a strap-down configuration, where the sensors are fixed to the aircraft and the platform is maintained mathematically by a microprocessor. The IRU is the heart of the system, producing attitude, heading, accelerations, angular rates, velocity, position, wind data, and calculated data — all feeding a wide range of aircraft systems. And it needs just three external inputs: initial position, barometric altitude, and TAS. That’s the foundation. The next step is to understand how the laser gyro itself actually works — how it measures that angular rate. That’s where we’ll go next.