Inertial Reference System — Page 254, Lesson 302

Let’s start with the big picture. The laser gyro has caused a technological revolution in the design of inertial reference and navigation systems. This solid-state, high-precision, angular rate sensor is ideally suited for a highly reliable strap-down system configuration. It eliminates the need for gimbals, bearings, torque motors, and other moving parts, and consequently changes the system operation considerably from conventional inertial navigation systems. Now, let me define what inertial navigation actually means. Inertial Navigation means the determination of a vehicle’s location without the aid of external references. Strap-down inertial navigation goes a step further by enabling navigation without the use of a mechanically stabilized platform. This has been achieved through the advent of laser gyros, or rate sensors, and powerful, high-speed microprocessors. The laser gyros allow a microprocessor to maintain a stable platform mathematically, rather than mechanically. So, the heart of the Inertial Reference System, the IRS, is the Inertial Reference Unit, the IRU. It provides all required inertial reference outputs for the aircraft’s avionics. Let me walk you through those outputs, because they are the core of what the system delivers. Primary attitude: that’s pitch and roll. Heading: that’s true and magnetic. Accelerations: lateral, longitudinal, and normal. Angular rates: pitch, roll, and yaw. Inertial velocity: north/south, east/west, ground speed, true airspeed, and vertical rate. Position: latitude, longitude, and inertial altitude. Wind data: wind speed, wind angle, and drift angle. And calculated data: flight path angle and acceleration, along and across track acceleration, inertial pitch and roll rate, vertical acceleration, and potential vertical speed. Now, who uses this inertial information? The flight management computer, the flight control computer, the thrust management computer, the stability augmentation system, the weather radar, the anti-skid 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. Let’s look at the primary sources of information for the IRU. They are its own internal sensors: three laser gyros and three inertial accelerometers. The only other inputs required are initial position, barometric altitude, and True Airspeed, or TAS. Initial position is required because present position is calculated from the distance and direction travelled from the initial start position entered. Barometric altitude stabilizes the vertical navigation, and thereby stabilizes the vertical velocity and inertial altitude outputs. The TAS input allows the IRU to calculate wind speed and wind direction. So, to tie it together: the IRU is a self-contained box that senses rotation with three laser gyros and acceleration with three accelerometers, and from those it derives attitude, heading, velocity, position, wind, and a whole set of calculated data — all without any external reference. The only things you feed it from outside are where you started, the barometric altitude, and true airspeed. That’s the essence of the Inertial Reference System.