Inertial Navigation Systems — Page 226, Lesson 270

Let's start with the very heart of this system. The fundamental element of the whole Inertial Navigation System is the Inertial Sensor System, or ISS. Think of the ISS as the sensing core. To build it, we take a stable platform, and on that platform we mount high-quality gyros and accelerometers, and we connect them to a computer. Now, what does that computer actually do? Its job is to take the raw acceleration data and turn it into useful navigation information. It does this through a process called integration. First, it integrates the accelerometer outputs with respect to time to give you velocity. Then, it integrates that velocity with respect to time to give you distance travelled. So we go from acceleration, to speed, to distance, purely by mathematical integration. From that integrated data, the system can derive a whole suite of information: pitch and roll attitude, true heading, true track, drift, present position in latitude and longitude, ground speed, and wind. That's the output of the ISS. But to turn the ISS into a full Inertial Navigation System, or INS, we add a second computer. This one lets the pilot inject and store waypoints. Once the waypoints are in, it computes track angle error, distance to go, and time to go to reach each one. And that information can be fed to the autopilot, the flight director, or used for normal manual flying. Now, a key point about the modern INS: it was the first self-contained single source of all navigation data. It didn't rely on external signals. It's now joined by the similar IRS, the laser gyro system, which we'll discuss later. The current state of the art has produced INS units whose performance, size, and weight far exceed older navigation systems. Let's move to the basic principles. This all rests on Newton's laws of motion. First law: a body continues in a state of rest, or uniform motion in a straight line, unless acted upon by an external force. Second law: the acceleration—the rate of change of velocity—of a body is directly proportional to the force acting on it, and inversely proportional to its mass. Third law: to every action there is an equal and opposite reaction. But here's the twist. Einstein, in 1905, destroyed the premise of absolute motion. His theory said nothing is truly at rest. "At rest" merely means the object is moving at the same velocity as its co-ordinate system and the observer. The accelerometer, the primary measuring device in an INS, demonstrates this perfectly. It makes no distinction between being at rest and being at any other fixed velocity. However, it does distinguish between truly fixed velocities and those we might regard as fixed but are really fixed speeds along curved paths. Now, the accelerometers themselves. Two are mounted at the heart of the inertial system. They sense any change in the aircraft's velocity, whether acceleration or deceleration, very accurately. One measures acceleration in the north-south direction, and the second in the east-west direction. Each accelerometer is basically a pendulous device. When the aircraft accelerates, the pendulum, due to inertia, swings off its null position. A signal pick-off device measures how far the pendulum is off null. That signal goes to an amplifier, and current from there drives the system. That's the core sensing mechanism we're building on.