Inertial Navigation Systems — Page 235, Lesson 273

Let’s pick this up right where the accelerometer signal leaves the amplifier. That acceleration signal, which is in feet per second squared, is sent to an integrator. And I want to be very precise about what an integrator is here: it is a time multiplication device. It literally multiplies the acceleration by time. So you start with feet per second squared, you multiply by time in seconds, and the result is velocity, in feet per second. That’s the first integration. Then that velocity signal goes through a second integrator. Again, it’s just a time multiplier. Now the input is feet per second, multiplied by time, and the result is distance — in feet, or in nautical miles. So the chain is: acceleration, integrate once to get velocity, integrate a second time to get distance. That’s the whole principle of how an inertial system turns sensed acceleration into position change. Now, where do these accelerometers live? They’re mounted on a platform, and there are two of them — one oriented in the north-south direction, the other in the east-west direction. Often a third accelerometer is fitted to measure vertical acceleration. So you have your two horizontal axes, and optionally a vertical one. Here’s the clever part. The computer associated with the inertial system knows the latitude and longitude of the take-off point. It takes the distance travelled north and the distance travelled east — which we just got from those double integrations — and it computes the new position of the aircraft. The result is given as a digital read-out, and I want you to note the precision: it’s to tenths of a minute. So the present position is displayed to one decimal place of a minute of latitude and longitude. And it doesn’t stop there. Using the stored velocity and the present position, the system can calculate additional navigation data and display it as requested by the operator. The information is chosen for display through the rotary switch at the bottom left of the control unit. So the operator selects what they want to see, and the system presents it. The full list of what can be obtained is described later in the book, but the key point for now is that the present position — the POS read-out — is the foundation, and everything else is derived from that stored velocity and current position. So to tie it together: accelerometers sense acceleration, two integrators convert that to velocity and then to distance, the computer knows the starting latitude and longitude, and from the north and east distances it computes the new position to tenths of a minute, with a rotary switch on the control unit letting the operator select which navigation data to display.