Inertial Navigation Systems — Page 235, Lesson 273

Let’s pick up with the accelerometer output and what happens to it after the amplifier. I want to walk you through the two integrators, because that’s the heart of how an inertial system turns raw acceleration into position. The acceleration signal from the amplifier is also sent to an integrator. An integrator is a time multiplication device. So we start with acceleration, which is in feet per second squared. In the integrator, that acceleration is literally multiplied by time, and the result is a velocity in feet per second. So the first integration takes you from acceleration to speed. Then that velocity signal is sent through a second integrator, and again, it’s just a time multiplier. Now the input is feet per second, multiplied by time, and the result is a distance in feet or nautical miles. So the second integration takes you from speed to distance travelled. Two integrations in series: acceleration → velocity → distance. Now, where are these accelerometers mounted? They’re mounted on a platform, and there are two of them: one 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 one sensing north-south motion, one sensing east-west motion, and optionally a third for vertical. The computer associated with the inertial system knows the latitude and longitude of the take-off point. It calculates that the aircraft has travelled so far in the north direction and so far in an east direction. From those two distances, the computer can compute the new position of the aircraft and give a digital read-out. And note this detail: the read-out is to tenths of a minute. In like manner, using stored velocity and present position, the system is able to 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. The full range of information that may be obtained will be described later in the chapter. So the whole chain is: accelerometer senses acceleration, first integrator gives velocity, second integrator gives distance, and the computer, knowing the starting latitude and longitude, resolves that distance into north and east components to update present position. That’s the core of inertial navigation.