Inertial Navigation Systems — Page 235, Lesson 279

I want to walk you through the alignment sequence of an Inertial Navigation System, and then the Schuler Period, which is the physical principle that makes the whole thing work. First, the big picture. The stable element — that's the gyro-stabilised platform inside the INS — has to be aligned accurately in two senses: azimuth, which is heading, and attitude, which is pitch and roll. Only then can the accelerometers measure acceleration along their chosen axes correctly. If the platform is tilted, the accelerometers will pick up components of gravity and give you false readings. So alignment is everything. The alignment sequence has a warm-up period first. This is the first stage in any alignment sequence. The fluid-filled components — the gyros and accelerometers are floated in fluid — must be brought to the correct operating temperature. This phase normally takes between 3 to 4 minutes. Then comes coarse alignment. The platform is roughly levelled and aligned in azimuth. The purpose here is to remove gyro alignment errors and cut the time to a minimum. Coarse alignment itself has two parts. First, coarse levelling: pitch and roll are driven until they are at 90° to each other. The platform is then roughly levelled using either the aircraft frame as reference, or using the outputs from gravity switches or the horizontal accelerometers. Second, coarse azimuth alignment: this is achieved by turning the platform until the heading output agrees with the aircraft's best known True Heading. The result of coarse alignment is that it levels and aligns the platform within 1° to 2° in a few seconds. Then we move to fine levelling. With zero output from the accelerometers, fine levelling is achieved. The process takes anything up to 1 to 1½ minutes, and it levels the platform to within 6 seconds of arc. That's a very tight tolerance — 6 seconds of arc is a tiny fraction of a degree. Next is gyro compassing. This is how the platform is aligned in azimuth to true north. The technique is to connect the gyro normally used to stabilize the platform about an east-west axis — that's the east gyro — to the azimuth gimbal motor. Here's the logic. With the platform correctly aligned in azimuth, the east gyro should not be subject to rotation of its input axis due to earth rotation. When the platform is out of alignment, the east gyro will detect a component of earth rotation, and the resultant output signal can be used to torque the azimuth gyro until the table is aligned. So the earth's rotation itself becomes the reference for finding north. Two final conditions complete the alignment. Accelerometers must be levelled, meaning velocity is set to zero. And the platform must be orientated to true north — that's the gyro compassing we just did, with the position verified. Now, the Schuler Period. This is the principle that explains why the platform stays horizontal even when the aircraft accelerates. Schuler postulated an earth pendulum with length equal to the radius of the earth, its bob at the earth's centre and point of suspension at the earth's surface. If the suspension point were accelerated around the earth, the bob would remain vertically below the suspension point because it is at the earth's centre of gravity. A platform mounted on the suspension point tangential to the earth's surface — that is, horizontal — would therefore remain horizontal irrespective of the acceleration experienced. The vertical defined by the normal to the platform is therefore unaffected by acceleration. If, for any reason, the bob on the earth pendulum became displaced from the earth's centre, the pendulum would start to oscillate. The oscillation period would be 84.4 minutes. That's the Schuler Period — 84.4 minutes. Now connect this to the INS. The stable element is maintained normal to the local vertical by feeding back the aircraft's radial velocity as levelling gyro signals. In this way, the north and east accelerometers are prevented from detecting components of the gravity acceleration. So the platform behaves like that ideal earth pendulum — it stays horizontal through acceleration, and the accelerometers only measure true horizontal acceleration, not gravity. Let me bring it together. The alignment sequence gets the platform level and pointing at true north, to within seconds of arc. The Schuler tuning — the 84.4-minute period — is what keeps it that way during flight, so the accelerometers stay honest. That's the heart of inertial navigation.