Inertial Navigation Systems — Page 244, Lesson 282

Let's start with the inherent errors of an inertial navigation system. I want you to understand that an INS is not perfect — it drifts, and the drift comes from the world itself. The earth is not a perfect sphere; it has an irregular shape and an irregular composition. The earth also moves through space, and there are other factors. All of these provide further possible sources of error. Now, here's the key point: these errors vary from system to system. Why? Because every manufacturer makes a trade-off. They balance accuracy on one hand against simplicity of design, reliability, ease of construction, and cost of production on the other. So one system might be extremely accurate but expensive and complex to build. Another might be simpler and cheaper but less accurate. That balance is what determines how much error a given system will have. Now let's move to the control and display panels. There are many makes and models of INS on the market. The modern trend — the "state-of-the-art" — is toward a single control and display unit. It has a standard keyboard, but instead of individual LED windows, it has one small video screen. With this modern video screen presentation, you load and extract information by selecting a "page number." Each page, displayed on the screen, deals with associated functions — so one page might be for position entry, another for waypoint selection, and so on. Here's a major advantage of this modern type: hundreds, even thousands, of waypoints can be stored in the machine's memory. These waypoints are normally airway reporting points — things like VORs and intersections. They are automatically loaded from a master disc, which is supplied and regularly updated by specialist agencies. So you don't type them in one by one; the disc does it for you. But — and this is important for you as a pilot — because of the high capital investment in the last generation of INS systems, and because they are proving extremely reliable, you are more likely to encounter the traditional type of control and display unit. And there's another very practical reason to study the older system: the JAA examination questions are based on the older type of INS. So we're going to focus on that. The traditional INS employs two panels for control and display. The simpler of the two is the mode selector panel. Its function is straightforward. In the standby mode, power is supplied to all parts of the system. And it is normal to insert the start position while the equipment is in this mode. The start position is the aircraft's ramp position, expressed in latitude and longitude to the nearest tenth of a minute of arc. So you're telling the system exactly where the aircraft is sitting on the ramp before it begins its navigation. One important caveat before we go further: these paragraphs are intended only as a general guide, not as definitive operating instructions for any particular model of INS. Different models will have their own specific procedures. Now, let me show you the Schuler period, because it's fundamental to understanding how an INS stays level and why it oscillates. That's the core of it — the inherent errors, the two-panel traditional layout, and the standby mode where you enter your start position.