Inertial Navigation Systems — Page 244, Lesson 282

Let’s start with the inherent errors of an inertial navigation system, because that’s where this part of the chapter opens. An INS works by sensing acceleration and integrating it to get velocity and position. But the real world isn’t a perfect sphere with uniform gravity. The irregular shape and composition of the earth, the movement of the earth through space, and other factors provide further possible sources of error. So even a perfectly functioning inertial platform will drift, because the model of the earth it assumes isn’t exactly the earth it’s flying over. Now, here’s the key point about these errors: they vary from system to system depending upon the balance achieved between accuracy on one hand and simplicity of design, reliability, ease of construction, and cost of production on the other. So you can build a more accurate INS, but you’ll pay for it in complexity, weight, and cost. Every manufacturer strikes a different balance, which is why no two systems drift identically. Now let’s move to the control and display panels. There are many makes and models of INS on the market. The state-of-the-art trend is towards a single control/display unit with a standard keyboard, but with a single small video screen, rather than the various individual LED windows you’d see on older units. With the modern video screen presentation, the loading and extraction of information is achieved by selecting a “page number,” and each page, which is displayed on the screen, deals with associated functions. So instead of dedicated windows for each parameter, you call up a page and everything related to that function is on it. One major advantage of this type of system is that hundreds or even thousands of waypoints can be stored in the machine memory. These waypoints, normally airway reporting points such as VORs and intersections, 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; you load a disc that carries the whole navigation database. Now, because of the high capital investment involved in the last generation of INS systems, and because they are proving to be extremely reliable, you are perhaps more likely to encounter the traditional type of control/display units described below. Another good reason for considering this system rather than the modern one is that the JAA examination questions are based on the older type of INS. So for your training, the older hardware is the reference. In any event, please appreciate that the following paragraphs are intended only as a general guide and not as a definitive operating instruction for any particular model of INS. Different manufacturers label things differently, so treat this as the common architecture. The traditional INS system employs two panels for control and display. The simpler of the two is the mode selector panel, shown at Figure 18.16. Its function is straightforward. In the standby mode, the power is supplied to all parts of the system. It is normal to insert the start position — the aircraft’s ramp position in latitude and longitude to the nearest tenth of a minute of arc — whilst the equipment is in this mode. So before you do anything else, you power up in standby, and while it’s in standby you enter where the aircraft is sitting on the ramp, to a tenth of a minute of arc precision. That’s the foundation: the errors that come from the earth’s imperfections and the design trade-offs, then the two-panel architecture of the traditional INS, starting with the mode selector panel and its standby mode where you enter the start position.