Inertial Navigation Systems — Page 248, Lesson 290

Right, let's get into the Inertial Navigation System, or INS. This is the heart of long-range navigation on modern aircraft, and I want to walk you through the two key display pages you'll see on the control unit. First, let's look at the Heading and Drift page. The INS gives us a derived true heading. Now, what exactly is that? It's the angle between the north-south axis of the platform and the aircraft's fore and aft axis, in a north-aligned system. In plain terms, it's the direction the aircraft's nose is pointing, measured from true north. This value is shown to the nearest tenth of a degree in the left-hand window. Next to it, in the right-hand window, we have the drift angle. This is also shown to the nearest tenth of a degree, and it's preceded by an 'L' for left or port drift, or an 'R' for right or starboard drift. So, if you see 'L 10.0', the aircraft is drifting 10 degrees to the left of its track. In the example we have, the heading is 050°(T) and the drift is 10°. Now, let's move to the second page: Cross Track Error and Track Keeping Error, abbreviated as XTK/TKE. This is where we see how well we're staying on our planned route. The cross track distance, or XTK, is the displacement of the aircraft perpendicularly from the direct great circle track between the two waypoints you've selected. Think of it as how far left or right you are from the line you're supposed to be flying. This is shown to the nearest tenth of a nautical mile in the left-hand window, and it's also preceded by an 'L' or 'R' to show which side of the track you're on. Then we have the track angle error, or TKE. This is the angle between the track the aircraft would require to make good, were it flying along the great circle route between the specified waypoints, and the track it is actually making good. In simpler terms, it's the difference between the direction you should be flying and the direction you are actually flying. This is shown to the nearest tenth of a degree in the right-hand window, again preceded by an 'L' or 'R'. Let's use the example from the display. We desire a track of 060°, but the indication shows we are tracking 20° to the left of that. So, we must be making good a track of 040°. The summary here is that the cross track error is 12 nautical miles to the right, and the track angle error is 20° to the left. That's the situation shown in Figure 18.20. So, to recap: the heading and drift page tells you your direction and how much you're being pushed off it, and the XTK/TKE page tells you how far off the line you are and at what angle you're diverging. Both are critical for maintaining accurate navigation.