Inertial Navigation Systems — Page 248, Lesson 290

Let's start with the heading and drift display, because that's the first thing you'll see on the INS control unit. The INS gives you a true heading — that'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 in the left-hand window, to the nearest tenth of a degree. Next to it, in the right-hand window, you get the drift angle — also to the nearest tenth of a degree. Drift is the angle between the aircraft's heading and its actual track over the ground, caused by wind. The display precedes this value with an L for left or port drift, or an R for right or starboard drift. So if you see "L 10", you're being pushed 10 degrees to the left of where the nose is pointing. The example given is a heading of 050°(T) — that's 050 degrees true — and a drift of 10 degrees. So the aircraft's nose is at 050 true, but the wind is pushing it 10 degrees off that heading. Now, the second part of this display is the cross track error and track keeping error, abbreviated XTK/TKE. This is a different pair of values, shown in the same two windows. The cross track distance — the XTK — is the displacement of the aircraft perpendicularly from the direct great circle track between the two selected waypoints. That's the straight-line route over the Earth's surface between where you are and where you're going. This distance is shown to the nearest tenth of a nautical mile in the left-hand window, and it's preceded by an L or an R to show whether you're left or right of that direct track. The track angle error — the TKE — is the angle between the track the aircraft would need to make good if it were flying along the great circle route between the specified waypoints, and the track it's actually making good. That's shown to the nearest tenth of a degree in the right-hand window. Again, the L or R in front tells you whether the actual track is to the left or right of the required track. Let me make that concrete with the example. Suppose your desired track is 060°. The display shows a track angle error of 20° to the left. That means you're actually making good a track of 040° — because you're tracking 20 degrees to the left of your desired track between consecutive waypoints. So in summary, for the situation shown in Figure 18.20: the cross track error is 12 nautical miles to the right, and the track angle error is 20 degrees to the left. The aircraft is 12 NM right of the direct great circle track, but it's heading 20 degrees to the left of where it needs to go to stay on that track. That's the key distinction to hold onto: the heading and drift display tells you about the aircraft's attitude relative to the air and the ground, while the XTK/TKE display tells you about your position and direction relative to your planned route between waypoints.