Inertial Navigation Systems — Page 248, Lesson 291

Let’s pick up with the Inertial Navigation System’s control display unit, because this is where the system talks back to you. I want to walk you through the pages that show you exactly what the INS displays, starting with present position. The aircraft’s present latitude is shown to the nearest tenth of a minute of arc in the left-hand window. The present longitude is shown to the nearest tenth of a minute of arc in the right-hand window. So a typical readout would be 34°31.5’N 117°11.3’W. Notice the precision — a tenth of a minute of arc. That’s a very fine resolution, and it’s the standard the system holds for position. Now, waypoint positions. These are also shown in latitude in the left-hand window and longitude in the right-hand window, again to the nearest tenth of a minute of arc. In the system we’re considering, there are 10 possible waypoint selections, numbered 0 through 9. Waypoints 1 through 9 are simply selected turning points, and they’re normally placed into the system by the operator before the flight. So those are your planned route points. Waypoint 0 is special. It represents the aircraft’s position at the last time a track change from present position to a specified waypoint was selected by the operator. Let me explain why that’s useful. Suppose you’re halfway between waypoints 3 and 4, and air traffic control clears you direct to waypoint 6. By selecting a track change from waypoint 0 — which is your present position — to waypoint 6 and inserting it, the aircraft will fly you directly to waypoint 6 if it’s coupled to the flight director or autopilot. So waypoint 0 is the computer’s way of saying “start from where I am right now.” And here’s the key limitation: waypoint zero is reserved for the computer to establish a track from the aircraft’s present position, and it will not accept operator-entered waypoint coordinates. You can’t type a lat/long into waypoint 0 — it’s the system’s own reference point. Next, distance and time. The distance to go from the aircraft’s present position direct to the next selected waypoint is shown to the nearest nautical mile in the left-hand window. The lapsed time from the present position to the next waypoint is shown to the nearest tenth of a minute in the right-hand window. So a typical readout would be 140 NM and 16.7 minutes. Note the units — distance in nautical miles, time in tenths of a minute. Then wind speed and direction. The INS-derived wind direction is shown in degrees true, to the nearest degree, in the left-hand window. The INS-derived wind speed is shown to the nearest knot in the right-hand window. So the wind velocity would read as 155°(T)/85 kt. The “(T)” is critical — that’s degrees true, not magnetic. Finally, desired track and status. The desired track — assuming the aircraft is on the direct great circle track between the two selected waypoints — is shown in degrees true to the nearest tenth of a degree in the left-hand window. The right-hand window will normally be blank, because the status check is generally only available with the equipment in the alignment mode. So during normal navigation, that right-hand window sits empty; you only get status information when you’re in alignment. So to tie it together: the left-hand window consistently carries latitude, distance, wind direction, and desired track. The right-hand window carries longitude, time, wind speed, and status. And the one thing to remember about waypoint 0 is that it’s the computer’s present-position reference — you can’t overwrite it with your own coordinates.