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PRELIMINARY SEXTANT CHECKOUT:
1. Test of installation and setup on runway
With sextant active in main window:
Use 'Go to airport' to get the game set up: Aircraft at Norfolk, Virginia. (KORF), runway 5 Note the location of the AC is 36*53N, 076*12W by using the Coordinates display on screen (for training purposes only). It is toggled by pressing Shift+Z. Set date/time to 14 March 1945, 0015 local time, 0515 Zulu Tune in the ADF on 329 Khz. The needle should swing to line up with the runway, 230 degrees. Its purpose is to help you visualize the line of position it provides, very similar to the LOP you will get from the sextant. Set Weather to clear On the sextant worksheet, set Assumed Position: 37*0' latitude, -076*0' longitude. **Don't forget the minus sign** Using Polaris for the star, set the Azimuth 0*0'; elevation 37*0' Using the UTC clock on the worksheet as a timer, click on the eyepiece hot-button. It should display the enlarged eyepiece for exactly 60 seconds. Click twice on the + bubble control to ensure you have a large bubble. Watch the bubble and star jitter, and note that, on average, the star is about 1/2 of the large bubble diameter below the center of the bubble. When the enlarged display extinguishes the display should show distance 0 nm. This says we measured our distance to be about 0 nm south of the assumed LOP. Because the star azimuth is true North, and the assumed Latitude is 37 degrees, this LOP is the 37th parallel of latitude. Start another 1 minute run by resetting and starting the sextant timer by clicking the eyepiece twice. During the run click the + fine elevation knob repeatedly but slowly 20 times. Notice the star is moving up the display (despite the jitter). This should move the star about halfway between the bubble and the top of the display, and leave it there till the light goes out. Now the distance should read about minus 100 NM since the measurement was deliberately miss-set. Next start another 1 minute run, and try to center the star in the bubble. Because for this training exercise we know our location is 7 NM south of the 37th parallel, keep your eye on the sextant elevation, and see if you like the sight picture when you are at 36*55' degrees of elevation. That is what you are looking for. Don't worry about the jitter. Don't chase the bubble! Make the corrections slowly over a few cycles, to end up at your best judgment. That is why real bubble sextants have averaging devices built in. They were not very accurate without them. Why 36*55'? 5 minutes of latitude is 5 NM, and your real location is further away from the star than your assumed position, so the angle will be smaller, and the distance will be negative on the worksheet. Your choices are -5 or -10 based on the coarseness of the fine elevation. -5 is the closest to the precise -7, which is the actual distance you are to the south of Latitude 37.0 North. If you happen to have the sectional chart for Washington, you can see this clearly on the chart. Otherwise, call up your FS map, and it shows the same information.
When you are comfortable with the way the fine elevation knob works, let's take a look at another neat feature of the sextant. Suppose you were 6 degrees east of where you thought you were. Say, your assumed longitude was 81W instead of 076W. Set that change into the sextant, and rerun your position. Note that it still accurately puts you on the east-west line of position 5 nm south of the 37th parallel. But you still don't have a clue, yet, as to where you are on that line.
This is where the second LOP comes in. Suppose that, to make it simple, a star due east of the airport was at 13 degrees above the horizon at the time you plan your shot. Lets plan a shot at 0530 hrs of this star. Reset your assumed latitude to 37N 076W. Set the star azimuth to 90, and elevation to 13 degrees. At 0529 hrs and 30 seconds, start your measurement. You should see the distance is -10 NM, which is to say that you are on a north/south line of position 10 NM west of the -076th longitudinal meridian. Check it with your ruler or dividers if you have a sectional chart, or eyeball it on your FS9 Map, or use the VE6B and enter the lat. and lon. of your true position and your assumed position, and see that 10 nm is pretty close (It shows 9.6 nm to the 37th parallel). So the intersection of these two lines of position, adjusted from the assumed position by the proper distances, is just where you expected them to be, near the aircraft.
One last observation. What if our assumed position was at 081 W, like before? Set it up, N37, W081, and now that star you used would appear lower in the sky. You may still use it, or pick another one, on the same azimuth (for this demo) Assume that at 0530Z, the star is at 11 degrees (We have grossly simplified this problem, but there are of course look up tables for the complete answer).
So, we set in an az of 90, and an el of 11, and take another shot.
The distance is 230 nm. Let's see how far off we are. Using the VE6B, 37N and 081W for departure 37N and 076.2W for destination, the distance is 230 nm. The significance of this is that even if our assumed position is off almost 5 degrees in longitude, the sextant will properly locate the aircraft. In real solutions, the stars are not lined up so well, so the LOPs don't line up exactly with latitude and longitude lines, but the accuracy is the same. Just harder to visualize and to draw. And Polaris is unique in that it doesn't move around very much.
2. Test of sextant at cruise altitude
So, let's get in the air from KORF, runway 5, turning due East (true), or 100 degrees magnetic at 500 ft AGL, turning on the autopilot, if desired, to hold your heading and climb to 2000 ft cruise altitude. Fly due East a while just south of the 37th parallel. Trim for cruise at 150 KTAS. The ADF needle should be swinging steadily toward 280 degrees, and if you take a shot of Polaris every four minutes using the original star and assumed position settings, you should see the distance remain at about -5 nm. Then decide to fly up to, but not beyond the 075th West meridian. This is of course before the days of DME, so a sextant is the gauge of choice.
Set the Assumed position at 37N, 075W. Select the star to the east at 25 degrees elevation. Now, every four minutes, take a sextant reading, and note the distance going toward zero. You can quickly get the ground speed by dividing the change in distance by the time change. Then you can calculate the time when you will be at 0 distance, which corresponds to the 075th Meridian. Since this is about a 20 minute flight, you may want to go to 4x speed till you get close to the meridian. Continue to take readings (now on one minute real time intervals). You should start your standard rate turn one minute before getting to zero miles. After completing the turn and starting back on course 280 degrees magnetic, you can do the same type of shot and calculation, watching your increasing distance. Now, for instance, you could plan your SOD based on calculated position, ground speed, and sextant readings. Take a peek at the track on the map and congratulate yourself for the nice looking track. You are now navigating!
3. A simple practical example daytime flight
Another example of sextant usage is a flight from the Naval Air Station, Ford Island, Oahu, Hawaii to Johnston Atol USA. NPS to PJON, 713 nm. The average magnetic bearing is 236 degrees, and you must use the compass, sextant and the Sun as the sole means of navigation. Using Elgen Long's rule that your final error (the distance between where your ded reckoning puts you and where you actually are) probably will not exceed one mile in ten, since the distance is 713 nm, you can expect to arrive in the vicinity of PJON with an error less than 71 nm. He means by this that 9 times in 10 you will arrive within a circle centered on PJON with a radius of 71 nm. This is called your circle of uncertainty, and unpredicted changes in wind strength or direction are a main contributor to it. Offset your heading to 242 degrees so your circle of uncertainty will lie just to the north of PJON.
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