Part IV · The fix · Page 1 of 5
Two lines make a fix
Each sight gives a line of position. You are on all of them at once, so you are where they cross: the fix. Plot each line from its own AP, exactly as in the intercept method, then mark the crossing.
- Two lines that cross near 90° give a good fix. Below about 30° a small error in either line moves the fix a long way.
- Three lines never quite meet: small errors make a triangle, the cocked hat. Take its center.
- At twilight, choose stars spread round the horizon: three about 120° apart, or four about 90° apart.
Predict Three stars all in the east, and every sight 1′ too high (a forgotten index error). What does the cocked hat look like?
- Bodies, and their bearings
- 030°, 150°, 270°
- Size of the cocked hat
- 0.8 nm
- Fix error
- 1.0 nm
Spread the bodies round the horizon, and the fix is good to about the size of the errors.
Try a constant error (every sight 1 mile out, like a forgotten index error), first with the bearings spread all round, then bunched on one side.