A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Gary LaPook
Date: 2021 Jan 23, 18:08 -0800
It's an "either or" proposition.
Since the sights are taken with a bubble sextant in a high speed vehicle we must apply some
additional corrections due to various accelerations to the liquid in the bubble chamber. The main
correction is Coriolis which makes altitudes measured to the right of the aircraft (in the northern
hemisphere, the opposite in the southern) read too low and those on the left side to read too high.
This can be handled in a number of ways. You can move the A.P. to the right (northern
hemisphere) at a 90º angle to the course (track) prior to plotting the LOPs by the amount of the
Coriolis correction shown in the table in the Air Almanac and in H.O. 249, page 3
file. Or you can move the final fix the same way. Or, the most complicated way, is to make a
correction to each Hc by multiplying the Coriolis correction by the sine of the relative Zn, and
adding it to altitudes measured on azimuths to the right and subtracting for observations to the
left (northern hemisphere), the Polhemus makes this relatively painless.
Rhumb line correction is similar to Coriolis and is caused by flying a rhumb line during the
shooting period by flying a compass heading. In high latitudes and high speeds it can exceed the
Coriolis correction and can act in either direction depending on the course, either adding to
Coriolis or canceling it. But this error can be avoided by steering by directional gyro during the
two minute shooting period and this is what is normally done anyway. The easiest way is to apply it to the
AP since you normally use just one AP for all the sights. Then that is one less step that you must take
after taking the sights and before you can plotting the fix.
On my Polhemus I added a " + C" and a "-C" on the face to tell me the sign of the coriolis correction when using the more complicated method in which the corrections applied to the individual observations. I also added little black arrrow heads at the 360 and 180 points using the standard that black is "PLUS" when applied to Hc.
And see this example:
Notice that in the exmple I leave the corriolis correction lines blank.