A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Ed Popko
Date: 2021 Jan 23, 16:09 -0800
I'm reacquainting myself with the Polhemus Celestial Computer - Air Navigation Type CPU-41/P
NavListers Gary LaPook and Tracy Shrier have posted extensively on this device and how air navigators
compute running fixes using bubble sextants. Unlike marine applications that happen at much lower
speeds, air navigators must content with a number of additional corrections to their sights. One correction is for the Coriolis' effect on the bubble in air navigator's sextants. The correction is proportional to the aircraft's speed and latitude. At higher speeds and latitudes, the correction can be considerable.
In reviewing the Polhemus manual (pdf attached), I have to say I'm confused about where the Coriolis Correction is applied. The manual example shows it applied to precomputed Hc's. But the manual also shows it applied again when the fix is plotted. Why both places. It seems appropriate to apply it to one or the other but not both. This is my confusion. Where to apply the correction.
On the Polhemus computation form example, Coriolis has been accounted for on the form’s left, line 9, Figure 11A . Line 22 show's it is part of the other corrections that are applied to Ha giving Hc (lines 4, 23 and 24 on the form’s right ). The Coriolis correction value is based on the relative Zn-Track and the plane's latitude. In the North hemisphere, it's correction sign is - when applied to Hc if the body was to the right of the track and + if body is left. At this point, it seems that Coriolis corrections have been accounted for.
Later in the manual, when three LOPs are plotted, we see that the cocked hat’s center is again adjusted for Coriolis. It moves the center point perpendicular to the flight track by so many minutes to get the final FIX. Again, this seems OK since Coriolis acts perpendicular to the fight track.
WHY is the Coriolis Correction applied again, highlight on plot template? Isn't it enough to apply the correction each sight's Hc before the LOPs are plotted OR to shift the FIX by the total effect but not both?
Any thoughts appreciated.