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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Polhemus computer
From: Gary LaPook
Date: 2008 Jul 24, 02:39 -0700

```Look at the thread "celestial up in the air" for more infomation about
the Polhemus computer.

gl

On Jul 15, 12:49 pm, "Gary J. LaPook"  wrote:
> Gary LaPook writes:
>
> As an addendum to my previous post, I forgot to point out that the
> central meridian on both the plotting sheet and the Polhemus computer
> were 119� 15' W and the central parallel was 34 � N although that should
> have been clear from the context.
>
> I also forgot to show how the final fix coordinates were determined. The
> latitude is easy, just read it off the central meridian scale and
> remember, for the plotting sheet, to divide by 4 since I multiplied the
> scale by 4 at the beginning. To determine the longitude you do the
> reverse of the process used to plot the A.P.s, set the scale to 56� (34�
> above the center parallel) and read straight down from the fix to where
> it strikes the diagonal scale and that is the longitude. On the plotting
> sheet do the same and place one leg of the dividers at that intersection
> and measure the distance from that intersection to the center of the
> plotting sheet  on the vertical scale, again dividing by 4. See figure 26.
>
> In addition to the plotting disk we just used, the Polhemus comes with 6
> other disks on which are drawn the graticle for 0�,  25�,  35�,  45�
> 55�,  and 65� latitudes for a Lambert projection at a scale of
> 1:5,000,000, a common scale used on the GNC series of aeronautical
> charts which allows you to use it at any latitude. (You use the 0� again
> for polar grid navigation.) Since the graticle is marked with latitude
> and longitude you just plot the A.P. on the graticle and read out the
> longitude also on the graticle, see figure 27 through 29. Figure 28
> shows the disk for 65� by itself and figure 29 shows it mounted on the
> Polhemus base.
>
> The Polhemus was used by the Air Force but the Navy also used similar
> devices such as the Mk5 and Mk6 plotting boards which are used in a
> similar fashion although they do not have the computer functions on the
> other side to do the in flight celnav calculation Figure 30 is a picture
> of a Mk6A plotting board. The Polhemus is 8 and a half inches in
> diameter while the plotting board is 12 inches across and is much
> heavier since it incorporates a storage compartment inside.
>
> Gary J. LaPook wrote:
> >Gary LaPook writes:
>
> >The Polhemus computer provides a convenient way  to plot celnav fixes
> >and this posting will show how you use it for this purpose. The other
> >side of the computer is used for in flight celnav and I will leave a
> >discussion of that use for later.
>
> >The first step in plotting a celnav fix is plotting the assumed
> >positions for each body and I will use the data from the "3-Star
> >Fix-'Canned Survival  Problem'" thread for this example.
>
> >Figure 1 shows the standard way of making a plotting sheet. A line is
> >drawn from the center at the same angle above above the horizontal that
> >is the same as the latitude of the center of the plotting sheet, in this
> >case, 34 degrees. The dividers are set to the difference in longitude
> >from the center meridian (in this case 119� 15') to the longitude of the
> >A.P. The first A.P. plotted is for Vega which is 119� 06.9' which is
> >7.9' east of the center meridian so the dividers are set to represent
> >7.9 as measured on the center meridian scale which I have multiplied
> >four times to make the scale of the plotting sheet larger so the
> >dividers were set to 31.6 and placed along the diagonal line. From this
> >point you go straight down and place the mark for the A.P. (an inverted
> >"V") on the central parallel of latitude.
>
> > Figure 2 shows the other two A.P.s plotted as well.
>
> >Figure 3 shows the base of the Polhemus computer which a vertical grid
> >marked in units, an unmarked horizontal grid and a surrounding azimuth
> >scale. ( On my computer I have added two scales near the center of the
> >grid for calculating the "motions" for in flight use and these scales
> >should be disregarded for this discussion..)
>
> >Figure 4 shows the transparent plotting surface that is mounted on the
> >central pivot of the base which has three vertical and three horizontal
> >lines lines forming a square and spaced to occupy 15 units on the
> >vertical scale on the base unit. (The plotting surface also has scales
> >marked along the lines but we will not make use of these tic marks.)
>
> >Figure 5 shows the plotting disk mounted on the base with the true index
> >set at 56� which lines up the numbered central line on the base 34�
> >above the horizontal and this causes the computer to be set in the
> >equivalent manner as the plotting sheet in figure 1. We use a similar
> >procedure and go straight down from 7.9 on the scale and place the Vega
> >A. P. on the horizontal line.
>
> >Figure 6 show the the other A.P.s plotted with the A.P. for Spica
> >plotted up from 7.9 since the A.P. is 119� 22.9 which is 7.9 west of the
> >center meridian; and Pollux plotted up from 24.1 representing 119� 39.1'.
>
> >Figures 7 through 12 show the plotting of the Spica line on the plotting
> >sheet using an aircraft plotter and the '"flip-flop" method. Figure 7
> >shows the plotter's edge passing through the Spica A.P. and set to the
> >azimuth of 170.5�, the azimuth of Spica.
>
> >Figure 8 shows the dividers set to a scaled intercept of 12.9 NM and set
> >along the straight edge with one leg on the A.P.. Holding the dividers
> >in place the the plotter is slid up so that the 270� mark on the plotter
> >scale is against the other leg of the dividers which is shown in figure 9.
>
> >Now carefully holding that leg and the plotter in place you move the leg
> >that had been at the A.P. so that is is on the reference line on the
> >other side of the azimuth scale on the plotter so that now the dividers
> >is at right angles to its previous position as shown in figure 10.
>
> >Carefully holding the dividers in place you slide the plotter out and
> >reposition it with the straight edge against the two divider legs so now
> >the straight edge is in position to draw the Spica LOP as shown in
> >figure 11 and 12.
>
> >Figure 13 shows the complete fix after carrying out the same steps for
> >the other bodies.
>
> >We will now go through the same process on the Polhemus computer. Figure
> >14 shows the true index set to 58� which is the azimuth of Vega. Figure
> >15 shows the A.P. for Vega which is at 5.5 on the base grid. Since the
> >Vega intercept is .5 away we move away from 58� half of a NM and trace
> >the LOP on top of the "5" grid line as shown in figures 15 and 16.
>
> >Figure 17 shows the true index set to 170.5� which is the azimuth of
> >Spica. We then count down (away) 12.9 NM from the Spica A.P. (which is
> >the "V" located on the "1" grid line, actually the "10" line which we
> >are scaling as "1") and trace the Spica LOP on top of the "14" line as
> >shown in figure 18. Figure 19 shows the Vega and Spica LOPs with the
> >plotting disk set to show north as up.
> >Figure 20 shows using the same procedure being used to plot the Pollux
> >line with an intercept of 13.6 away from an azimuth of 290�.
>
> >Figure 21 shows the completed fix with the plotting disk set to north up.
>
> >After carefully plotting these two examples I decided to go for "time."
> >I started over again with a fresh plotting sheet and an erased Polhemus
> >plotting disk. It took 2 minutes and 10 seconds to plot the three A.P.s
> >on the plotting sheet; an additional 1 minute 25 seconds to plot the
> >Vega LOP; an additional 1 minute 30 seconds to to plot the spica LOP; 58
> >seconds more to plot the Pollux LOP and finally another 40 seconds to
> >derive the fix for a total time of 6 minutes and 45 seconds. The fix is
> >34� 13'N, 119� 16.5' W. This is shown in figures 22 and 23.
>
> >I then did the same exercise on the Polhemus computer. It took 22
> >seconds to plot the three A.P.s; 40 seconds to plot the first LOP; 28
> >seconds for the second LOP; 18 seconds for the third LOP; then 41
> >seconds to derive the fix for a total of just 2 minutes and 29 seconds
> >which is 4 minutes and 16 seconds faster than using the traditional
> >plotting sheet. The fix is 34� 12.5'N, 119� 16' W a half  mile south and
> >a half mile east of the fix as plotted on the traditional plotting
> >sheet. This is shown in figures 24 and 25.
>
> >gl
>
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> ...
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>
>  26.jpg
>
>  27.jpg
>
>  28.jpg
>
>  29.jpg
>
>  30.jpg
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