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    Re: Polhemus computer
    From: Gary LaPook
    Date: 2008 Aug 6, 02:19 -0700

    August 6, 2008.
    Gary LaPook writes:
    
    Look at the new posting on the "Celestial up in the air" thread for an
    example of how the Polhemus is used.
    
    gl
    
    On Jul 24, 2:39�am, glap...---.net wrote:
    > 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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    > read more �
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