# NavList:

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

**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 > > > > ------------------------------------------------------------------------ > > > > ------------------------------------------------------------------------ > > > > ------------------------------------------------------------------------ > > > > ------------------------------------------------------------------------ > > > > ------------------------------------------------------------------------ > > > > ------------------------------------------------------------------------ > > > > ------------------------------------------------------------------------ > > > > ------------------------------------------------------------------------ > > > > ------------------------------------------------------------------------ > > > > ------------------------------------------------------------------------ > > > > ------------------------------------------------------------------------ > > > > ------------------------------------------------------------------------ > > > > ------------------------------------------------------------------------ > > > > ------------------------------------------------------------------------ > > > > ------------------------------------------------------------------------ > > > > ------------------------------------------------------------------------ > > > > ------------------------------------------------------------------------ > > ... > > read more � --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To unsubscribe, email NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---