# NavList:

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

**Re: Celestial up in the air**

**From:**Gary LaPook

**Date:**2008 Jul 28, 05:30 -0700

For example, using the page from the Air Almanac found on page 206, a day when the H.P is 60', and an altitude of 36� we find the parallax in altitude correction to be 48' and this would be the correction to use with a bubble sextant. (Page 206 of AFPAM 11-216.) Additionally, formulas for these correction are found on pages 393 and 394 of the same manual. gl On Jul 28, 5:24 am, glap...@pacbell.net wrote: > One more thing to discuss before giving an example of in flight celnav > is corrections to sights taken in flight. We discussed this back on > December 14, 2007 in the thread "additional corrections... (just > search "additional corrections") which include an excerpt from AFPAM > 11-216. You should download the entire manual here:http://www.e-publishing.af.mil/shared/media/epubs/AFPAM11-216.pdf > > Review chapters 10 through 13. > > I want to add to the manual on this. > > Coriolis can be handled in a number of ways. You can move the A.P. to > the right (northern hemisphere) 90� to the course (track) prior to > plotting the LOPs by the amount of coriolis correction shown in the > table in the Air Almanac and in H.O. 249 (previously posted). 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, the Polhemus makes this relatively > painless. > > Rhumb line correction is avoided by steering by directional gyro > during the two minute shooting period and this is what is normally > done anyway. > > Wander correction is small at low airspeeds and it can be avoided by > making sure the heading is the same at the end of the shot as it was > at the beginning of the shot. It doesn't matter how the heading > changes during the shot (within reason) as the errors will average > out. > > Ground speed correction can also be avoided by making sure the > airspeed is the same at the end as at the beginning, any changes in > between will also average out. > > Auto pilots do a good job of maintaining airspeed and heading for the > two minute shooting period so eliminating the need for the above > corrections. > > The AFPAM states you must figure the refraction correction based on > the actual Hs as opposed to using the refraction correction based upon > the Hc but this is a needless refinement and keeps you from completing > the pre computation prior to the shot. Look at the refraction table in > H.O. 249 (previously posted) and you will see for altitudes exceeding > 10� that the brackets are at least two degrees wide. So only in the > rare cases where the altitude is almost exactly at the break point > could you come up with a different refraction correction using Hc > rather than Hs and even then it could only be a difference of one > minute of altitude. For example the break point between a 5' > correction and a 4' refraction correction is 12� so if Hs were 11� 50' > and Hc were 12� 15' then using Hc would get you a 4' correction and > using Hs would get you a 5' correction. This is actually only 1/2 of a > minute error because the corrections are rounded to the nearest full > minute. > > The parallax in altitude correction for the moon is printed on each > page of the Air Almanac based upon the horizontal parallax (H.P.) for > the moon on that particular day. This parallax varies with the > distance to the moon and moves in lock step with the S.D. since they > are both related to the distance to the moon. The H.P varies from 54' > to 61' during the year. For example, using the page from the Air > Almanac found on page 206, a day when the H.P is 60', and an altitude > of 36� we find the parallax in altitude correction to be 48' and this > would be the correction to use with a bubble sextant. If using a > marine sextant and shooting the lower limb we would add the S.D. of > 16' to produce a total correction (but not including refraction yet) > of 64'. Subtract the refraction correction of 1' gives the total > correction of 63'. Using the correction table in the Nautical almanac > for the identical parameters you get 63.5'. The Nautical Almanac moon > correction table includes a procedure for using it with a bubble > sextant and what this does is just backs out the S.D. correction which > is included in the correction table and not needed for a bubble > observation. Using this procedure produces a correction for a bubble > observation of 47.2' which compares with the 48' from the Air Almanac. > > Remember to reverse the signs of these corrections and apply them to > Hc to produce Hp (pre computed altitude) which you then compare > directly with Hs to compute intercept. > > gl > > On Jul 25, 7:48 pm, Gary LaPookwrote: > > > We can also use the Polhemus computer to calculate the MOO adjustment. > > We do this by setting the ground speed in the setting window and read > > out the MOO in the "ZN-TR" window adjacent to the relative Zn. (See > > Pol1.jpg) (Zn-TR is another way of saying "relative Zn" since you > > calculate relative Zn by subtracting Track from Zn.) Looking at the top > > of the TR-ZN window where the relative Zn of 000� is adjacent to "5" in > > the MOO window showing that the aircraft moves 5NM per minute which > > causes the altitude to also change 5' every minute when the body is > > directly ahead of or directly behind the aircraft. This MOO is > > equivalent to the MOO table at page 6 of the original PDF which > > tabulates the MOO adjustment per minute. Multiplying this 5' times the > > same eight minute period gives the same 40' adjustment we got from the > > MOO table on page 4 of the PDF. You will also find that the adjustment > > is 2.5' adjacent to the relative Zn of 60� which multiplied by eight > > minutes gives the 20' adjustment we found in the table on page 4. > > > The Polhemus makes it easy to figure the relative Zn. You place the "SET > > TRACK" pointer on the track of the aircraft ,130� as shown in the > > attached image. (see Pol2.jpg) Look at the next image (Pol3.jpg) for the > > second case, a track of 70� and you find the relative Zn, 60� on the > > inner scale. > > > The Polhemus also makes it easy to figure the sign to use for the > > adjustment, if the relative Zn is on the white scale, meaning the body > > is ahead, then the sign is minus and if found on the black scale (the > > body is behind) then the sign is plus when these adjustments are made to > > Hc, the normal method. This same pattern is revealed in the two MOO > > tables, the top of the tables show the body ahead and the bottom has the > > body behind. > > > gl > > > glap...@pacbell.net wrote: > > > Now let's talk about the "motion of the observer" (MOO) adjustment. > > > Every fix in the air is a running fix because the aircraft moves a > > > considerable distance between the first and last sight. Assuming the > > > normal eight minute spacing between the first and last shot, a slow > > > airplane, say 100 knots, will have traveled 14 NM while a 450 knot > > > plane will have traveled 60 NM. In marine practice the navigator will > > > advance the earlier LOPs to cross them with the last shot. The MOO > > > adjustment accomplishes the same thing. > > > > As an example of how this works consider a sun shot taken at 1000Z > > > resulting in an observed altitude, Ho, of 35� 55'. After doing the > > > normal sight reduction you end up with an Hc of 35� 45' at the chosen > > > A.P and a Zn of 130�. This results in an intercept of 10 NM toward the > > > body, 130�. To plot this LOP you draw the azimuth line from the A.P > > > and measure off the 10 NM intercept toward the sun and plot the LOP > > > perpendicular to the Zn. > > > > Then, two hours later at 1200Z you take another altitude of the sun > > > and to obtain a 1200Z running fix you must advance the 1000Z sun line > > > to cross the 1200Z line. There are three ways to advance the LOP. > > > First, you can pick any spot on the LOP and lay off a line in the > > > direction of travel of the vessel, measure off the distance traveled > > > along that line, make a mark there and then draw a line through that > > > mark that is parallel to the existing LOP and label the advanced LOP > > > "1000-1200Z SUN." A second way is to advance each end of the LOP and > > > then just draw a line through these two points, this avoids having to > > > measure the azimuth when laying down the advanced line. The third way > > > is to advance the original A.P and then from the ADVANCED A.P. plot > > > the LOP using the ORIGINAL intercept and Zn. Any of these methods will > > > produce the same advanced LOP. > > > > Now let's consider a simple case. Suppose the vessel's course is the > > > same as the Zn, in this case, 130� and the vessel's speed is 20 knots > > > meaning it has traveled 40 NM in the two hour period. In this simple > > > case we can just extend the Zn line an additional 40 NM and then plot > > > the advanced LOP at that point. So, the LOP is now 50 NM from the > > > original A.P., the original 10 NM intercept plus the additional 40 NM > > > that the vessel has traveled on the same course as the azimuth. Since > > > we have no interest in actually plotting the 1000Z LOP, as we are just > > > planning on having the 1200Z running fix, we can skip drawing the > > > earlier LOP and just plot the advanced LOP by adding the distance > > > traveled to the original intercept to get a total intercept now of 50 > > > NM and using that adjusted intercept to plot the advanced LOP using > > > the ORIGINAL A.P. This method also creates the exact same advanced LOP > > > as the other three methods. This last described procedure is how the > > > MOO table is used. > > > > Look now at the MOO table, page 4 of the PDF in my original post. > > > Assume now we are in 300 knot airplane and the first sight is taken at > > > 1152Z, eight minutes prior to the planned fix time. At the top of the > > > column marked "300" knots ground speed you find the number "20" > > > showing that the plane will travel 20 NM (and so the altitude of the > > > body should change by 20 minutes of arc) in a 4 minute period. Also > > > notice that the top row of values are marked for a relative Zn of 000� > > > meaning the body is directly ahead, as in our example. The plane will > > > obviously travel 40M in the normal 8 minute period from the first to > > > the last shot of a three star fix. The sign convention is the same as > > > that for the MOB table so simply draw a horizontal line > > ... > > read more � --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To unsubscribe, email NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---