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    Re: Celestial up in the air
    From: Hewitt Schlereth
    Date: 2008 Jul 28, 10:53 -0400

    Hi Gary -
    
    I'd probably missed the answer to this question because I picked up
    the thread in the middle:  Is celestial part of your routine duty of
    navigating an airliner or is it something you do for its own sake?
    
    Thanx,  Hewitt
    
    PS  It's been an absorbing thread to follow.  Keep it coming. HewS
    
    On 7/28/08, glapook@pacbell.net  wrote:
    >
    >   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 LaPook  wrote:
    >  >
    >  > > 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 �
    >
    > >
    >
    
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