Welcome to the NavList Message Boards.


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

Compose Your Message

Add Images & Files
    Re: A-10 Sextant Manual
    From: Gary LaPook
    Date: 2009 Jun 11, 09:12 -0700

     From looking at his chart and the Nautical Almanac it is clear that the
    highest altitude he measured on sun was about 51 degrees which would
    produce a three-tenths of a mile offset at 45 miles (Table 19) and maybe
    one NM two degrees along the LOP, nothing to worry about.
    For those curious about computing winds based on drift sights I have
    attached an excerpt from Air Force Manual 51-40.
    Gary LaPook wrote:
    > Chichester did use  Mercator charts, actually plotting sheets, for
    > this flight. I have attached his charts. The first two are
    > illustrations from his books. The third is figure 41 from Chichester's
    > own explanation of the navigation of this flight published in the
    > Observer's Book, which I have also attached. This chart is a
    > simplified copy of the following charts, the actual chart that he used
    > on his flight from Norfolk to Lord Howe. The last chart is his actual
    > chart used for flying to Norfolk from New Zealand. I have also
    > attached Hughes' explanation of the navigation.
    > The actual charts may look confusing because of the arcs and the
    > radial lines which was Chichester's method of determining the wind
    > encountered in flight based on his observations of his angle of drift
    > on two different headings. More modern practice is to plot the drift
    > lines on an E-6B rather than on the chart itself to determine the
    > winds. If you are just interested in the celnav aspects, you can
    > ignore all of these lines as Chichester did in his simplified figure
    > 41 in the Observer's Book.
    > Your concern over plotting a curved LOP as a straight line on a
    > Mercator chart is unwarranted. I have attached Table 19 from Bowditch
    > which shows how much the curved LOP departs from a straight line. You
    > can see that, except for very high altitudes, this factor can be
    > ignored especially given the lower level of precision available with
    > celnav in flight.
    > You can download the complete American Practical Navigator for free
    > from this website, choose in the "select" box.
    > Scheele wrote:
    > gl
    >> Hello Gary,
    >> I hear you and I largely agree with you but please bear with me for a
    >> moment while I try to work something out. If you're assuming that a
    >> navigator is plotting on a Mercator chart ( I think Chichester was),
    >> I wouldn't say that deliberate offset navigation is a modern
    >> implementation of latitude sailing. Latitude sailing, using the noon
    >> method, gives you a latitude line and this means a position line that
    >> can be plotted on a Mercator chart as a straight line without the
    >> need for any corrections. When you find a position line on the basis
    >> of a sun shot at any other time other than noon, your plotted
    >> position line, because it is plotted as an uncorrected straight line
    >> on a Mercator chart, will always be "a bit" assumed unless..... you
    >> are actually plotting a real great circle arc, i.e. a position line
    >> corrected for the map projection. Could the astrograph do this? I
    >> assume not, because the curves on its foils are rigid, unless it
    >> actually came with maps to fit the curves, rather than curves to fit
    >> the maps, but that would be impractical way to do something I would
    >> imagine the astrograph was supposed to make easy in the first place.
    >> I am assuming, therefore, that the astrograph was, from a precision
    >> point of view, not a "perfect" plotter. I know that in world war 2
    >> some navigators had an astrograph over their tables, but how many I
    >> don't know, I don't know much about the military stuff. In any event,
    >> Chichester didn't use an astrograph. I'm still assuming that
    >> Chichester used a Mercator projection. Chichester was using sun, not
    >> star shots. When using the stars, if you have a reasonably good idea
    >> of where you are, say you are sure that you must be within about 2
    >> degrees, depending on which region you are in, of a point on the the
    >> position line which, again, is a small arc of a great circle track,
    >> and assuming that the global position of the sun is far away enough,
    >> then the aforementioned problem should not exist as you can then plot
    >> your position more or less as a straight line - the great circle
    >> radius is so big - just like Weems did in his books plotting star
    >> altitude curves. I say 2 degrees because I am taking an extract of
    >> Weems pages showing a latitude and great circle radius for which this
    >> arc size to gradient of position line relationship holds. You can't
    >> plot the "sun curves" in the same way as you would star curves,
    >> because of the rapid change of declination. When I said Chichester
    >> had an assumed  position line rather than a position line, I was
    >> assuming that such a relationship which allows one to plot a great
    >> circle arc as a straight line may not have been available to
    >> Chichester, had his D.R. navigation not been so good, all the while
    >> still assuming Chichester was plotting position lines as
    >> "uncorrected" straight lines onto a Mercator map.... But I am not
    >> entirely sure, maybe I should have phrased it as a question...here's
    >> what I was thinking.......Let's say his D.R. had not been so good and
    >> he had been on the unfavourable side of the position line, that is
    >> away from the island, further than he had planned to be at the
    >> precomputed time. Let us assume that this error had brought him
    >> beyond 2 degrees (120 miles) or so of the predetermined point on the
    >> position line.   Assuming this scenario, how serious would it have
    >> been had he plotted his position line as a straight line on Mercator
    >> chart? Imagine: The straight now really becomes a curve if, only a
    >> very slight one. Could this produce a substantial error? Norfolk
    >> island is 35sqkm. Lord Howe is 56sqkm. For this last stage of the
    >> flight, D.R. is of course used again. Did he have the drift, wind
    >> speed indicators that should have been around by the time of world
    >> war 2? What about his compass? Did he have more than one? An
    >> induction compass like Lindbergh - who did not use astro on his epic
    >> flight - to check the magnetic one? Let's say some of these issues
    >> come in and another error, albeit a much smaller one, is added on to
    >> the existing one. Could a pilot miss the island due to the combined
    >> sum of such errors? Very important in this regard is: How high was
    >> Chichester flying, i.e. what what was the range of his visible
    >> horizon? Perhaps I'm wrong and it wasn't that serious and any error
    >> would have been "innocuous" due to the far visible horizon. In
    >> referring to Chichester's navigational method, I was implicitly
    >> including this speculation....
    >> Again, I largely agree with you  ....but there's just that shred of
    >> doubt.
    >> I'll take a thorough look at your website as soon as possible.
    >> Thanks, I appreciate the tip.
    >> Christian Scheele
    >> ----- Original Message ----- From: "glapook@pacbell.net"
    >> To: "NavList" 
    >> Sent: Thursday, June 11, 2009 4:01 AM
    >> Subject: [NavList 8612] Re: A-10 Sextant Manual
    >> Well, I don't know that you can call it "reckless" since Chichester's
    >> deliberate offset navigation procedure became the standard navigation
    >> method for finding islands and was taught to thousands of navy and air
    >> corps navigators during the second world war. It was actually a just a
    >> modern implementation of the centuries old method of latitude sailing,
    >> approaching to one side of a destination, east or west, and then
    >> following the latitude LOP to the destination.
    >> See the texts I have posted on my website at :
    >> http://www.geocities.com/fredienoonan/
    >> Go to "List of topics" then to "Single LOP landfall procedure."
    >> gl
    >> >>
    Navigation List archive: www.fer3.com/arc
    To post, email NavList@fer3.com
    To unsubscribe, email NavList-unsubscribe@fer3.com


    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site