Bubble sextant for Celestial Navigation in FlightGear
-----------------------------------------------------

Copyright (C) 2007 - 2013  Anders Gidenstam

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Yes, the stars on the night sky in FlightGear can be used for navigation.
This short text gives a hint to how it is done. The references explain it
much better, though, so have a look there both for introduction and more
advanced topics. 


Part 1 - Taking sextant sights in FlightGear
--------------------------------------------

What is needed?
- An aircraft with the RAF mk9 bubble sextant instrument.
  Short_Empire is one such aircraft.

- Using the sextant one needs to measure and record the celestial altitude
  (~angle from the horizon) for some (at least 2) stars and the precise time
  of observation. 

How?

- For the Short_Empire: Go to the copilot view.

- Pick up the sextant by clicking on either of its handles.

- For the Short_Empire: move (using w,a,d,s) to the observation hatch
                        and open it by clicking on it.

- Enter the sextant view by clicking on the sextant eye piece.

- Obtain a bubble by turning the bubble knob on the left side of
  the instrument. It is possible to reach the bubble knob while in
  sextant view.

- Aim the center of the view on the star. Shift + move mouse in pointer mode
  enables high precision view rotation.

- Level the instrument. This is done by changing the altitude setting,
  first in 10 degree steps by the coarse motion knob on the right side
  of the instrument (reachable also while in sextant view) and then
  with the slow motion knob (or shift+ctrl+move mouse up/down).
  Tilt the instrument (shift+ctrl+move mouse right/left) as needed to
  center the bubble.

- Time + celestial altitude constitutes a sighting. Make sure that
  the instrument is level and aimed at the star when you note the time
  and altitude.


But where are those bl**dy stars?

- I use the star charts around page 256 in "The American Practical Navigator",
  http://www.nga.mil/MSISiteContent/StaticFiles/NAV_PUBS/APN/pub9.zip
  to find some. For me the ones in Ursa Major was easiest to find (though
  one should use a wider spread).
 
  Suggestion (northern hemisphere):
  * Dubhe  (Right upper corner star of the Ursa Major "wagon".)
  * Alkaid (Third and last (leftmost) star in the Ursa Major "wagon" handle.)
  * Arcturus (Brightest star found to the "left" of Alkaid just below the
              extended line between Alkaid and the star immediately to the
              right in the Ursa Major "wagon" handle.
              See star chart in [Bowditch] page 256.)
              

Part 2 - Computing a position
-----------------------------

The Intercept method

- Assume a position. It should be close to ones actual position - usually one
  has a dead reckoning position to start from. (The assumed position need
  not be equal to the DR one - one can choose a more convenient one e.g.
  when using tables.) 

- Compute the expected locations of the observed stars for
  that position.
  Use a program/website, like http://www.tecepe.com.br/cgi-win/cgiasvis.exe ,
  or a Nautical Almenac + tables.
- Compare the observed star locations with the computed ones.
  This gives a position line for each star. Plot the position
  lines to (hopefully) get a fix. 
  Further info and plotting sheets: http://www.efalk.org/Navigation/ 



Appendix - Installing the tools: 3d bubble sextant model for FlightGear
-----------------------------------------------------------------------

  - The bubble sextant 3d model RAF_Mk9_bubble_sextant.xml need to be
    loaded from the aircraft 3d model file and placed at the center of
    the view it is intended to be used from. For example for a view located
    2.5 meter behind and 2.0 meter above the aircraft origin:
    (Note that any offsets of the main 3d model needs to be accounted for
    in translating the view position to a 3d model position and that the
    axes order differs.)

 <!-- Star observer's instruments -->
 <model>
  <name>Bubble_sextant</name>
  <path>Aircraft/Instruments-3d/RAF_Mk9_bubble_sextant/RAF_Mk9_bubble_sextant.xml</path>
  <offsets>
   <x-m> 2.50 </x-m>
   <y-m> 0.50 </y-m>
   <z-m> 2.00 </z-m>
  </offsets>
 </model>

  - The Nasal module needs to be loaded from the main aircraft -set file.
    The name of the view in which the sextant will be used and 
    the stowed (= inactive) location of the instrument needs to be set.
    The stored location is specified in a frame axis parallel with the
    aircraft frame but centered at the location given for the sextant
    3d model in the 3d model file.
    For example:
 <nasal>
  <RAFmk9sextant>
   <file>Aircraft/Instruments-3d/RAF_Mk9_bubble_sextant/RAF_Mk9_bubble_sextant.nas</file>
   <script>
    var VIEW_NAME = "Star Observation View";
    var STOWED = { position    : {x:-0.3, y: 0.0, z: -0.2},
                   orientation : {heading : 320.0,
                                  pitch   : -45.0,
                                  roll    : 0.0}
                 };
   </script>
  </RAFmk9sextant>
 </nasal>

  - To allow fine control of the sextant using the mouse add the following
    XML snippet to the aircraft's -set file.
    It binds moves movements in mode 0/pointer mode to the sextant.
    alt + move mouse         - precision movement of view direction.
    alt + shift + move mouse - up/down changes the altitude setting of the
                               sextant.
                             - left/right tilts the sextant to align with
                               the horizon.

 <input>
  <!-- Install the sextant controls in mouse mode 0. -->
  <mice n="0">
   <mouse n="0">
    <mode n="0">
     <x-axis-shift>
      <binding>
       <command>nasal</command>
       <script>
         RAFmk9sextant.mouseXmove();
       </script>
      </binding>
     </x-axis-shift>
     <x-axis-ctrl-shift>
       <binding>
        <command>nasal</command>
        <script>
         RAFmk9sextant.mouseXtilt();
       </script>
      </binding>
     </x-axis-ctrl-shift>
     <y-axis-shift>
      <binding>
       <command>nasal</command>
       <script>
         RAFmk9sextant.mouseYmove();
       </script>
      </binding>
     </y-axis-shift>
     <y-axis-ctrl-shift>
      <binding>
       <command>nasal</command>
       <script>
         RAFmk9sextant.mouseYaltitude();
       </script>
      </binding>
     </y-axis-ctrl-shift>
    </mode>
   </mouse>
  </mice>
 </input>


  To correctly measure the altitude of a heavenly body the instrument
  must be level, i.e. the bubble must be centered.
  The instrument is configured for an aim-then-level method, i.e.
  one first center the view on the desired object then change the altitude
  and tilt settings until the instrument is level. (I.e. changing altitude
  behaves as if the user changes the setting and rotates the instrument so
  that it remains aimed in the same direction).

  NOTE: Tilting of the instrument might still be somewhat buggy.



References
----------

http://www.celestialnavigation.net/
  Good place to start.

http://www.efalk.org/Navigation/
  Intercept method tutorial and downloadable plot sheets. 

http://www.ludd.luth.se/users/kavli/peck_celestial/space.html
  Longer tutorial and description. Focuses on the use of tables for sight
  reductions.

http://www.tecepe.com.br/cgi-win/cgiasvis.exe
  Computes star locations. Use instead of tables.

[Bowditch]
  "The American Practical Navigator",
  http://www.nga.mil/MSISiteContent/StaticFiles/NAV_PUBS/APN/pub9.zip
  For those that really really want to learn it all :)

RAF Mk9 bubble sextant manuals and more information
  http://www.users.bigpond.com/bgrobler/sextant/RAF_Mk9_manual.pdf
  http://www.users.bigpond.com/bgrobler/sextant/sextant.html
  http://www.physics.uq.edu.au/physics_museum/bblsex.html