210 lines
7.6 KiB
Text
210 lines
7.6 KiB
Text
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Bubble sextant for Celestial Navigation in FlightGear
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-----------------------------------------------------
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Copyright 2007 - 2010 Anders Gidenstam
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* These programs are free software; you can redistribute them and/or modify
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* them under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
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Yes, the stars on the night sky in FlightGear can be used for navigation.
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This short text gives a hint to how it is done. The references explain it
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much better, though, so have a look there both for introduction and more
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advanced topics.
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Part 0 - Installing the tools: 3d bubble sextant model for FlightGear
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---------------------------------------------------------------------
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- The bubble sextant 3d model RAF_Mk9_bubble_sextant.xml need to be
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loaded from the aircraft 3d model file and placed at the center of
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the view it is intended to be used from. For example for a view located
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2.5 meter behind and 2.0 meter above the aircraft origin:
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(Note that any offsets of the main 3d model needs to be accounted for
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in translating the view position to a 3d model position and that the
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axes order differs.)
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<!-- Star observer's instruments -->
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<model>
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<name>Bubble_sextant</name>
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<path>Aircraft/Short_Empire/Instruments/BubbleSextant/RAF_Mk9_bubble_sextant.xml</path>
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<offsets>
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<x-m> 2.50 </x-m>
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<y-m> 0.50 </y-m>
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<z-m> 2.00 </z-m>
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</offsets>
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</model>
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- The Nasal module needs to be loaded from the main aircraft -set file.
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The name of the view in which the sextant will be used and
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the stowed (= inactive) location of the instrument needs to be set.
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The stored location is specified in a frame axis parallel with the
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aircraft frame but centered at the location given for the sextant
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3d model in the 3d model file.
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For example:
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<nasal>
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<RAFmk9sextant>
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<file>Aircraft/Short_Empire/Instruments/BubbleSextant/RAF_Mk9_bubble_sextant.nas</file>
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<script>
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var VIEW_NAME = "Star Observation View";
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var STOWED = { position : {x:-0.3, y: 0.0, z: -0.2},
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orientation : {heading : 320.0,
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pitch : -45.0,
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roll : 0.0}
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};
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</script>
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</RAFmk9sextant>
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</nasal>
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- To allow fine control of the sextant using the mouse add the following
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XML snippet to the aircraft's -set file.
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It binds moves movements in mode 0/pointer mode to the sextant.
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alt + move mouse - precision movement of view direction.
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alt + shift + move mouse - up/down changes the altitude setting of the
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sextant.
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- left/right tilts the sextant to align with
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the horizon.
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<input>
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<!-- Install the sextant controls in mouse mode 0. -->
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<mice n="0">
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<mouse n="0">
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<mode n="0">
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<x-axis>
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<binding>
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<command>nasal</command>
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<script>
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RAFmk9sextant.mouseX();
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</script>
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</binding>
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</x-axis>
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<y-axis>
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<binding>
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<command>nasal</command>
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<script>
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RAFmk9sextant.mouseY();
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</script>
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</binding>
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</y-axis>
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</mode>
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</mouse>
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</mice>
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</input>
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To correctly measure the altitude of a heavenly body the instrument
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must be level, i.e. the bubble must be centered.
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The instrument is configured for an aim-then-level method, i.e.
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one first center the view on the desired object then change the altitude
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and tilt settings until the instrument is level. (I.e. changing altitude
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behaves as if the user changes the setting and rotates the instrument so
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that it remains aimed in the same direction).
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NOTE: Tilting of the instrument might still be somewhat buggy.
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Part 1 - Taking sextant sights in FlightGear
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-------------------------------------
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What is needed?
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- The celestial altitude (~angle from the horizon) for some (at least 2)
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stars and the precise time of observation.
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How?
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- Pick up the sextant by clicking on either of its handles.
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- Enter the sextant view by clicking on the sextant eye piece.
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- Obtain a bubble by turning the bubble knob on the left side of
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the instrument. It is possible to reach the bubble knob while in
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sextant view.
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- Aim the center of the view on the star. Alt + move mouse in pointer mode
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enables high precision view rotation.
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- Level the instrument. This is done by changing the altitude setting,
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first in 10 degree steps by the coarse motion knob on the right side
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of the instrument (reachable also while in sextant view) and then
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with the slow motion knob (or alt+shift+move mouse up/down).
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Tilt the instrument (alt+shift+move mouse right/left) as needed to
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center the bubble.
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- Time + celestial altitude constitutes a sighting. Make sure that
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the instrument is level and aimed at the star when you note the time
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and altitude.
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But where are those bl**dy stars?
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- I use the star charts around page 256 in "The American Practical Navigator",
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http://www.nga.mil/MSISiteContent/StaticFiles/NAV_PUBS/APN/pub9.zip
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to find some. For me the ones in Ursa Major was easiest to find (though
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one should use a wider spread).
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Suggestion (northern hemisphere):
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* Dubhe (Right upper corner star of the Ursa Major "wagon".)
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* Alkaid (Third and last (leftmost) star in the Ursa Major "wagon" handle.)
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* Arcturus (Brightest star found to the "left" of Alkaid just below the
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extended line between Alkaid and the star immediately to the
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right in the Ursa Major "wagon" handle.
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See star chart in [Bowditch] page 256.)
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Part 2 - Computing a position
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-----------------------------
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The Intercept method
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- Assume a position. It should be close to ones actual position - usually one
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has a dead reckoning position to start from. (The assumed position need
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not be equal to the DR one - one can choose a more convenient one e.g.
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when using tables.)
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- Compute the expected locations of the observed stars for
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that position.
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Use a program/website, like http://www.tecepe.com.br/cgi-win/cgiasvis.exe ,
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or a Nautical Almenac + tables.
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- Compare the observed star locations with the computed ones.
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This gives a position line for each star. Plot the position
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lines to (hopefully) get a fix.
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Further info and plotting sheets: http://www.efalk.org/Navigation/
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References
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----------
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http://www.celestialnavigation.net/
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Good place to start.
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http://www.efalk.org/Navigation/
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Intercept method tutorial and downloadable plot sheets.
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http://www.ludd.luth.se/users/kavli/peck_celestial/space.html
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Longer tutorial and description. Focuses on the use of tables for sight
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reductions.
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http://www.tecepe.com.br/cgi-win/cgiasvis.exe
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Computes star locations. Use instead of tables.
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[Bowditch]
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"The American Practical Navigator",
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http://www.nga.mil/MSISiteContent/StaticFiles/NAV_PUBS/APN/pub9.zip
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For those that really really want to learn it all :)
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RAF Mk9 bubble sextant manuals and more information
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http://www.users.bigpond.com/bgrobler/sextant/RAF_Mk9_manual.pdf
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http://www.users.bigpond.com/bgrobler/sextant/sextant.html
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http://www.physics.uq.edu.au/physics_museum/bblsex.html
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