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    Re: Artificial Horizon
    From: M Spencer
    Date: 1997 Sep 21, 1:23 AM

    Bill and Diane,
    
    You asked about artificial horizons (see following) but you did not indicate 
    the reason for your interest.  I can assume two reasons off the top of my 
    head:  (1) you think this might allow you to take celestial sights at sea 
    when the actual horizon is obscured by fog or haze, or (2) you would like to 
    "practice" taking observations from a land-locked position where the actual 
    horizon may not be visible due to the surrounding mountains, hills or other 
    terrain.
    
    I have had some experience with celestial navigation using an artificial 
    horizon as a military aviation navigator in the mid-1960's.  The sextant that 
    we used had a bubble chamber showing a small air bubble that was superimposed 
    into the field of view of the telescope.  The telescope itself, about an inch 
    and half in diameter and twelve inches long, was inserted through an opening 
    in the roof of the aircraft so that we had access to the celestial canopy.  
    The field of view was perhaps 15 degrees of arc.  When the "bubble" was 
    centered in the field of view, the sextant was oriented perpendicular to the 
    surface of the world, and an accurate observation of the height of the 
    celestial body above the "horizon" could be made.  Because of the restricted 
    field of view, we would usually pre-compute the zenith and height of the 
    desired celestial body based on our dead-reckoning position, and pre-set the 
    sextant with these values.  Usually, the desired object would be the 
    brightest body in the field of view, although clouds or turbulence could make 
    star identification "iffy" every once in a while.  Whoever created our 
    air-navigation tables gave some thought to listing potential objects so that 
    we could actually identify them through this tiny pipe.
    
    The air-navigation sextant was (is?) actually very practical.  For one thing, 
    it permitted us to take observations at any time celestial bodies were 
    visible - no actual horizon was ever needed (and probably not visible from 
    the navigation station inside an aircraft at any rate).  In the time period 
    of my experience (1964-1970+)  electronic means of navigation were often 
    "iffy" at best, so that celestial work was critical to our arriving at the 
    wherever we were headed.  We used Loran A in those parts of the pacific that 
    were covered, for example, but this did not include the Pacific Ocean south 
    of the equator, nor the Arctic or Antarctic oceans.  We had something called 
    a "doppler" radar system that was supposed to give us a read-out of ground 
    speed and angle of drift, but I can only remember one flight when the thing 
    actually worked, so I never relied on it.
    
    As far as I know, the accuracy of the air-navigation sextant was as good as 
    any surface sextant, but the conditions of its use would introduce a number 
    of errors.  First, it's pretty rare to find perfectly placid air for an 
    airplane trip, and we would get bounced around quite a lot.  Probably half 
    the flights I made were "bumpy"  enough that I would have to hang on the 
    sextant for dear life just to keep from being thrown onto the cockpit floor.  
    To compensate for any violent motions and the normal vibration, we had a 
    timer associated with the sextant.   Under good conditions we would observe 
    the body for at least 2 minutes - keeping the celestial body in the middle of 
    the "bubble" by cranking the "height" knob up or down, and/or swinging the 
    sextant left or right.  The mechanism would automatically "average" the 
    height over the period of the observation.  If we wanted the sighting as of 
    2200, for example, we would start the observation at 2159 and end at 2201.  
    In worse conditions, we would observe the body for longer periods of time - 
    say four or six minutes.  Of course, bumps and clouds go together, so in 
    severe conditions we might not be able to see anything at all and would take 
    no observation.
    
    Another factor we learned about  was related to the motion of the plane 
    through the air.  This is not "straight as an arrow" as you might think, but 
    a sort of cork-screw motion.  This would be very noticeable to the navigator 
    in fine weather conditions because of the very regular "up and down" 
    correction that would be required to track the body.  Also, flying along at 
    300 kts or so, coriolis has its affect on the bubble.  I don't remember any 
    formal
    correction for this affect, however.  Considering all the various motions that 
    could affect the aircraft, a 2 minute observation period was generally 
    accepted as an absolute minimum to average them out.  Longer periods ran the 
    risk of losing sight of the body or otherwise screwing up the sight.
    
    Overall, we learned to become pretty handy with this device.  An experience 
    navigator, in good conditions could easily determine his position within 2 or 
    3NM's with a standard three lines-of-position fix.  That was rare, however.  
    For one thing, half our flight time was during daylight when we had only 
    sun-lines to work with, and at night, we would often could experience 
    turbulence  or obscuring clouds etc., and our celestial accuracy was 
    correspondingly compromised.
    
    Later, while trying to take a morning sun-line from the deck of a small boat 
    in the Atlantic Ocean, as I was alternately down in a 12 foot trough, or at 
    the top of a 12 foot swell, I had occasion to think about that boat's motion 
    as compared to an aircraft.  I decided that it would be entirely impractical 
    to compensate for the very severe accelerations to which the boat was 
    subjected on the water.  I was very happy indeed to be able to take an 
    "instantaneous" observation of the sun and to skip all the averaging.  Of 
    course, the other side of that coin is that I never was able to get a "star" 
    line of position on that trip, since the horizon was never visible when the 
    stars were.  Nevertheless, we got to Bermuda without incident.
    
    As to "practicing" with a bubble sextant, my experience was limited to my 
    training days.  The navigation school provided four-legged "tri-pods" 
    surmounted with a 12" square platform through which we could insert a 
    standard air-navigation bubble sextant.  I suppose this worked out OK as a 
    method of familiarizing ourselves with the equipment and procedures, but I 
    don't think any of us got particularly adept at celestial work (observations 
    could easily be miles off; our ability to recognize navigational stars though 
    a partial cloud cover wasn't too hot, etc.) until much later, after 
    considerable experience.  I think if I were inclined to teach some-one how to 
    use a celestial sextant now, I would simply head over to the sea-shore and 
    use a regular sextant on the sun, moon or whatever  was visible at the time.  
    Star identification can be done anywhere, and the exercises for calculating 
    or plotting can be done separately as well.
    
    Finally, I have sometimes wondered how surveyors manage to keep track of where 
    they are.  They undoubtedly use a number of techniques, but their standard 
    transit would make a mighty fine sextant for the land-locked to my way of 
    thinking.  The transits I am familiar with have leveling bubbles attached to 
    their base at right angles to each other to allow the surveyor to orient the 
    telescope perfectly parallel to the surface of the earth.  Once this is done, 
    it doesn't take much (filters mainly) to observe the height of the sun or 
    whatever happens to be handy, day or night, and calculate a line-of-position 
    with a high degree of accuracy.  Of course, the surveyor is NOT moving when 
    he does this!  It's the motion that kills the utility of the liquid based 
    artificial horizons more than anything else.  But if it's practice you're 
    after, you might try to borrow or rent one of these.
    
    Henning Umland mentions some devices available through the "CELESTAIRE, INC".  
      The catalog descriptions accompanying the artificial horizons reinforce 
    what I have been describing above.
    
    For the $49 item Celestaire says:
    
    "Today, artificial horizons are used by those wanting to use or practice 
    celestial navigation inland. There are two kinds of artificial horizons to be 
    had: a bubble attachment for the sextant, and the flat artificial horizon 
    shown on this page. Actually, some people have luck with a simple pie pan of 
    motor oil if conditions are not windy. There are two major drawbacks involved 
    with the flat artificial horizon: it is difficult to use with stars, and 
    actual altitudes are limited to about 60� (half the range of the sextant). 
    Therefore, every effort should be made to acquire a natural horizon rather 
    than resorting to a flat artificial one. The Practice Bubble Horizon shown on 
    the next page is most effective for practice because it allows the sextant to 
    be used in a normal manner (i.e. not pointed at the ground) The DIP SHORT 
    table (reprinted from a 1919 issue of Bowditch) is shown here for your 
    convenience. It will allow you to correct for the use of lakes, rivers, and 
    even ponds as natural horizons. It gives an effective dip correction to use 
    when sighting above an opposite bank, or the waterline of another boat."
    
    For $1500 or so you can get the CASSENS & PLATH - HORIZON ULTRA sextant from 
    Celestaire which comes with the following  attachments...
    
    "Optional items include a 6x30 telescope, zero magnification sight tube (see 
    page 21), a practice bubble horizon (see page 13) which facilitates effective 
    back-yard practice without the need for a sea horizon, and a more expensive 
    Professional Artificial Bubble Horizon. The Professional Artificial Bubble 
    Horizon provides an artificial horizon for use on land, or extremely calm sea 
    conditions when the natural horizon is not visible. It connects to batteries 
    in the handle for night lighting, and incorporates a dimming rheostat for use 
    with faint stars."
    
    -----Original Message-----
    From 	Bill Allan [SMTP:ballan{at}bserv.com]
    Sent:	Friday, September 19, 1997 7:34 PM
    To:	navigation{at}ronin.com
    Subject:	[Nml] Artificial Horizon
    
     << File: ATT00001.html >> I would appreciate any comments or advice on 
    artificial horizons. Where can I buy one? Can I make one? How do I do that? 
    Thanks,
    
    Bill and Diane Allan
    t-7.5 #239 "Getaway"
    
    
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