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    Re: Aviation Bubble Sextant
    From: Frank Reed
    Date: 2010 Aug 28, 23:06 -0700

    mjans, you wrote:
    "I have recently acquired my grandfathers WWII Bubble sextant. He has long passed away and left me no instruction on how to use it. No manuals, no spoken wisdom, nada... I've figured out how to make the bubble and how to collapse it, etc.. but still haven't quite figured out what corrections I need to apply to the sighting. When figuring out Ha from Hs, would I need to apply an Index correction? Since there doesn't seem to be an index mirror, I don't really see why?"

    Gary Lapook will no doubt have lots of advice for you, but I'll throw in my two or three cents before he gets back.

    For instructions and manuals, you can pick up old air navigation textbooks on ebay and at abebooks.com. One that I like is the "American Air Navigator" published during the Second World War. It's beautifully illustrated, and it is a detailed, complete account of the methods of airborne celestial navigation. Also, ask away here on NavList. There are plenty of people who would be happy to discuss any questions, from the most basic to the most esoteric.

    Think of index error as being like "tare weight" on a scale. We need to check the offset when the instrument should be reading zero. The exact geometry and construction of the device is immaterial. For a bubble sextant, make some observations for known altitudes. Gary has often suggested using the altitude of Polaris when it is near the meridian since it is changing very slowly and easily calculated for a known latitude.

    You asked:
    "When taking a sighting, how big should the bubble be and where do I place the object? In the bubble or on top?"

    The bubble should be about a degree or so in diameter. It doesn't seem to be critical. You can place the object on the upper edge of the bubble, the side of the bubble, the center of the bubble, the lower edge of the bubble as you find convenient, but it needs to be consistent with your measurement of the index error. For some reason, using the center of the bubble seems not to have been recommended historically. I don't understand that since it actually seems to give better sensitivity in altitude observations with it centered since the bubble acts like a little lens. On land, you can take an altitude at the top edge of the bubble, then one from the bottom edge of the bubble, and average the two. That yields excellent results.

    You asked:
    "This one seems to have a timer too, is there anyway to mechanically change the timing interval? It seems to only take a sight for about a minute!"

    The timer is not for taking extended sights, like LAN sights (which you mentioned later). It's for averaging. The idea is that your hand will be shaking, the airplane is vibrating from its engines and machinery, and in smaller planes and/or rougher air the airplane may be rolling and pitching around quite a bit. So you look through the instrument and do your best to keep the Sun aligned with the bubble for a minute (or two, depending on the type of averager). The average altitude observed over that interval will generally be much better than any single instantaneous altitude

    You asked:
    "Assuming I was in an old DC-3 flying at 5000 ft, taking a LAN sight... What corrections would I apply.. I assume, temperature, pressure and altitude (pressure or density alt??) but would airspeed apply too? If so.. True Airspeed or Ground Speed?"

    Yes, temp, pressure, and altitude. You can find tables for this in many places. There is a fairly important correction for the Coriolis acceleration which depends on ground speed.

    You asked:
    "Would the correction be altitude only instead of Dip?"

    Since you don't use the visible horizon (and it's essentially never visible from flight altitudes anyway), there is no dip correction. The altitude correction is a correction for air density which could, in theory, be made by adjusting the air pressure, but in practice it's a separate correction.

    -FER


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