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    Re: Azimuth Circle compass error.
    From: Nicol�s de Hilster
    Date: 2009 Nov 19, 20:17 +0100

    byronink@netzero.com wrote:
    > The bearing taker with Alidade can call in a round (3 or more) of bearing in 
    a few seconds, with the accuracy of 0.1 or .2. (Looking back, a good azimuth 
    accuracy may set the gyro to 0.5 + or -.)  The Azimuth Circle used in the 
    yard to set up the gyro is known to be inaccurate. Bowditch "it is reasonable 
    to round calculations to the nearest half or perhaps whole degree for most 
    purposes."   We have the Alidade that can take bearings to 0.1 or 0.2 and 
    helmsman that can steers less than 0.5 and we introduce large error because 
    of past azimuth practice.
    > ...
    > My best example was the USS INTREPID.She left the Philideliy Yards after 
    overhaul. Sailed to her home port of Norfolk VA. Sailed to her new home in 
    RI. And ran aground in Narragansett Bay RI. Than to her birth at the pier. 
    She ran aground in heavy fog, I am sure that is the main reason. After I went 
    aboard and she got underway, I saw two small triangles on the chart, her 
    second fix. What I saw was an east error of approximately 1.5 degrees. 
    What would be interesting in this respect is the heading accuracy 
    specified by the gyro's manufacturer used on the vessel(s) in question.
    Modern state of the art FOG's (Fiber Optic Gyro's) are accurate to about 
    0.03xSEC(LAT) and these are only around quite recently. The next, lesser 
    order, FOG's perform to about 0.2xSEC(LAT) degrees and both are known to 
    be quite a bit better than most conventional (spinning) gyro's, but 
    still far too young for a vessel like the USS INTREPID. Now I am not too 
    well informed about naval gyro's from those days, so there might have 
    been models around I am not aware of that could have paralleled these 
    FOG figures, but in general I would say 0.5 degree accuracy seems to be 
    quite something for those conventional ones.
    One of the gyro's widely used on navy vessels I know of was the Sperry 
    MK29 (which was, I believe, even quite modern compared to the USS 
    INTREPID). Although I do not know it's specs, I do know that it was 
    replaced by (among others) the Kearfott KN-5054 Seanav Gyrocompass 
    (accuracy: 0.5xSEC(LAT)) and the SITEP Gyrocompass Unit GB-7371(V) 
    (accuracy: 0.2xSEC(LAT)), so obviously the MK29 was not (much) better 
    than that Kearfott's 0.5xSEC(LAT). At Narragansett Bay (app. 41.5N) the 
    MK29 would thus not be better than about 0.7 degrees.
    As far as I know these figures are only 1SD, so 68% probability, which 
    means that 98% of the readings (3SD) would be within 1.5xSEC(LAT) 
    degrees (or about 2 degrees for Narragansett Bay). Therefore occasional 
    deviations of 1.5 degrees for a conventional gyro like the MK29 or 
    similar old type gyro - no matter how well calibrated - do not surprise 
    me too much, seem to be quite normal even.
    Apart from this I find it surprising that a navy vessel (or rather any 
    vessel) would be navigated through thick fog by compass bearing only. 
    One would expect those waters had a well marked navigable channel with 
    radar reflector equipped buoys and that - under dense fog conditions - 
    one would pay attention to the radar (and a keel clearance meter), not 
    only relying on a steered course. At first sight it seems to me that 
    other factors may have contributed to her running aground.
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