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    Re: Azimuth Circle compass error.
    From: Nicol�s de Hilster
    Date: 2009 Nov 21, 10:27 +0100

    byronink@netzero.com wrote:
    > Maybe I didn't make the problem clear in the write up. The gyro was fine, it 
    was the yard set up that was in fault. I saw this same problem of the gyro in 
    error , not because of a faulty gyro but because of reference "the AZIMUTH 
    CIRCLE ERROR" and the yard set up. The Intrepid and the pilot did not know of 
    the error. The ship normally would not have gone up the bay but some one mad 
    a costly decision to travel in fog, pilot on board to the new home port with 
    the Goveror all kind of dignitaries and other people were there to greet her.
    > The other point is why use The Azimuth that is known to be at it's best not 
    as accurate as the fixes required by the Navy. Navy exercises in harzard 
    water uses under 50 yrd as zero error in a fix position, any thing over is 
    marked againt the ship. I know that the other large ships, don't have the 
    personel equipment and requirements of the Navy. They may not have the 
    > and newworth of a war ship especially a Carrier or Atomic Sub. I am really 
    saying that the reference for setting gyro equipment should be the same as 
    taking bearing in hazard water. Of causes I want information from others 
    also. If they (INTREPID) had used my piloting tectnique, that I found the 
    error with they would not have had the gyro error.    
    Even if the yard had this Utopian, absolute, error free, device that was 
    used to calibrate the gyro, you would - in my opinion - still observe 
    errors up to 1.5 - 2 degrees every now and then at Narragansett Bay. It 
    simply is not possible to make a device more accurate by using more 
    accurate calibration methods. The other problem is that you do not know 
    the gyro error during its calibration in respect to the average gyro 
    error. What I mean is: the gyro might be wrongly oriented in the vessel 
    (say 10 degrees to starboard) and have a random error on top of that 
    (say 1 degree to portside). If you now calibrate the gyro you will find 
    9 degrees deviation, but that is for that moment only. When calibrating 
    gyro's for survey vessels it is standard procedure to do that in two 
    directions, simply because the gyro will show a different error in those 
    two directions, even when using the Utopian device (and even at the same 
    location at the same day and within the hour). In above example with the 
    vessel in an opposite direction the gyro might still have its fixed 10 
    degrees offset to starboard but 0.5 degree error to starboard on top of 
    that, resulting of a 10.5 degree total error. So even when you take the 
    average of the two, you will not have the gyro better aligned than to 
    0.25 degrees. Then when you start sailing again the gyro will still add 
    up to 1.5 degrees in each direction making the gyro producing errors up 
    to between -1.25 and +1.75 degrees. In respect to this all I guess they 
    were probably not able to do opposite calibrations at the yard with a 
    vessel this size.
    It simply comes down to this: if a device is accurate to n units at the 
    1SD level (whether it is degrees, metres or whatever), it should never 
    be relied on within 3n units for critical operations. So if the vessel 
    did run aground due to the steered course being off at 1.5 degrees, it 
    most probably was not because of how the yard calibrated the gyro.
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