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    Azimuth Circle compass error.
    From: Byron Franklin
    Date: 2009 Nov 19, 09:36 -0800

             Azimuth of the sun and gyro/compass error                             
                                                    While I was crew on NAVY 
    Ships undergoing overhaul in the Shipyard, the yard worker align the gyro and 
    related systems.  The azimuth of the sun is the preferred reference to setup 
    the system and my pet peeve.
    The Azimuth Circle used to align the gyro is a delicate instrument that is 
    with mirrors and a prism that throws a beam of light on the compass for a 
    reading of the compass/ gyro. The Azimuth is timed and the true bearing of 
    the sun is worked out using reduction tables for a tabulated Azimuth. The 
    light beam on to the compass card has a thick beam of about .3 degrees, the 
    Circle leveling bubbles must be level to get a good reading.   There exists a 
    reasonable error of 0.5 degrees of error when the sun is high. Even so, the 
    delicate, mirrors and prism may be unknowingly out of alignment (small knock 
    can do this) for additional error.  After the ship is out of the yards and 
    underway, the Azimuth circle is use to further check on the compass/gyro 
    error.  At sea everything is fine, the ship may often correct course to close 
    the Dead Reckoning (DR), because of set and drift found by underway fixes. 
    There is no problem. After the ship enters  to the hazard waters and the 
    ships is to transit for a distance to the birth or anchorage, another set of 
    rules are in place, large NAVY ships are to take on a pilot to advise the 
    conn of harbor up dates , recommend courses and place tugs.  The navigation 
    team will use visual, radar, and electricronic fixes ever three minutes, 
    every third fix must be of different equipment (according to naval 
    instructions.) The compass, helmsman and the visual bearing s are a big part 
    of the navigation aids used in these more dangerous, waters. The bearing 
    takers for visual bearing are stationed with telescopic Alidades not the 
    Azimuth circles. 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. I have seen the 
    results many times in my past, when I evaluated piloting problem aboard both 
    submarines and surface ships. 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. I requested to the Navigator that he add 1.5 
    degrees to the next round of bearings. With that correction the next fix was 
    a three point intersection, that 1.5E correction was used for fixes until 
    months later when the gyro error was manually corrected. The AZIMUTH CIRCLE 
    use at the overhaul was the original problem. The grounding in the fog was 
    due to the visibility, although the bottom INTREPID hit was to the right of 
    the center channel. With a 1.5 East error the ship will track you to the 
    right of your ordered course (17 YARD FOR EVERY 1000 YDS TRAVELED.)  INTREPID 
    traveled the ordered course for about 6000yrds and tracked 150 yds to the 
    right of the desired track. The Pilot didn't know this compass error nor did 
    the crew.  I found it after the grounding. Reconditions, don't use the 
    Azimuth to set the compass. Use the Alidade and bearing to chart NAVAIDS to a 
    three point fix. (Use the two Alidades (use in transiting hazardous water,) 
    to ensure they are the same. See Franklin piloting technique. I may rewrite 
    this for the naval proceedings (a naval Magazine.)I would like to get 
    Merchant / Navy /every one, ideas and experience on this subject.
    
    
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