On 23 August at 0604 ZT, in DR position LAT 16° 42.3' S, LONG 28° 19.3' W, you observed an amplitude of the Sun. The lower limb was a little above the horizon, and the Sun bore 076.0° pgc. At the time of the observation, the helmsman reported that he was heading 143° pgc and 167° per standard magnetic compass. The variation in the area was 23° W. What were the gyro error and deviation for that heading?
• Amplitude of the Sun and how it gives you the true bearing of the Sun at the time of observation • Relationship between gyro course (pgc), standard compass course, variation, and deviation • How to extract gyro error from the difference between observed and calculated true bearings
• From the DR position, date, and time, what is the Sun’s true bearing at the moment of the amplitude? Compare that to the observed bearing 076.0° pgc to infer gyro error. • How do you convert the standard magnetic compass heading plus known variation and unknown deviation to a true heading? Set that equal to the true heading you found from the Sun to solve for deviation. • Once you know whether the gyro is reading high or low compared to true, and whether the compass is east or west of magnetic, which sign conventions for E/W error and E/W deviation match the situation?
• Confirm which way east and west errors are defined: Does ‘gyro error east’ mean gyro reading is higher or lower than true? • Verify the correct formula links: True = Gyro ± gyro error and True = Magnetic ± variation = Compass ± deviation using consistent sign convention. • Double-check that you are using the correct limb (lower) and correct hemisphere (South) in the amplitude calculation, so your computed true bearing of the Sun is in the correct quadrant.
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