Calculate Gyro Error and Compass Deviation: Step-by-Step Guide with the ABC Method

Summary

This video tutorial demonstrates how to determine a ship's gyro error while underway through astronomical observation using the ABC method. The process includes gathering key data such as the ship’s position, time, and variation, and calculating values such as the sun’s GHA, declination, and LHA from a nautical almanac. The steps lead to finding the true azimuth of the sun and gyro error, which is then used to correct the ship’s course. The video offers a detailed yet clear explanation of each step, helping navigators ensure accurate course adjustments for safe and efficient operations.

Takeaways

• 😀 Determining a ship's gyro error while underway is essential for accurate navigation and safety during operations.
• 😀 Even minor gyro errors can accumulate over time and lead to significant deviations from the planned route.
• 😀 To calculate the gyro error, a celestial body (the sun) is observed using the ship's gyro repeater.
• 😀 Key data for the observation includes universal time, ship's position, variation, gyro course, compass course, and the date.
• 😀 The Greenwich Hour Angle (GHA), Local Hour Angle (LHA), and declination of the sun are essential values that need to be calculated using the Nautical Almanac.
• 😀 Corrections must be applied to the GHA and declination values to ensure accuracy, including adjustments for time and other corrections.
• 😀 The true bearing or azimuth of the sun is determined using the ABC method and various nautical tables (A, B, and C).
• 😀 The gyro error is found by calculating the difference between the true bearing of the sun and the gyro bearing.
• 😀 The direction of the gyro error (East or West) is determined using a rhyming rule based on the relative values of the true bearing and gyro bearing.
• 😀 To calculate the magnetic compass error, the compass course is compared to the true course, with further corrections for deviation and variation applied.
• 😀 The deviation of the ship's heading is calculated by adjusting for the compass error and variation, and the final magnetic course is determined.

Q & A

Why is determining a ship's gyro error important while underway?

Determining the ship's gyro error is crucial for accurate navigation and maintaining safe operations. Even minor errors in the heading can accumulate over time, leading to significant deviations from the planned route.

What is the first step in determining the gyro error using the ABC method?

The first step is to gather the necessary data, including the celestial body (in this case, the Sun), the gyro repeater bearing, universal time, the ship's position, variation in the locality, gyro course, and compass course.

What tools can be used to determine the ship’s position and universal time?

A GPS receiver can be used to determine the ship's position, and universal time can be obtained from a GPS receiver or other accurate time sources.

How is the Greenwich Hour Angle (GHA) of the Sun determined?

The GHA of the Sun is determined by consulting the nautical almanac. The almanac provides values for the GHA at specific times, and any adjustments for minutes or seconds of time can be calculated using the increments and correction tables.

How do you calculate the Local Hour Angle (LHA) of the Sun?

The LHA of the Sun is calculated by applying the ship's longitude to the GHA. For easterly longitudes, you add them to the GHA, and for westerly longitudes, you subtract them from the GHA.

What is the significance of the declination correction in the calculation?

The declination correction accounts for the changes in the Sun's declination. It is either added or subtracted based on whether the declination is increasing or decreasing at the time of observation.

What is the ABC method used for in this process?

The ABC method is used to calculate the true azimuth or true bearing of the Sun by using Nori’s nautical tables. This involves determining values based on the Local Hour Angle (LHA), latitude, and declination of the Sun.

What does the term ‘true bearing’ refer to?

The true bearing refers to the accurate direction of a celestial body, in this case, the Sun, as calculated from the observation, corrected for any magnetic or gyro error.

How do you calculate the gyro error from the true bearing and gyro bearing?

The gyro error is determined by subtracting the gyro bearing from the true bearing. If the true bearing is greater than the gyro bearing, the error is positive, and if the gyro bearing is greater, the error is negative.

What is the purpose of applying the rhyming rule (best vs least) in this method?

The rhyming rule helps determine the direction of the gyro error. If the gyro bearing is greater than the true bearing (best), the error is west. If the gyro bearing is lesser than the true bearing (least), the error is east.