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GYROCOMPASS ERRORS

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sam kristopher barrioga

on 2 February 2016

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Transcript of GYROCOMPASS ERRORS

The total of the all the combined errors of the gyrocompass is called gyro error and is expressed in degree E or W, unlike magnetic compass error, and being independent of Earth's magnetic field, will be constant in one direction; that is, an error of one degree east will apply to all bearings all around the compass.

The errors to which a gyrocompass is subject are speed error, latitude error, ballistic deflection error, ballistic damping error, quadrantal error, and gimballing error. Additional errors may be introduced by a malfunction or incorrect alignment with the centerline of the vessel.

Speed error is caused by the fact that a gyrocompass only moves directly east or west when it is stationary (on the rotating earth) or placed on a vessel moving exactly east or west. Any movement to the north or south will cause the compass to trace a path which is actually a function of the speed of advance and the amount of northerly or southerly heading.












BALLISTIC DEFLECTION ERROR
Ballistic deflection error occurs when there is a marked change in the north-south component of the speed. East & West accelerations have no effect. A change of course or speed also results in speed error in the opposite direction, and the two tend to cancel each other if the compass is properly designed. This aspect of design involves slightly offsetting the ballistics according to the operating latitude, upon which the correction is dependent. As lattitude changes, the error becomes apparent, but can be minimized by adjusting the offset.
QUADRANTAL ERROR
Quadrantal error has two causes. The first occurs if the center of gravity of the gyro is not exactly centered in the phantom. This causes the gyro to tend to swing along its heavy axis as the vessel rolls in the sea. It is minimized by adding weight so that the mass is the same in all directions from the center. Without a long axis of weight there is no tendency to swing in one particular direction.

The second source of quadrantal error is more difficult to eliminate. As a vessel rolls in the sea, the apparent vertical axis is displaced first to one side and then the other. The vertical axis of the gyro tends to align itself with the apparent vertical. On northerly or southerly courses, and on easterly or westerly courses, the compass precesses equally to both sides and the resulting error is zero. On intercardinal courses, the N-S and E-W precessions are additive, and a persistent error is introduced which changes direction in different quadrants.

This error is corrected by use of a second gyroscope called floating ballistic which stabilizes the mercury ballistic as the vessel rolls eliminating the error. Another method is to use two gyros for the directive element which tend to precess in opposite directions neutralizing the error.
GIMBALLING ERROR
Gimballing error is caused by taking readings from the compass card when it is tilted from the horizontal plane. It applies to the compass itself and to all repeaters. To minimize this error the outer ring of the gimbal oach repeater should be installed in alignment with the fore-and-aft line of the vessel. The lubber's line must be exactly centered as well.
BALLISTIC DAMPING ERROR
Ballistic damping error is a temporary oscillation introduced by changes in course or speed. During a change in course or speed the mercury in the ballistic is subjected to centrifugal and acceleration/deceleration forces. This causes a torquing of the spin axis and subsequent error in the compass reading. Slow changes do not introduce enough error to be a problem, but rapid changes will. This error is counteracted by changing the position of the ballistics so that the true vertical axis is centered, thus not subject to error, but only when certain rates of turn or acceleration are exceeded.
TANGENT LATITUDE ERROR
Tangent latitude error is a property only of gyros with mercury ballistics, and is easterly in north latitudes and westerly in south latitudes. This error is also corrected internally, by offsetting the lubber's line or with a small movable weight attached to the casting.
GYROCOMPASS ERRORS
GYROCOMPASS ERRORS
ERROR OF A GYROCOMPASS
It has shown that a gyrocompass does not point to north unless it has been designed to do so. Even then from the instant its rotor starts spinning it takes a certain amount of time (usually about one and a half to two hours) to settle. Once settled the axis of the gyro might not align exactly to the north-south meridian in which case it will either settle to the east or west of the meridian. If this happens it is called a gyro-error as it not pointing to true north.
Figure 1 shows zero gyro error as the gyro's axis of rotation is aligned to true north.
Figure 2 shows the spin axis of the gyro settled to the east of meridian ie gyro error high (or east).
Figure 3 shows the spin axis settled to the west of meridian ie gyro error low (or west).
There are several methods of ascertaining gyrocompass error that do not involve variation or deviation (used for a magnetic compass).
Azimuth and amplitude methods.
Transit bearings, a comparison of true bearing observed from the chart and compared with gyro bearing.
Comparing the ship's heading on the gyrocompass with that measured from a navigational chart when the ship is in port and berthd alongside.
Comparing two gyrocompasses where the error of one is known.
The amount of gyrocompass error is dependent on the speed course and latitude of the ship. Usually on northerly courses between 270-090 the spin axis of the gyro settles to the west of the meridian because of speed error and on southerly courses between 090-270 it would settle to the east of the meridian.
To obtain a gyro bearing or courses to set on the compass (given true course/bearing).
Gyro Error High - Add to the given true bearing/course to set on the steering gyro compass or azimuth circle (Gyro High Steer High).

Gyro Error Low - Subtract from the given true bearing/course to set on the steering gyro compass or azimuth circle (Gyro Low Steer Low).
GYRO ERROR HIGH
GYRO ERROR LOW
To obtain a true bearing or course
Gyro Error Low - Add to the given gyro bearing/course.

Gyro Error High - Subtract from the given gyro bearing/course.
GYRO ERROR LOW
GYRO ERROR HIGH
Questions
It is the total of the all the combined errors of the gyrocompass and is expressed in degree E or W?
What are the errors to which a gyrocompass is subject? (Give 6 examples)
Is caused by the fact that a gyrocompass only moves directly east or west when it it stationary (on the rotating earth) or placed on a vessel mving exactly east or west?
It is a temporary oscillation introduced by changes in course or speed. During a change in course or speed the mercury in the ballistic is subjected to centrifugal and acceleration/deceleration forces?
What are the two things to use to obtain gyro bearing/true bearing or course?
ANSWER
GYRO ERROR
SPEED ERROR,
LATITUDE ERROR
,
BALLISTIC DEFLECTION ERROR
,
BALLISTIC DAMPING ERROR
,
QUADRANTAL ERROR
AND
GIMBALLING ERROR
SPEED ERROR
BALLISTIC DAMPING ERROR
GYRO ERROR HIGH AND
GYRO ERROR LOW
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