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NAU 102
Lesson 18
Gyro Error
Properly adjusted, gyro errors seldom exceed 1°
Any error will be the same on all headings.
Determining Gyro Errors
Comparing gyro bearing to charted true bearing of a range or object.
Comparing gyro bearing to computed true bearing of a celestial object.
Trial and error adjustment of bearings of 3 lines of position.
Comparison with a compass of known error.
Gyro Error
Gyro Error is labeled East (E) if the gyro points to the east or right of the
true meridian.
Labeled West (W) if the gyro points to the west or left of the true
meridian.
Memory Aid
“Naming” Gyro Error
Compass Least, Error East
Compass Best, Error West
Example
If the true bearing of a light is 161° T and the gyro bearing is 159.5° pgc (per gyro
compass), what is the gyro error?
T
G
161° T
159.5° pgc
1.5°E or W?E
Example
3 LOP’s from Gyro Bearings
Example
Add 1° to each bearing
Example
Gyro Error = 1° W
Subtract 1° from each bearing
Applying Known Error
Once error is determined, it should be applied to all gyro directions.
GET
Gyro + East = True
Therefore, Gyro - West = True
And, True - East = Gyro
Example
You are steering course 195° pgc. The gyro error is 2° W. What true course are you
making good?
G
+E
T 193°
2° W195°-
Homework Problem #1
The true course between two points is 057°. Your gyrocompass has an error of 3° east and you make an allowance of 1° leeway for a north-
northwest wind. Which gyro course should be steered to make the true course good?
G
+E
T
054° pgc
3° E
057°T
-054° pgc
1°-
053° pgcAnswer
Homework Problem #3
You are steering 154° per gyrocompass. The wind is northeast by east, causing 4° leeway. The gyro
error is 3° east, variation is 11° west, and deviation is 7°E. What is the true course made good?
G
+E
T
154° pgc
3° E
157°T
+157° T
4°+
161° TAnswer
Deviation Table Construction
One method to determine the deviation of the magnetic compass is to compare it with a gyrocompass of
known error.
Homework Problem #7
You are on course 251°pgc and 241° psc, when you observe a range in line bearing 192°pgc. The chart indicates that the range is in line on a bearing of 194°T. The variation is 16°E. What is the deviation of the magnetic compass?
G
+ET
192° pgc
2°194°T
E (Compass Least)
Step 1 – Determine Gyro Error
Homework Problem #7
You are on course 251°pgc and 241° psc, when you observe a range in line bearing 192°pgc. The chart indicates that the
range is in line on a bearing of 194°T. The variation is 16°E. What is the deviation of the magnetic compass?
G
+ET
251° pgc
2° E253° T+
Step 2 – Compute True Heading
Homework Problem #7
You are on course 251°pgc and 241° psc, when you observe a range in line bearing 192°pgc. The chart indicates that the range is in line on a bearing of 194°T. The variation is 16°E. What is the deviation of the magnetic compass?
TVM
16° E237° M
253° T-
Step 3 – Compute Deviation
DC 241° psc
4° WE or W?
Homework Problem #20
You swung ship and compared the magnetic compass against the gyro compass to find deviation. Gyro error is 2°E.
The variation is 8°W. Find the deviation on a true heading of 187°.
HEADING HEADING HEADING PSC PGC PSC PGC PSC PGC 358.5° - 350° 122.5° - 110° 239.5° - 230° 030.5° - 020° 152.0° - 140° 269.0° - 260° 061.5° - 050° 181.0° - 170° 298.0° - 290° 092.0° - 080° 210.0° - 200° 327.5° - 320°
Magnetic Compasses
Disadvantages
Deviation
No digital output
Gyrocompasses
Disadvantages
Expensive
Difficult and expensive maintenance
Requires stable power and backup
Takes a long time to stabilize
Introduction to Navigation
Questions?