AP-7
APPENDIX 3 CALIBRATION
For an accurate display of speed, a speed trial test to find the difference between your actual
speed and the speed calculated by the equipment is necessary. Since DS-85 can measure only
the speed against water, calibration is performed using round trip data in the same sea area to
minimize the effect of tidal currents.
Note: To obtain accurate offset values, it is recommended to conduct the test using similar con-
ditions and speeds to a regular voyage.
Calibration from the speed trial
Calibration values can be calculated by comparing the doppler speed log with DGPS or other sim-
ilar speed measuring devices.
1) Calibration from the speed trial
Speed trial result conducted using a GPS milepost may differ depending on the ship-
yard'smeasurement methods, as shown below:
a) The ship’s speed is calculated based on the time traveled.
b) The ship's fore/aft (Y-axis) speed is calculated based on the time traveled.
c) Calculation is based on a one-mile course, traveled three times.
d) Calculation is based on the time traveled over a predetermined time.
Offset calculation for the above methods is described below.
a) Calculate the ship’s speed with the measured total time of trip distance
Vectors are combined to calculate the ship speed. The DS-85 can only measure the speed
of the ship bow and stern, so errors may occur depending on the sea conditions.
Steer the ship at a steady speed for 10 minutes or longer on the test course (EG: A to B in
the figure). The ship’s speed data is collected as VBW fore/aft speed (taken immediately
after switching to water-based speed) data.Calculate the error with the average speed,
from the GPS mile post meter and the speed data which DS-85 outputs, then enter the cor-
rection value so that the error are eliminated.
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