Simrad IS20 Manual page 59

Calibrate by speed over ground
With a GPS connected to the system, the speed may be
automatically set identical to the speed over ground
value.
This adjustment should be made in calm sea with no
effect from wind or tidal current.
1. Bring the boat up to cruising speed (above 5 knots)
2. Select Calibrate w/SOG and press the Menu/Enter
key to start the calibration
3. When this calibration is completed the SPD
Correction factor is set to 1.00
Manually adjust the speed value
If you experience an incorrect speed reading this can be
manually re-adjusted as follows:
− Compare the speed reading with that on another
boat
− Run the boat at constant speed over a known
distance in both directions and average the speed
reading
Then adjust the SPD Correction factor to get the correct
speed reading.
Range
-0.50 - +1.50
Change per step
0.01
Default value
1.00
Configuration | 59