Raymarine Marine GPS System - Manual (Page 160)

Advanced Navigator Functions
5-69
True Wind Speed Calibration
True Wind Speed calibration assumes that the True Wind Speed
upwind reflects the correct value. However, when sailing downwind,
airflow over the masthead unit is affected by the spinnaker. Therefore,
downwind Tws must be corrected so that your instruments show the
same Tws value upwind and down.
The simplest method to achieve this is to measure wind speed upwind
and then calculate an average using the wand method. Once the
spinnaker is up and the boat is stable downwind, take another wand
method reading and average it. The difference between the two
values is the correction to be subtracted in the Tws Corrections
screen (Figure 5-23). Enter your corrections into the 180° column
for the wind speed you are experiencing.
Figure 5-23. Tws Corrections Screen