Airmar PB200 Owner's Manual & Installation Instructions page 25

(or to the optional Combiner), in order to enable the true wind capabilities of the
WeatherStation Instrument.
Even if your WeatherStation Instrument includes a built-in GPS receiver, you may
wish to use a separate external GPS receiver instead, for the determination of true
wind. If the WeatherStation Instrument receives speed over ground and course
over ground (SOG and COG) data on its NMEA input from an external GPS, these
data will override the data from the built-in GPS for the purpose of calculating true
wind speed and direction. In addition, the WeatherStation Instrument will
automatically suppress transmission of GPS messages from its own built-in GPS
receiver.
About True Wind Relative to Water
If a fix from a GPS receiver is not available, it is still possible for the
WeatherStation Instrument to determine a value for true wind, if the speed of the
vessel through the water is known. In this case, it is necessary that a water-speed
sensor with an NMEA output (such as an Airmar Smart™ Sensor) be connected
to the NMEA input on the WeatherStation Instrument (or to the optional
Combiner).
The WeatherStation Instrument's calculation for true wind relative to water makes
the significant simplifying assumption that the vessel's course is the same as its
heading. That is, the effects of wind and current on the motion of the boat are
ignored. The direction of the true wind relative to water is referenced only to the
bow of the vessel, not to true or magnetic north.
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