Furuno WS-200 Operator's Manual page 22

The GPS receiver in the Weather Station instrument takes approximately one
minute on average to achieve a position fix after power is first applied. This is
known as the "time to first fix."
The GPS receiver synchronizes itself to the atomic clocks on board each satellite.
This allows the GPS receiver to accurately determine the date and time as well.
If the GPS receiver is mounted on a moving vessel, its changing position over time
allows the speed and course over ground to be calculated. The course reported by
a GPS is always with respect to true north.
The ability of the Weather Station instrument to calculate true wind speed and di-
rection depends on the presence of a GPS fix. If the GPS receiver is not tracking
at least three satellites, then the Weather Station instrument will be unable to pro-
vide true wind data. (Apparent wind data should always be available, regardless of
the status of the GPS receiver.)
True Wind Relative to Water
If a fix from a GPS receiver is not available, it is still possible for the Weather Station
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 be connected to the NMEA input on the Weather Station instrument.
The Weather Station 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 ig-
nored. 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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