Airmar PB200 Owner's Manual & Installation Instructions - page 24
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The WeatherStation Instrument calculates two values for wind-chill temperature:
one using the apparent wind-speed, and one using the true wind-speed. The
apparent wind-chill temperature is relevant to what an observer is currently
experiencing on the vessel. The true wind-chill temperature indicates what the
wind chill would be if the vessel were not moving.
Wind chill temperature is only defined for temperatures at or below 10°C (50°F)
and wind speeds above 2.6 knots (3MPH).
By default, transmission of wind-chill data is disabled by the WeatherStation
Instrument. When used with WeatherCaster software, the wind-chill data will be
automatically enabled.
About the Barometric Pressure Sensor
The WeatherStation Instrument contains a temperature-compensated, silicon,
piezoresistive, pressure sensor. It measures atmospheric pressure for use as a
digital barometer. While a single measurement of air pressure at a given location
has little value, the trend of changing pressure and wind over time can be a useful
tool in performing basic weather forecasting.
About the GPS
Some WeatherStation Instruments have a built-in Global Positioning System with
their own antenna, receiver, and position determining electronics. The GPS
receiver receives radio signals from a constellation of orbiting satellites
maintained by the U.S. government. By accurately measuring the time it takes for
a transmission to travel from each satellite to the receiver, the unit is able to
determine the distance between the satellite and the receiver. When the distance
is known to three satellites, the unit is able to calculate the latitude and longitude
of the receiver. This is known as a 2D fix. If the distance is known to four or more
satellites, then the unit is additionally able to calculate the altitude of the receiver.
This is known as a 3D fix.
The GPS receiver in the WeatherStation 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 WeatherStation Instrument to calculate true wind speed and
direction depends on the presence of a GPS fix. If the GPS receiver is not tracking
at least three satellites, then the WeatherStation Instrument will be unable to
provide true wind data. (Apparent wind data should always be available,
regardless of the status of the GPS receiver.)
Certain models of the WeatherStation Instrument do not include a built-in GPS
receiver. In this case, if the true wind capabilities of the WeatherStation
Instrument are desired, it will be necessary to connect the output from an external
NMEA 0183-capable GPS to the NMEA input on the WeatherStation Instrument