Airmar PB100 WeatherStation Owner's Manual - page 33
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About the Global Positioning System
Some WeatherStation instruments have a built-in GPS 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 (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.