Airmar PB100 Technical Information PDF herunterladen (Seite 55)

About the Barometric Pressure Sensor

The WeatherStation 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 WeatherStations 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 takes approximately 1 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 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 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 do not include a built-in GPS receiver. In

this case, if the true wind capabilities of the WeatherStation are desired, it will be

necessary to connect the output from an external NMEA 0183-capable GPS to

the NMEA input on the WeatherStation (or to the optional Combiner), in order to

enable the true wind capabilities of the WeatherStation.

Even if your WeatherStation 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 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 will automatically suppress transmission

of GPS messages from its own built-in GPS receiver.

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Airmar PB100 Technical Information Seite 55