Airmar PB100 Technical Information PDF herunterladen (Seite 53)

About the Electronic Compass

The WeatherStation includes a pair of magnetoinductive sensors that measure

magnetic ﬁeld strength in two axes on the horizontal plane of the WeatherStation.

From these measurements, it calculates the resultant magnetic heading angle,

thereby providing a built-in electronic compass.

Like all magnetic compasses, the WeatherStation compass will be affected by any

ferrous or magnetic materials in the vicinity, such as metal structures, motors,

speakers, etc. It will also be affected by nearby electric ﬁelds, such as the wiring

for navigation lights or radar domes. These nearby sources of magnetic

interference will distort the magnetic ﬁeld and produce errors in the compass

heading. These errors are known as magnetic deviation.

Although the WeatherStation compass is a 2-axis device, the earth's magnetic

ﬁeld occurs in three dimensions. That is, part of the earth's magnetic ﬁeld is

oriented in the vertical direction. The closer one's location is to the north or south

pole, the stronger this vertical component becomes in comparison to the

horizontal components. The effect this has on the WeatherStation is to introduce

an error in the compass reading if the WeatherStation is tilted from the horizontal

plane. Therefore, it is important when installing the WeatherStation to ensure the

support pole is mounted vertically, and not tilted to one side. Also, keep in mind

that when your vessel experiences pitch and roll, the compass heading will be

affected accordingly.

Because the compass heading is used in the calculations for true wind, any errors

in the compass heading will also produce errors in the reported true wind speed

and direction.

If you have another electronic compass on board your vessel that is capable of

providing the NMEA 0183 HDG sentence, you can connect the output from this

external compass to the NMEA input on the WeatherStation (or to the optional

Combiner), and the data from the external compass will override the data from the

built-in compass for the purpose of calculating true wind speed and direction.

About Magnetic Variation and True Heading

The earth acts like a giant magnet, with a magnetic north pole and a magnetic

south pole. The axis of the magnetic poles is offset approximately 11.5° from the

axis of the earth's rotation. Therefore, the earth's magnetic north and south poles

are in different locations than the earth's geographic north and south poles. In

addition, the earth's magnetic ﬁeld is non-uniform, and changes over time.

Magnetic variation, also known as magnetic declination, is the angle between

magnetic north and true (or geographic) north, at the observer's current location.

A magnetic compass measures heading with respect to magnetic north. To

convert this magnetic heading to true heading (that is, heading with respect to true

north), the magnetic variation must be added to the measured magnetic heading

value.

Because magnetic variation changes with location and gradually over time, it is

necessary to calculate the magnetic variation using the user's present position

and the current date. Therefore it is necessary to have a GPS with a ﬁx in order to

provide magnetic variation and heading with respect to true north.

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