Airmar PB100 WeatherStation Owner's Manual - About The Electronic Compass
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About the Electronic Compass
The WeatherStation instrument includes a pair of magnetoinductive sensors that
measure magnetic field strength in two axes on the horizontal plane of the
WeatherStation instrument. 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 instrument is to
introduce an error in the compass reading if the WeatherStation instrument is
tilted from the horizontal plane. Therefore, it is important when installing the
WeatherStation instrument 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 instrument (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.