Airmar LB150 Technical Information download pdf (Page 19)

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 lights. 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 vehicle 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. This is adjusted for in the WeatherStation instrument by using the

GPS-sourced course over ground when the vehicle is moving.

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.

About the Air Temperature Sensor

The WeatherStation instrument includes a built-in negative-temperature-

coefficient thermistor that measures the ambient air temperature. This NTC

thermistor is located in a thermally isolated region of the WeatherStation housing

that is open to the outside air.

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Airmar LB150 Technical Information Page 19

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