Airmar PB200 Owner's Manual & Installation Instructions - page 23
23
About the Electronic Compass
The WeatherStation Instrument includes three magnetoinductive sensors that
measure magnetic field-strength in three axes relative to the instrument. From
combined measurements of the three-axis magnetic and tilt sensors, it calculates
the resultant magnetic heading angle, thereby providing a built-in three-axis
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.
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.
About Wind Chill Temperature
Wind Chill is a term that describes the heat loss on the human body resulting from
the combined effects of low temperature and wind. As wind speed increases, heat
is carried away from the body at a faster rate, causing a reduction in skin
temperature. Because the face is the part of the human body that is most likely to
be exposed, the wind-chill index is adjusted for the average adult face.
The concept of wind chill does not apply to inanimate objects, such as a boat. The
only effect that wind chill has in this case is to shorten the time it takes the object
to cool to the actual air temperature––wind chill does not cause an object to cool
below that temperature. For example, fresh water freezes at 0°C (32°F)
regardless of what the wind chill is.