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Consider the case of a vehicle proceeding at a speed of 15 mph in
calm air. An observer on board would experience a wind of 15 mph
from dead ahead. This apparent wind would be due solely to the mo-
tion of the vehicle. If a true wind of 15 mph was blowing from the
rear, an observer would experience dead calm—no apparent wind.
That is because the vehicle is moving at the same speed and in the
same direction as the surrounding air.
Now, consider the more complicated situation of a vehicle proceeding
at 15 mph with a true wind of 15 mph blowing from the side. To an
observer on board, the apparent wind would be 21.2 mph blowing
from an angle 45º off the front.
In order to calculate the true wind speed and direction when on board
a moving vehicle, it is necessary to know the apparent wind speed and
direction, the speed and course over ground of the vehicle, the com-
pass heading, and the local magnetic variation. Note that heading and
course are not the same thing: heading is the direction the vehicle is
pointing, while course is the direction the vehicle is traveling. On
land, heading and course differ only when the vehicle is stationary.
The Sprayer Station can provide true wind speed and direction only if
all of the data is available. The speed and course over ground must be
provided by a GPS receiver––either built-in or networked. The head-
ing may be provided by either the built-in electronic compass or by an
external networked compass.
Because true wind is calculated using the data from several sensors,
its accuracy depends on the accuracy of all the raw data used in the
calculation. For instance, if the electronic compass is located near iron
or a similar magnetic disturbance, the heading will be incorrect, and
the true wind calculation will therefore be in error, perhaps by quite a
bit. In another example, the speed and course over ground (SOG and
COG) provided by the GPS receiver are averaged over time. If the
vehicle is performing maneuvers, changing speed and/or direction,
then it will take a few seconds for the SOG and COG values to "catch
up". The reported true wind values will therefore also be incorrect
until the vehicle reaches a steady-state condition, traveling in a
straight line at a constant speed.
About the Electronic Compass
The Sprayer Station includes a pair of magnetoinductive sensors that
measure magnetic field strength in two axes on the horizontal plane of
