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north. To convert this magnetic heading to true heading (that is, head-
ing 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 Sprayer Station 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
Sprayer Station housing that is open to the outside air.
About the Relative Humidity Sensor
The Sprayer Station contains a capacitive cell humidity sensor that
measures the relative humidity of the air. Humidity refers to the
amount of water vapor in the air. Relative humidity is the percentage
of saturation of the water vapor in the air. It is the ratio of the mois-
ture content of the air to the saturated moisture level at the same tem-
perature and pressure.
About the Barometric Pressure Sensor
The Sprayer Station contains a temperature-compensated, silicon,
piezoresistive, pressure sensor. It measures atmospheric pressure for
use as a digital barometer
About the GPS
The Sprayer Station has a built-in Global Positioning System with its
own antenna, receiver, and position determining electronics. The GPS
receiver receives radio signals from a constellation of orbiting satel-
lites maintained by the U.S. government. By accurately measuring the
time it takes for a transmission to travel from each satellite to the re-
ceiver, the unit is able to determine the distance between the satellite
and the receiver. When the distance is known to three satellites, the
unit is able to calculate the latitude and longitude of the receiver. This
is known as a 2D (2 dimensional) fix. If the distance is known to four
