Orion Car Audio 420 - Manual (Page 13)

Orion 420 Weather Station 13
________________________________________________________________________
Columbia Weather Systems, Inc.
Principles of Measurements
Wind Measurement
Both wind speed and direction are measured using advanced ultrasonic
technology. The sensor utilizes ultrasound to determine horizontal wind
readings. The array of three equally-spaced ultrasonic transducers on a
horizontal plane is an ideal design that ensures accurate wind
measurement from all directions, without blind angles or corrupted
readings.
The wind sensor has no moving parts, which makes it virtually
maintenance free.
Wind speed and wind directions are determined by measuring the time it
takes the ultrasound to travel from each transducer to the other two.
The wind sensor measures the transit time (in both directions) along the
three paths established by the array of transducers. This transit time
depends on the wind speed along the ultrasonic path. For zero wind
speed, both the forward and reverse transit times are the same. With
wind along the sound path, the up-wind direction transit time increases
and the down-wind transit time decreases.
The wind speed is calculated from the measured transit times using the
following formula:
V
w
= 0.5 x L x (1/ t
f
– 1/tr
where:
V
w
= Wind speed
L = Distance between the two transducers
t
f
= Transit time in forward direction
t
r
= Transit time in reverse direction
Measuring the six transit times allows V
w
to be computed for each of the
three ultrasonic paths. The computed wind speeds are independent of
altitude, temperature and humidity, which are cancelled out when the
transit times are measured in both directions, although the individual
transit times depend on these parameters.
Using V
w
values of two array paths is enough to compute wind speed
and wind direction. A signal processing technique is used so that wind
speed and wind direction are calculated from the two array paths of best
quality.
The wind speed is represented as a scalar speed in selected units (m/s,
kt, mph, km/h). The wind direction is expressed in degrees (°). The wind