HS-50 & HS-100
Doc. No 1199-PS-0035 Issue 3
June 2021
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Page 5
5 PRINCIPLE OF OPERATION
Figure 1 Time of Flight details
The HS measures the times taken for an ultrasonic pulse of sound to travel from an upper
transducer to the opposite lower transducer, and compares it with the time for a pulse to travel from
lower to upper transducer.
Likewise times are compared between each of the other upper and lower transducers.
As Figure 1 shows, the air velocity along the axis between each pair of transducers can then be
calculated from the times of flight on each axis.
This calculation is independent of factors such as temperature.
From the three axis velocities, the wind speed is calculated, as either signed U, V, and W, or as
Polar and W.
It can be seen from Figure 1 that the speed of sound in air can be calculated from the times of flight.
From this the sonic temperature can be derived from the formula
T
S1 =
C
1
2
/ 403
Where
T
S1
=
Sonic temperature
C
1
= Speed of sound
Note the formula does not account for the effect of Humidity
Crosswind correction is applied to results.
Summary of Contents for 1199-PK-085
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