WindSonic - How it works

The WindSonic sensor uses the time of flight method of air velocity measurement. There are four ultrasonic transducers arranged as two pairs at right angles to each other. Each pair is used to measurer the component of the wind in the direction between the transducers. For example if one pair is on a North South line and the other pair is on an East West line then the North South component V_N and the East West component V_E are measured. These are then combined to give the actual wind speed and direction.

Wind angle  A = arctan ( V_E / V_N )

Wind speed  S = sqrt ( V_E ² + V_N ² )

To look at how the component measurements are done we will take the example of the North South transducer pair. A pulse of ultrasonic sound is transmitted by the North transducer and the time T₁ it takes to travel to the South transducer is accurately measured. The time T₂ is then measured for an ultrasonic pulse to travel in the opposite direction from the South transducer to the North transducer. These times are affected by the wind blowing along the line between the transducers. For example if there is some wind blowing from a Northerly direction, then the time taken for the ultrasonic pulse to travel from N to S will be shorter than from S to N.

The time for the pulse to travel from N to S is given by:

  T₁ = L / ( C + V_N )

and the time for the pulse to travel from S  to N  is given by:

  T₂ = L / ( C - V_N )

where L is the distance between the transducers and C the speed of sound.

From this the northerly component of the wind  can be found from

V_N = L ( 1/T₁ - 1/T₂ ) / 2

and the speed of sound from

C = L ( 1/T₁ + 1/T₂ ) / 2