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Sodar Test

 

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This is an AeroVironment Sodar. It is an acoustic radar.  It uses sound to determine what the winds are doing at high altitudes.  Inside there is an array of audio speakers (tweeters) like a checkerboard.  The speakers transmit a tone in a burst  of sound (a loud beep) up into the air. The moving air reflects the sound back to the antenna.  The frequency of  the returning sound shifts according to the movement of the air in the same manner as the tone of a  moving motorcycle changes when it passes you (known as Doppler shift).  By measuring the change in the frequency you can calculate the speed of the moving air.  By aiming the transmit burst in 3 directions (up, forward, and to either side)   the wind can be triangulated in 3 dimensions. The trick  is how the transmit burst is aimed.  This is done by changing the phase of the signal going to certain speakers in the array thus bending the direction of the sound.  Altitude is determined by sampling the return signal at regular intervals determined by the speed of sound and the distance it travels.   

The testing we did for Energy Unlimited was very successful.  We ran the Sodar next to 2 different station to make projection of the winds at higher altitudes using historical data collected by the meterlogical towers.

  

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