Campbell Scientific CSAT3 CSAT3 3-D Sonic Anemometer - Page 22

CSAT3 Three Dimensional Sonic Anemometer Unlike CSAT3s running embedded code version 3, embedded code version 4 does not use a library of calibration templates and pattern recognition to determine the ultrasonic time of flight. Embedded code versions 4 and 3 have the same wind resolution. The resolution of the speed of sound measurement using embedded code version 4 is 15 times worse than embedded code version 3 (0.002°C to 0.025°C RMS); however, the resolution of the speed of sound measurement is still a factor of two better compared to any other commercially available sonic anemometer. Field comparisons of measurements made with a CSAT3 anemometer containing embedded code version 4 and one containing embedded code version 3 show that the new algorithm significantly improves the anemometer's ability to obtain measurements under conditions of low signal to noise ratio, e.g. during rain events. Additionally, the water wicks installed on the CSAT3 transducers contribute to improved performance during rain events by preventing large drops from accumulating on the transducer faces. 7.2 Embedded Code Version 3 The CSAT3 measures wind speeds and the speed of sound by determining the time of flight of sound between pairs of transducers, as described in Appendix C. However, unlike many other commercial anemometers, it does not use simple threshold detection to determine the ultrasonic times of flight. Instead, it uses advanced digital signal processing techniques and pattern recognition algorithms which, in comparison to other systems, results in more accurate, lower noise measurements. The pattern recognition algorithms allow the system to quickly detect and indicate a degradation of signal quality and system performance. As a consequence, a CSAT3 running embedded code version 3 was highly sensitive to rain droplets on the transducers faces. WARNING Placing wicks on a CSAT3 running embedded code version 3 will not improve the CSAT3s performance in the rain. The wicks prevent large drops of water that completely obscure the sonic signals from accumulating on the transducer faces and are part one of a two part solution in operating in rainy conditions. The second part is in the digital signal processing in embedded code version 4. The CSAT3 signal processing techniques result in slight differences in operation when compared to simpler sonic anemometers. These differences include a start-up delay before the system is fully functional plus a requirement for the correct triggering of the sensor to ensure a constant flow of synchronized data. The general principles of operation are described in the following section. After the CSAT3 transmits an ultrasonic signal from one of its transducers, it records the received signal from the opposing transducer. To optimize system performance, the CSAT3 only looks for the received signal during a limited window in time. The exact start and end times of the window is determined not only by the spacing between transducers but also by the current wind speed and speed of sound conditions. 14