Modeling and characterization of gas coupled ultrasonic transducers at low pressures and temperatures and implications for sonic anemometry on Mars

A sonic anemometer targeted at wind speed measurements on the surface of Mars is described. This environment requires transducer operation in 4-10 mbar CO2 at temperatures between 143 and 293 K (-130 °C and 20 °C, respectively). Over these ranges, transducer pressure and temperature sensitivity could be a source of measurement error. To investigate this, four candidate transducers were tested using transmission mode ultrasonic testing and impedance measurements: two narrowband piezoelectric transducers, a broadband capacitive transducer, and a micromachined capacitive ultrasound transducer. A system model was used for comparison and interpretation, and implications for a sonic anemometer were examined. Variation of transducer characteristics, including diffraction effects, across 2-10 mbar in CO2 and 190-293 K (-83 °C-20 °C) result in ±2.3% error in wind speed measurement and ±1.1% error in speed of sound measurement for the worst case but only ±0.14% error in wind and ±0.07% error in speed of sound for the best transducer operated off resonance. The acoustic conditions on Mars are similar to those in Earth's stratosphere at 30-42 km of altitude. Hence, testing was also conducted in dry air over the same range of pressures and temperatures with relevance to a secondary application of the instrument as a stratospheric anemometer for high altitude balloon missions on Earth.