Flow Testing of a Sonic Anemometer for the Martian Environment

Test results for a sonic anemometer targeted at the Martian environment are reported. The system was tested in the Mars Simulation Wind Tunnel at the University of Aarhus in dry carbon dioxide (CO2) at pressures between 2 and 20 mbar and temperatures between 15 C and 27 C; this paper focuses on the 6 mbar results. Narrowband piezoelectric ultrasonic transducers operating at voltages less than 15 V peak-to-peak provide gas coupled ultrasound between 40.3 and 41.1 kHz. An acoustic transmission path length of approximately 12 cm is used. The sensor head masses 300g including frame, transducers and internal cables. Flow velocity is determined from the difference in phase shift between the forward and reverse propagating ultrasound. High resolution, low power analog phase measurement electronics are employed with only a low power microcontroller required for communication back to the host computer. Random noise in a no-flow scenario (in 6 mbar CO2) is 0.7-0.9 cm/s (RMS, sampled at 0.4 S/sec). In flow at low speed (0.6 m/s, in 6 mbar CO2) offset is less than 5 cm/s with rms variations of less than 3.5 cm/s in a static configuration. Variation increases to 13 cm/s when rotating through a 340 degree dynamic turn in the flow. At speeds up to 12 m/s (in 6 mbar CO2), for the X and Y axes, offsets are at most 15 cm/s and RMS variation is at most ±3.6% of full scale when compared to laser doppler anemometery measurements. The Z axis exhibits offsets as high as 17 cm/s and RMS variation as high as ±10% due to flow interactions with the system mounting structure. Advantages to the system include high accuracy, high update rate, and low power. Challenges to overcome include the effects of self-wind shadowing created by the mechanical structure, and system temperature sensitivity. To the best of our knowledge, this work represents the first demonstration of a three-axis sonic anemometer operating in 6 mbar CO2 using low voltage transducers.