Flow Testing of a Digital Sonic Anemometer for Martian and Stratospheric Environments

Reported here are the results of wind tunnel tests for a digital sonic anemometer targeted at the Martian surface and Earth's stratosphere. The anemometer system uses ultrasound in the 35 to 55 kHz band to measure three-dimensional wind velocity and the speed of sound. The instrument has been designed to operate in low pressure environments, specifically at 6 mbar in CO2 of relevance to the surface of Mars, and dry air at pressures as low as 2 mbar of relevance to Earth's stratosphere at altitudes up to 42 km. Advantages to the system include high accuracy and resolution, high update rate, and low power consumption. The current prototype produces 4.2 three-dimensional wind speed measurements per second and has a "no windflow" RMS noise level of 1.4 cm/s in 6 mbar CO2 and 2.5 cm/s in 4 mbar air. The sensor head has an acoustic path length of 19 cm and a total mass of 400g. The primary challenge that had to be overcome in designing the instrument was operation in low pressure environments with the correspondingly low acoustic signal level. Electronics had to be carefully designed to avoid electrical and structural cross-talk, and to reduce noise. Secondary challenges for calibration include transducer pressure and temperature sensitivity, and self wind-shadowing produced by the wake of the flow over the sensor head. In this paper we report on system architecture and test results in the Mars Simulation Wind Tunnel at the University of Aarhus, Denmark. Testing was performed both in 6 mbar dry CO2 and in 4 or 11 mbar dry air at flow speeds from 0 to 11 m/s in various orientations at temperatures between 10 degrees C and 23 degrees C. The system produced measurements in three dimensions that agreed well with a laser doppler anemometry system, providing wind speeds that were within expectations. Differential phase measurements and cross-correlation techniques using linear chirps and multi-tone windowed bursts were explored. Both methods perform comparably.