Objective   The wind field is one of the most critical impacts in the specifying stratospheric airships and plays a practical fundamental role in flying safety. The airship has attracted increasing interest and made it a tremendous potential of long-time fix-station residence missions at nearly the altitude range of the stratosphere bottom 18-22 km. Its integrating superiority, such as broad covering, and large load capacity, has been proven as effective and powerful operational buoyant platform to contribute to more accurate remote sensing of environment monitoring, resource exploration, and meteorological research. Due to the Earth’s periodic rotation, there is an extremely strong west wind region in low stratosphere while the wind speed changes along with the season, the latitude and longitude, and the height. The key challenges for applying a wind measurement system in the low stratosphere (20 km), which has significant differences from the terrestrial environment, are the low densities and the low pressures at the working height of the high-altitude airship. This represents a precise measurement that is essential to detect the atmosphere parameters of wind. For this purpose, a laser wind velocimetry system for the stratospheric airship is proposed.

  Methods   A compact laser anemometer is designed using dual-channel Fabry-Perot etalon to analyze Doppler shift due to high-speed thermal motion at molecular scale. Atmospheric molecular scattering mainly consists of a strong Rayleigh-Brillouin spectrum (Fig.1). The system generally consists of four major subsystems, which are the 532 nm fiber pulsed laser transmitter subsystem, the telescope subsystem, a photocounting detection (PD) subsystem and computer controlling subsystem (Fig.5). The liquid crystal phase variable retarder (LCVR) is chosen to be a controller between two lines-of sight (LOS) for the successive horizontal speed estimates with a novel non-mechanical structure in the detection. The radial wind velocity can be uniquely determined by measuring the ratio of the two channel edge signals of Fabry-Perot etalon.

  Results and Discussions   In the beginning of the experiment, it is an indispensable step of the laser anemometer system design and study that the theoretical parameters must be preliminarily confirmed (Tab.1). According to the thin air characteristics at the airship flying altitude of 20 km, the improvement of signal noise ratio (SNR) and velocity errors are simulated. Assuming the Doppler shift is 0, the FWHM of the outgoing laser spectrum is 400 MHz, and the low stratosphere temperature is set at 216.5 K. The radial wind measurement error as a function of the free spectral range (FSR) of the Fabry-Perot etalon is given (Fig.8). As shown, to fully analyze the Rayleigh-Brillouin spectrum, the free spectral range (FSR) of the Fabry-Perot etalon is chosen to be 6.5 GHz with the minimum error occurrence in the system (Fig.8). At the same time, the spectral spacing of the Fabry-Perot etalon is required to be 3.25 GHz due to the same velocity sensitivity for both molecular and aerosol backscattering (Fig.9). The SNR of more than 180 is obtained due to the fact that the spectral radiance value of the solar background is still very small in the daytime and nighttime (Fig.10). The velocity error is less than 1 m/s up to 500 m distance with wind velocity of 100 m/s (Fig.11).

  Conclusions  Using dual-channel Fabry-Perot etalon with fixed cavity length for frequency discriminator and a 532 nm fiber pulsed laser, the structural design of the laser wind velocimetry was completed. The system referred to optical path of coherent wind measurement lidar and adopted monostatic telescope, which had no blind area and smaller receiving field of view, thus improving performance of all-sky detection. The polarization property of liquid crystal variable retarder was used to control direction of the detected optical path. The system performance of wind field detection was also analyzed. The average power of 500 mW laser, the integration time of 10 s, and the range resolution of 100 m during simulation are selected. The analysis results illustrate the maximum wind speed error of 1 m/s and wind direction error of 5° under the wind speed condition of larger than 10 m/s, respectively. The theoretical results highly meet wind detection requirements in navigation environment of stratospheric airships.