Doppler Lidar - Fixed Pointing mode - dlfpt

The Doppler lidar (DL) is an active remote-sensing instrument that provides range- and time-resolved measurements of the line-of-sight component of air velocity (i.e., radial velocity) and attenuated aerosol backscatter. The DL operates in the near-infrared and is sensitive to backscatter from atmospheric aerosol, which are assumed to be ideal tracers of atmospheric wind fields. \n \nThe DL works by transmitting short pulses of infrared laser light into the atmosphere. Atmospheric aerosols scatter a small fraction of that light energy back to the transceiver, where it is collected and recorded as a time-resolved signal. From the delay between the outgoing pulse and the backscattered signal, the instrument infers the distance to the scattering volume. \n \nCoherent detection is used to measure the Doppler frequency shift of the backscatter signal. This is accomplished by mixing the backscatter signal with a reference laser beam (i.e., local oscillator) of known frequency. The onboard signal processor then determines the Doppler frequency shift from the spectrum of the mixed signal. The Doppler frequency shift and thus the radial air velocity is determined from the peak of the Doppler spectrum. The attenuated backscatter is determined from the energy content of the Doppler spectra. \n \nThe DL provides accurate measurements of radial velocity in regions of the atmosphere where aerosol concentrations are high enough to ensure good signal-to-noise ratio. Thus, valid data are usually limited to the atmospheric boundary layer where aerosol is ubiquitous. Valid measurements can also be obtained in elevated aerosol layers or in optically thin clouds above the boundary layer. Most of the ARM DLs have full upper-hemispheric scanning capability, enabling 3D mapping of turbulent flows in the atmospheric boundary layer. With the scanner pointed vertically, the DL provides height- and time-resolved measurements of vertical velocity.多普勒激光雷达（Doppler lidar, DL）是一种主动遥感仪器，可实现距离与时间分辨的视线方向风速分量（即径向速度）以及衰减气溶胶后向散射信号的测量。该设备工作于近红外波段，对大气气溶胶的后向散射信号敏感，而气溶胶被视为大气风场的理想示踪物。 多普勒激光雷达通过向大气发射短脉冲红外激光实现工作。大气气溶胶会将一小部分激光能量后向散射至收发一体机，该信号会被收集并记录为时间分辨信号。通过比对发射脉冲与后向散射信号之间的时延，仪器可反推散射体所在的距离。 采用相干检测技术测量后向散射信号的多普勒频移。具体实现方式为：将后向散射信号与已知频率的参考激光束（即本地振荡器）进行混频。随后，机载信号处理器可通过混频信号的频谱计算出多普勒频移。通过多普勒频谱的峰值可得到多普勒频移，进而解算出径向风速；而衰减后向散射信号强度则可通过多普勒频谱的能量占比反推得到。 多普勒激光雷达可在气溶胶浓度足够高、信噪比良好的大气区域实现精准的径向速度测量。因此，有效数据通常仅局限于气溶胶分布普遍的大气边界层。在高空气溶胶层或边界层上方的薄光学云区中，也可获取有效测量数据。多数ARM部署的多普勒激光雷达具备完整的半球扫描能力，可实现大气边界层内湍流场的三维测绘。当扫描仪垂直指向时，该设备可开展高度与时间分辨的垂直速度测量。