The Doppler Aerosol WiNd Lidar (DAWN) is an active remote, airborne lidar developed by NASA Langley Research Center. The laser is pulsed, 2-micron, and solid-state. It pulses at 10 Hz with 250 mJ pulses that are 200 ns long full width at half maximum (FWHM). The laser beam expanded to 12 cm e-2 (13.53% of center) intensity diameter with a 15-cm diameter, reflective, off-axis, afocal telescope. The 12-cm beam is eyesafe at any range. An optical wedge that is mounted in the center bottom of DC-8 Port Nadir 7 rotates around a vertical axis (if DC-8 horizontal) to select the laser’s azimuth angle. The wedge causes a fixed approximately 30-degree laser nadir angle. The transmitted light backscatters off natural aerosol and cloud particles which move with the wind. The backscattered light is directed onto a room temperature, round 75-micron diameter, InGaAs detector where heterodyne detection with a continuous-wave (CW) local oscillator (LO) laser is performed.
The heterodyne (or coherent) detection provides excellent photon efficiency, immunity to background light, a reduction in signal dynamic range, and a downward translation of the entire signal light spectrum intact to much lower frequencies capable of being captured. The receiver electronics and analog-to-digital converter (ADC) capture the backscattered light signal as a function of time since pulse started, which is proportional to the range of the pulse’s leading edge (t = 2R/c).
In the computer, the captured signal is divided into range gates. A discrete Fourier transform (DFT) is used to produce a periodogram for each range gate. The periodograms of several laser pulses are added to increase the signal-to-noise ratio (SNR). The summed periodograms are processed to find the peak frequency and hence line-of-sight (LOS) wind velocity. For one azimuth angle, a range profile of LOS wind velocity is attained. Using the wedge scanner, five different azimuth angles are measured: 1) to end up with five equations for the three unknown components of wind vs. altitude, 2) to mitigate cloud obscurations, and 3) to measure the atmospheric variability.