DAWN

DAWN

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-deg. 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.

DAWN Wind Data Parameter Trades

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