Analysis and implementation of signal processing strategies for a three-dimensional Doppler lidar wind profiler
Item
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Title
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Analysis and implementation of signal processing strategies for a three-dimensional Doppler lidar wind profiler
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Identifier
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d_2009_2013:8f83fbfe22e4:11489
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identifier
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12001
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Creator
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Abdelazim, Sameh,
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Contributor
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Sam Ahmed | Fred Moshary
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Date
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2012
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Language
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English
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Publisher
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City University of New York.
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Subject
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Remote sensing | Electrical engineering | Environmental engineering | Doppler Lidar | Fiber laser | FPGA | wind profiler | wind sensing
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Abstract
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A heterodyne detection fiber optic based wind lidar system has been developed and tested, which benefits from unique field programmable gate array (FPGA) signal processing techniques and leverages devices from the telecommunication industry to make it particularly cost efficient. A narrow band stabilized fiber laser, polarization maintaining fiber amplifiers, acousto-optic modulators and an optical circulator comprise the transmitter which is coupled to free space using refractive optics. The collinear propagating lidar return signal that scatters off of atmospheric aerosols and in a heterodyne arrangement beats with a local oscillator and is then detected using a shot noise limited polarization maintaining balanced receiver. The system, which operates at a 20 kHz pulse repetition rate and acquires lidar return signals at 400 MSample/second, accumulates signals that are as much as 20 dB lower than the receiver noise power by using embedded programming techniques. For this reason two FPGA embedded programming approaches are considered and compared. In the first approach, the acquired return signal is gated in time and the square modulus of the fast Fourier transform is accumulated for each range gate, producing a series of power spectra as a function of range. Wind speed estimates based on numerical estimators can then be made after transferring the range gated accumulated power spectra to a host computer, enabling line of sight wind speed to be calculated as a function of range gate and stored for additional processing. In the second FPGA approach, a digital IQ demodulator and down sampler reduces the data flow requirements so that an autocorrelation matrix representing a pre-selected number of lags can be accumulated, allowing for the process of range gating to be explored on the host computer. The Fourier transform of the autocorrelation produces the power spectrum and, in the same manner as the first approach, estimates can then be made regarding the line of sight wind speed. The added feature of the second approach is that it allows for an additional capability to adjust the range gate period dynamically as the state of the atmospheric boundary layer (e.g. backscatter coefficient and stability condition) changes. A simple manual beam scanning technique is used to sample three line of sight directions and, by making suitable assumptions regarding the coherence of the averaged wind fields, the three dimensional wind field vector (representing both the horizontal wind speed and direction and the vertical wind speed and direction) is calculated and graphically displayed on time-height cross section plots. Precision in the velocity measurements is estimated to be on the order of 0.08 m/sec and the precision in the measured horizontal wind direction is estimated to be to be about 2 degrees, where both of these estimates are made assuming a relatively short 3-beam cycle time (less than 2 minutes) and a typical backscatter coefficient and atmospheric stability condition. A comparison to other observed wind information is presented which indicates that this lidar will open new doors for the practical characterization of microscale meteorology.
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Type
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dissertation
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Source
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2009_2013.csv
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degree
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Ph.D.
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Program
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Engineering
