Pulsed Coherent Doppler Lidar Research Articles

1.55-μm pulse coherent LIDAR with 10-km detection range

Recently, a 1.55-μm pulsed coherent Doppler LIDAR system using all fiber optical components was developed to achieve real-time measurements of wind fields. The system employs 100-μJ pulse energy at 10 kHz pulse repetition rate. In addition, the system consists of a fiber-based optical transceiver unit, a two-axis scanner, and a multicore digital signal processor (DSP) for real-time signal processing. With compact and mobile design, the LIDAR is easy to transport and deploy for different field campaigns. For a different application of LIDAR, the range resolution can change manually. A horizontal detection range of 10 km is achieved with the temporal and spatial resolution of 1 s and 30 m, respectively. Field experiments compared with an anemometer show that correlation coefficient of the different wind speed measurements is 0.953; the correlation coefficient for wind direction values is 0.967. Continuous wind profiles of the planetary boundary layer are presented to demonstrate the stability of the system.

Adaptive iteratively reweighted sine wave fitting method for rapid wind vector estimation of pulsed coherent Doppler lidar.

Wind vector estimation method that provides more available data in low signal-to-noise ratio (SNR) regime improves the performance of pulsed coherent Doppler lidar. The adaptive iteratively reweighted sine wave fitting (airSWF) method proposed here reweights the contribution of each radial wind velocity adaptively and iteratively when estimating the wind vector. Based on the processing results of both the simulated and real-captured signal, the airSWF method provides more available wind vector estimates with little computational time increment, compared with the direct sine wave fitting (DSWF) and weighted DSWF methods. Specifically, the proportion of available wind vector estimates determined using the airSWF method increases by >20% when the detection height exceeds 1 km. Another significant advantage of the airSWF method over the filtered sine wave fitting (FSWF) method is that no prior knowledge is required. Moreover, the computational complexity of the airSWF method is lower than that of the FSWF and maximum of the function of accumulated spectra methods.

Aircraft wake vortex and turbulence measurement under near-ground effect using coherent Doppler lidar.

This paper evaluates the wake vortex characteristics using pulsed coherent Doppler lidar (PCDL) under near-ground effect (NGE). A wake vortex visualization demonstrator (V2D) is developed in order to visualize wake vortex in real-time. The combination of radial velocity distribution and FFT spectrum characterization are used to identify the core position of wake vortex. The velocity envelope and Burnham-Hallock model correction are used to retrieve the circulation of wake vortex under NGE. The circulation error, which is caused by PCDL scanning mode, is simulated and corrected. To investigate the dissipation rate's effect on wake vortex in real atmosphere, the cross wind and atmospheric turbulence are concurrently retrieved from the same measurement of wake vortex by using structure function. The statistics of wake vortex parameters are analyzed, based on the measurement campaign at Beijing Capital International Airport (BCIA) in 2017.

Airport low-level wind shear lidar observation at Beijing Capital International Airport

The low-level wind shear is defined as a sharply change of the wind speed or wind direction within 600 m above the ground level. Due to affecting the airspeed of aircraft during take-off and landing phases, the wind shear can result in the disastrous aircraft accidents. The lidar team at Ocean University of China (OUC) carried out the particular research of the low-level wind shear at Beijing Capital International Airport (BCIA) in 2015 and 2016 by operating a pulse coherent Doppler lidar (PCDL) system. In this field experiment, the radial resolution is 30 m while the angle resolution is 0.1° and the updating rate of observation data is 4 Hz. On account of the high spatial and temporal resolution, the small-scale meteorological phenomenon can be captured. The low-level wind shear is observed by different scanning modes including 5-Doppler beam swing (5-DBS), step-wise plane-position indicator (PPI), multi-elevation PPI, range-height indicator (RHI) and glide path scan. The experiment configuration, the PCDL system, the observation modes and the case study of the wind shear are proposed and experimentally demonstrated. The quick-look algorithms which identifies the wind shear alarm based on the radial velocity data have been developed and tested. A case study of the wind shear shows that the events of low-level wind shear at the southern end of 18R/36L are suspected to be caused by buildings and rows of trees along the glide path under strong northwest wind conditions. Meanwhile, more than 2000 PPI scans have been analyzed to indicate the relationship between wind shear intensity and ramp length. Preliminary validations of the algorithms against wind shear reports from the crew and the terminal control office indicated positive skill. Totally, 14 wind shear cases have been validated by reports from the crew during field observation campaign.

Investigation of feasibility of wind turbulence measurement by a pulsed coherent doppler lidar in the atmospheric boundary layer

Feasibilities of determination of the wind turbulence parameters from data measured by the Stream Line coherent Doppler lidar under different atmospheric conditions have been studied experimentally. It has been found that the spatial structure of the turbulence is described well by the von Karman model in the layer of intensive mixing. From the lidar measurements at night under stable conditions the estimation of the outer scale of turbulence with the use of the von Karman model is not possible.

Airport low-level wind shear lidar observation at beijing capital international airport

Ocean University of China lidar team operated a pulse coherent Doppler lidar (PCDL) for the low level wind shear monitoring at the Beijing Capital International Airport (BCIA) in 2015. The experiment configuration, observation modes is presented. A case study shows that the low level wind shear events at the southern end of 18R/36L runway were mainly caused by the trees and buildings along the glide path under strong northwest wind conditions.

Measurements of wind turbulence parameters by a conically scanning coherent Doppler lidar in the atmospheric boundary layer

Abstract. The method and results of lidar studies of spatiotemporal variability of wind turbulence in the atmospheric boundary layer are reported. The measurements were conducted by a Stream Line pulsed coherent Doppler lidar (PCDL) with the use of conical scanning by a probing beam around the vertical axis. Lidar data are used to estimate the kinetic energy of turbulence, turbulent energy dissipation rate, integral scale of turbulence, and momentum fluxes. The dissipation rate was determined from the azimuth structure function of radial velocity within the inertial subrange of turbulence. When estimating the kinetic energy of turbulence from lidar data, we took into account the averaging of radial velocity over the sensing volume. The integral scale of turbulence was determined on the assumption that the structure of random irregularities of the wind field is described by the von Kármán model. The domain of applicability of the used method and the accuracy of the estimation of turbulence parameters were determined. Turbulence parameters estimated from Stream Line lidar measurement data and from data of a sonic anemometer were compared.

Stream Line Doppler lidar measurements of wind speed and direction with the duo-beam method in the surface air layer

The results of retrieval of wind speed and direction from measurements of radial velocity by a Stream Line pulsed coherent Doppler lidar using the duo-beam method and conical scanning are presented. These results are compared with data of a sonic anemometer (point sensor).

Wind turbine wake visualization and characteristics analysis by Doppler lidar

Wind power generation is growing fast as one of the most promising renewable energy sources that can serve as an alternative to fossil fuel-generated electricity. When the wind turbine generator (WTG) extracts power from the wind, the wake evolves and leads to a considerable reduction in the efficiency of the actual power generation. Furthermore, the wake effect can lead to the increase of turbulence induced fatigue loads that reduce the life time of WTGs. In this work, a pulsed coherent Doppler lidar (PCDL) has been developed and deployed to visualize wind turbine wakes and to characterize the geometry and dynamics of wakes. As compared with the commercial off-the-shelf coherent lidars, the PCDL in this work has higher updating rate of 4 Hz and variable physical spatial resolution from 15 to 60 m, which improves its capability to observation the instantaneous turbulent wind field. The wind speed estimation method from the arc scan technique was evaluated in comparison with wind mast measurements. Field experiments were performed to study the turbulent wind field in the vicinity of operating WTGs in the onshore and offshore wind parks from 2013 to 2015. Techniques based on a single and a dual Doppler lidar were employed for elucidating main features of turbine wakes, including wind velocity deficit, wake dimension, velocity profile, 2D wind vector with resolution of 10 m, turbulence dissipation rate and turbulence intensity under different conditions of surface roughness. The paper shows that the PCDL is a practical tool for wind energy research and will provide a significant basis for wind farm site selection, design and optimization.

Aircraft Wake Vortex Parametrization Based on 1.5-μm Coherent Doppler Lidar Data

A strategy of measurement by a 1.5-μm pulsed coherent Doppler lidar “Stream Line” has been developed, and a method for estimation of aircraft wake vortices from the lidar data has been proposed. The principal possibility of obtaining the information about the vortex situation over an airport airfield with the Stream Line lidar has been demonstrated.

Observations and Analysis of Turbulent Wake of Wind Turbine by Coherent Doppler Lidar

Turbulent wake of wind turbine will reduce the power output of wind farm. The access to real turbulent wake of wind turbine blades with different spatial and temporal scales is provided by the pulsed Coherent Doppler Lidar (CDL) which operates by transmitting a laser beam and detecting the radiation backscattered by atmospheric aerosol particles. In this paper, the authors discuss the possibility of using lidar measurements to characterize the complicated wind field, specifically wind velocity deficit by the turbine wake.

Aircraft Wake Vortex Measurement with Coherent Doppler Lidar

Aircraft vortices are generated by the lift-producing surfaces of the aircraft. The variability of near-surface conditions can change the drop rate and cause the cell of the wake vortex to twist and contort unpredictably. The pulsed Coherent Doppler Lidar Detection and Ranging is an indispensable access to real aircraft vortices behavior which transmitting a laser beam and detecting the radiation backscattered by atmospheric aerosol particles. Experiments for Coherent Doppler Lidar measurement of aircraft wake vortices has been successfully carried out at the Beijing Capital International Airport (BCIA). In this paper, the authors discuss the Lidar system, the observation modes carried out in the measurements at BCIA and the characteristics of vortices.

All-Fiber Airborne Coherent Doppler Lidar to Measure Wind Profiles

An all-fiber airborne pulsed coherent Doppler lidar (CDL) prototype at 1.54μm is developed to measure wind profiles in the lower troposphere layer. The all-fiber single frequency pulsed laser is operated with pulse energy of 300μJ, pulse width of 400ns and pulse repetition rate of 10kHz. To the best of our knowledge, it is the highest pulse energy of all-fiber eye-safe single frequency laser that is used in airborne coherent wind lidar. The telescope optical diameter of monostatic lidar is 100 mm. Velocity-Azimuth-Display (VAD) scanning is implemented with 20 degrees elevation angle in 8 different azimuths. Real-time signal processing board is developed to acquire and process the heterodyne mixing signal with 10000 pulses spectra accumulated every second. Wind profiles are obtained every 20 seconds. Several experiments are implemented to evaluate the performance of the lidar. We have carried out airborne wind lidar experiments successfully, and the wind profiles are compared with aerological theodolite and ground based wind lidar. Wind speed standard error of less than 0.4m/s is shown between airborne wind lidar and balloon aerological theodolite.

All‐fiber pulse coherent Doppler LIDAR and its validations

An all‐fiber pulsed coherent Doppler LIDAR (CDL) system is described. It uses a fiber laser as a light source at a 1.54‐μm wavelength, producing 200 μJ pulses at 10 kHz. The local oscillator signal is mixed with the backscattered light (of different frequency) in the fiber. The atmospheric wind speed is determined through the fast Fourier transform applied to the difference frequency signal acquired by an analog‐to‐digital converter card. This system was used to measure the atmospheric wind above the upper‐air meteorological observatory in Rongcheng (37.10°N, 122.25°E) of China between January 7 and 19, 2015. The CDL data are compared with sounding‐ and pilot‐balloon measurements to assess the CDL performance. The results show that the correlation coefficient of the different wind‐speed measurements is 0.93 and their discrepancy 0.64 m/s; the correlation coefficient for wind‐direction values is 0.92 and their discrepancy 5.8 deg. A time serial of the wind field, which benefits the understanding of atmospheric dynamics, is presented after the comparisons between data from CDL and balloons. The CDL system has a compact structure and demonstrates good stability, reliability, and a potential for application to wind‐field measurements in the atmospheric boundary layer.

Method of radial velocities for the estimation of aircraft wake vortex parameters from data measured by coherent Doppler lidar.

The method of radial velocities (RV) is applied to estimate aircraft wake vortex parameters from measurements conducted with pulsed coherent Doppler lidar (PCDL). Operations of the Stream Line lidar and the 2-µm PCDL are simulated numerically to analyze the accuracy of the estimated wake vortex parameters with the RV method. The RV method is also used to estimate wake vortex trajectories and circulation from lidar measurements at Tomsk and Munich airports. The method of velocity envelopes and the RV method are compared employing data gathered with the 2-µm PCDL. The domain of applicability of the RV method is determined.

Estimation of aircraft wake vortex parameters from data measured with a 1.5-μm coherent Doppler lidar.

A strategy of measurement by a 1.5-μm pulsed coherent Doppler lidar "Stream Line" has been developed, and a method for estimation of aircraft wake vortices from the lidar data has been proposed. The principal possibility of obtaining the information about the vortex situation over an airport airfield with the Stream-Line lidar has been demonstrated.

Joint radiosonde and doppler lidar measurements of wind in the atmospheric boundary layer

Results of joint measurements of height profiles of wind velocity and direction by the Stream Line pulse coherent Doppler lidar and RS92-SGP radiosonde in Tomsk from 23 to 27 of September, 2013, are presented. It has been established that wind profiles can be retrieved up to heights from 400 to 1100 m depending on the aerosol concentration in the atmospheric boundary layer from lidar data measured at an elevation angle of 45°. It is shown that the coefficient of correlation between lidar and radiosonde measurements of wind velocity and direction is equal to 0.97. The mathematical expectation and standard deviation of the difference between estimates for the wind velocity and direction from the radiosonde and lidar data amount to 0.1 and 0.7 m/s, respectively, for the velocity and 0.8° and 4°, respectively, for the wind direction.

Noise modeling by the trend of each range gate for coherent Doppler LIDAR

A denoising method of all-fiber pulsed coherent Doppler LIDAR (CDL) is investigated. The goal is to enhance the signal-to-noise ratio (SNR) in the weak signal regime. Based on differential detection theory, the total noise expression of CDL with a dual-balanced detector is introduced and analyzed. The conclusion is drawn that the total noise can be acquired under the local oscillator laser exposure conditions by reasonable simplification. Using the actual measured data, the ratio of the standard deviation to the mean value of the total noise in each range gate is obtained and is up to approximately 11%. In order to suppress the jitter of the noise, an effective noise modeling by the trend of each range gate is developed. The feasibility of this method is verified by a long set of measured data. The spatial and temporal distribution of wind speed is illustrated with 400 pulses accumulation. Compared to the noise modeling of the tail, the detection range of wind speed using the proposed method can be improved by 35.3%.

All fiber pulsed coherent lidar development for wind profiles measurements in boundary layers

An all fiber pulsed coherent Doppler lidar (CDL) system at 1.54 ?m wavelength is developed for wind profiles measurements. This lidar affords 43.0-?J pulse energy at 10-kHz pulse repetition frequency with 500-ns pulse width. The lidar is operated in monostatic mode with 50-mm diameter telescope. The heterodyne mixing signals are acquired with 500 M/s analog to digital converter and 2048 points fast Fourier transform (FFT) is implemented. Line of sight wind speeds are measured with more than 3.0-km range in a horizontal direction and about 1.9 km in the vertical direction with 75-m range resolution. Systematic error of speed measurement of 0.2 m/s is validated.

Accuracy of estimation of the turbulent energy dissipation rate from wind measurements with a conically scanning pulsed coherent Doppler lidar. Part II. Numerical and atmospheric experiments

Using numerical simulation, possibilities of measurements of the turbulent energy dissipation rate with a pulsed coherent Doppler lidar (PCDL) are studied in the case of conical scanning with a probing beam, with allowance for the averaging of the radial velocity over the sensing volume and error of estimates of the radial velocity. The error of the lidar estimate of the dissipation rate was calculated as a function of the number of full scans with a probing beam and of the signal-to-noise ratio. A comparative analysis was performed for the results of joint measurements of the dissipation rate by sonic anemometers and 2-μm PCDL.