Coherent Doppler Lidar System Research Articles

Using optimal estimation to retrieve winds from velocity-azimuth display (VAD) scans by a Doppler lidar

Abstract. Low-powered commercially available coherent Doppler lidar (CDL) wind profilers provide continuous measurement of vertical profiles of wind in the lower troposphere, usually close to or up to the top of the planetary boundary layer. The vertical extent of these wind profiles is limited by the availability of scatterers and thus varies substantially throughout the day and from one day to the next. This makes it challenging to develop continuous products that rely on CDL-observed wind profiles. In order to overcome this problem, we have developed a new method for wind profile retrievals from CDL that combines the traditional velocity-azimuth display (VAD) technique with optimal estimation (OE) to provide continuous wind profiles up to 3 km. The new method exploits the level-to-level covariance present in the wind profile to fill in the gaps where the signal-to-noise ratio of the CDL return is too low to provide reliable results using the traditional VAD method. Another advantage of the new method is that it provides the full error covariance matrix of the solution and profiles of information content, which more easily facilitates the assimilation of the observed wind profiles into numerical weather prediction models. This method was tested using yearlong CDL measurements at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) Central Facility in 2019. Comparison with the ARM operational CDL wind profile product and collocated radiosonde wind measurements shows excellent agreement (R2>0.99) with no degradation in results where the traditional VAD provided a valid solution. In the region where traditional VAD does not provide results, the OE wind speed and wind vector have uncertainties of 3.44 and 4.33 m s−1, respectively. As a result, the new method provides additional information over the standard technique and increases the effective range of existing CDL systems without the need for additional hardware.

Field programmable gate array-based coherent lidar employing the ordinal statistics method for fast Doppler frequency determination

We report the development of a portable coherent Doppler lidar (CDL) system incorporating onboard field programmable gate array for real-time data processing. An ordinal statistic-based mean Doppler frequency estimation algorithm with high accuracy and low computational cost is proposed. The basic characteristics of the proposed method were specified with the use of mathematical modeling through the algorithm, realized as MATLAB computer code that includes parts simulating lidar signals and data processing. A comparison of the proposed method with established and well-used determination methods of the maximum Doppler frequency is accomplished. The ordinal statistics method is easy to implement and facilitates obtaining computational results swiftly. To demonstrate the performance of this lidar system, an intercomparison field campaign between lidar and ultrasonic anemometer is carried out. The excellent degree of correlation experimentally confirmed the feasibility of developing affordable, low-power consumption, compact, and robust CDL systems for various industrial and scientific applications.

Effects of buildings on wind shear at the airport: Field measurement by coherent Doppler lidar

Wind shear has a negative impact on aircraft safety, particularly during takeoff and landing processes. The buildings inside or around the airport can induce wind shear by disturbing the airflow along the glide path. This paper investigates the building-induced wind shear along the glide path of aircraft by utilizing two coherent Doppler lidar systems deployed at Beijing Capital International Airport. A well-designed coordinated scanning strategy of lidar was used to measure the building-induced wind shear along the glide path and the wind field influenced by buildings. With a correlation coefficient of 0.97 for wind speed and 0.99 for wind direction, the wind field estimated by lidar was well validated by those from sonic anemometers. In addition, the lidar-observed wind shear events are verified by pilot reports. The results show that building wakes bring the velocity deficit and turbulence increase along the glide paths. It is susceptible to wind shear due to the rapidly changing wind speeds when the aircraft flies through these wake regions. The relative frequencies of wind shear events reached above 70 and 50 at 36L and 01, respectively, in the wind direction range of 270°∼300° with a wind speed greater than 12 m/s. The statistical analysis of lidar-observed wind shear reveals that the wind shear frequently occurs within the height of 40 ∼ 80 m and the distance of 763 ∼ 1526 m at both 36L and 01.

Statistical interpolation technique based on coherent Doppler lidar for real-time horizontal wind shear observations and forewarning

Coherent Doppler lidar (CDL) has been considered a powerful and reliable tool for horizontal wind shear monitoring since the 1990s. However, radial observations obtained by Doppler lidar are limited in determining sophisticated variations in horizontal wind field and thus the wind shear. In our work, this problem is solved by applying a statistical interpolation technique to a CDL system, which is implemented at Lanzhou Zhongchuan Airport (103°E, 36°03′N) since August 2019, for routine observation of the horizontal shear along the pathway of the aircraft. By means of that, the horizontal wind vectors are retrieved in real time and the wind shear is also obtained simultaneously. Compared with radial observations, the accuracy of real-time horizontal wind vectors is indicated with the root mean squared error of 0.504 m · s − 1. During the field campaign, a typical wind shear induced by wind gust is obtained by the CDL system, which moves from south to north with the maximum up to 9.6 m · s − 1 · km − 1. Also, five cases resulting in flights going around are described in detail and compared with the reports from the airport terminal control office. The results reveal that the CDL system is quite suitable for wind shear forewarning for demonstrating the time, position, and intensity of wind shear with acceptable accuracy.

Research on technology of 1550nm all-fiber continuous wave coherent Doppler wind lidar

In order to obtain the telemetry data of wind field 80 m ahead of the wind turbine, an all-fiber continuous wave coherent Doppler lidar (CW-CDL) system is demonstrated in this paper. The CW-CDL of all-fiber structure has advantages of the compact structure, good stability and low cost. However, there is inherent crosstalk in the optical circulator of the optical fiber module, the crosstalk light and the reference light enter the photon detector together to produce phase induced intensity noise (PIIN). Aiming to effectively solve the noise, we proposed a polarization control method by combining a polarization beam splitter and a quarter wave plate to reduce the PIIN in the system. The wind speed Doppler signal at 80 meters is obtained on a sunny day, and the typical signal-to-noise ratio can reach 21.7 dB.

Low-Level Wind Shear Identification along the Glide Path at BCIA by the Pulsed Coherent Doppler Lidar

Low-level wind shear is usually to be a rapidly changing meteorological phenomenon that cannot be ignored in aviation security service by affecting the air speed of landing and take-off aircrafts. The lidar team in Ocean University of China (OUC) carried out the long term particular researches on the low-level wind shear identification and regional wind shear inducement search at Beijing Capital International Airport (BCIA) from 2015 to 2020 by operating several pulsed coherent Doppler lidar (PCDL) systems. On account of the improved glide path scanning strategy and virtual multiple wind anemometers based on the rang height indicator (RHI) modes, the small-scale meteorological phenomenon along the glide path and/or runway center line direction can be captured. In this paper, the device configuration, scanning strategies, and results of the observation data are proposed. The algorithms to identify the low-level wind shear based on the reconstructed headwind profiles data have been tested and proved based on the lidar data obtained from December 2018 to January 2019. High spatial resolution observation data at vertical direction are utilized to study the regional wind shear inducement at the 36L end of BCIA under strong northwest wind conditions.

All-fiber coherent lidar Doppler imaging based on phase correction with matching method

For a coherent Doppler lidar system, it is extremely important to accurately extract the heterodyne signal while improving the system’s speed measurement accuracy. In this paper, All-fiber coherent lidar Doppler image based on phase correction with matching method is proposed to solve this problem. First, the interpolation method is adopted to improve the system speed measurement accuracy. In addition, the matching method is adopted to initially locate the target heterodyne signal. Finally, the phase correction is adopted to accurately locate the heterodyne signal to extract the target speed value. In order to verify the method, a preliminary simulation analysis of the simulated heterodyne signal localization is performed. Based on this, real experimental data were obtained by pulse heterodyne coherent laser imaging radar, and the correctness of the proposed method was verified by comparison. The results show that the method improves the velocity accuracy of weak signal coherent Doppler lidar.

A Compact, Flexible, and Robust Micropulsed Doppler Lidar

AbstractThis work details a master oscillator power amplifier (MOPA) microjoule-class pulsed coherent Doppler lidar system configuration designed to measure line-of-sight wind velocities and backscatter intensity of atmospheric aerosols. The instrument is unique in its form factor. It consists of two physically separated modules connected by a 10 m umbilical cable. One module hosts the transceiver, which is composed of the telescope, transmit/receive (T/R) switch, and high-gain optical amplifier, and is housed in a small box (34.3 cm × 34.3 cm × 17.8 cm). The second module contains the data acquisition system and several electro-optical components. This form factor enables deployments on platforms that are otherwise inaccessible by commercial and research instruments of similar design. In this work, optical, electrical, and data acquisition components and configurations of the lidar are detailed and two example deployments are presented. The first deployment describes measurements of a controlled wildfire burn from a small aircraft to measure vertical plume dynamics and fire inflow conditions during summer in Florida. The second presents measurements of the marine boundary layer height and vertical velocity and variance profiles from the Research Vessel (R/V) Thomas Thompson. The new instrument has enabled greater flexibility in field campaigns where previous instruments would have been too costly or space prohibitive to deploy.

Performance validation on an all-fiber 1.54-μm pulsed coherent Doppler lidar for wind-profile measurement

The characteristics and capability of a homemade all-fiber 1.54-μm pulsed coherent Doppler lidar (CDL) were validated in field experiments by comparing the detection results with a collocated lidar and sounding balloons. With the range gate of 30 m and temporal resolution of 16 s at velocity–azimuth display mode, the detection capability of the CDL ranged from 0.1 to 5 km, and the time sequence and height position of this CDL were calibrated by the collocated lidar. In the intercomparison experiments with sounding balloons, the discrepancy of 30-s averaged measurement results of horizontal wind speed and wind direction was nearly 0.7 m / s and 5.3 deg, respectively. The good agreement achieved in such a short averaged time period was a convincing case of intercomparison experiments between CDL and sounding balloon. The CDL system demonstrated good reliability and operational stability in field experiments.

Vehicular Motion Experiment and Data Retrieval of a Compact Floating Lidar System

Accurate and rapid observation of sea surface wind is important for the research of ocean dynamic prediction model, offshore wind resource assessment, air-sea interaction and flux. A compact floating coherent Doppler lidar system named WindMast 350-M was developed by Ocean University of China (OUC) and Leice Transient Technology Co. LTD (LEICE) for the observations of sea surface wind profiles. As an observation device installed on buoy platforms, the first vehicular motion experiment was conducted at Laoshan campus of Ocean University of China(120.49°E , 36.16°N) on 06 and 12 March, 2019. During the first experiment, the wind profiles measured by the WindMast 350-M were compared with the results from a well calibrated Ground-based Coherent Doppler lidar WindMast WP350. In this contribution, the systematic design and the specifications of 350-M are presented in detail. The preliminary results of the vehicular motion experiment are discussed as well.

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.

Wind turbine load validation using lidar‐based wind retrievals

AbstractWe define and demonstrate a procedure for carrying out wind turbine load validation based on measurements from nacelle‐mounted scanning lidars. Two coherent Doppler lidar systems, a pulsed lidar and a continuous‐wave lidar, are mounted on a 2.3‐MW wind turbine equipped with load measurement sensors. Wind measurements from a meteorological mast mounted at 2.5 rotor diameters distance are used as reference. The study shows how lidar measurements are processed and applied as inputs to aeroelastic load simulations, and the results are then compared with simulations where the wind inputs have been determined using the meteorological mast data in compliance with the IEC61400‐13 standard. For the majority of simulation cases considered, the use of nacelle‐mounted lidar measurements results in load estimation uncertainties lower or equal to those that are based on measurements from cup anemometers on the mast. These results demonstrate the usefulness of nacelle‐mounted lidars as tools for carrying out load validation without the need of meteorological masts.

A Hardware Implemented Autocorrelation Technique for Estimating Power Spectral Density for Processing Signals from a Doppler Wind Lidar System.

A signal processing technique utilizing autocorrelation of backscattered signals was designed and implemented in a 1.5 µm all-fiber wind sensing Coherent Doppler Lidar (CDL) system to preprocess atmospheric signals. The signal processing algorithm’s design and implementation are presented. The system employs a 20 kHz pulse repetition frequency (PRF) transmitter and samples the return signals at 400 MHz. The logic design of the autocorrelation algorithm was developed and programmed into a field programmable gate array (FPGA) located on a data acquisition board. The design generates and accumulates real time correlograms representing average autocorrelations of the Doppler shifted echo from a series of adjustable range gates. Accumulated correlograms are streamed to a host computer for subsequent processing to yield a line of sight wind velocity. Wind velocity estimates can be obtained under nominal aerosol loading and nominal atmospheric turbulence conditions for ranges up to 3 km.

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.

Signal to Noise Ratio Characterization of Coherent Doppler Lidar Backscattered Signals

An eye-safe coherent Doppler Lidar (CDL) system for wind measurement was developed and tested at the Remote Sensing Laboratory of the City College of New York (CCNY). The system employs a 1542 nm fiber laser to leverage components’ availability and affordability of the telecommunication industry. A balanced detector with a bandwidth extending from dc to 125 MHz is used to eliminate the common mode relative intensity noise (RIN). The system is shot noise limited i.e., the dominant component of received signals’ noise is the shot noise. Wind velocity can be measured under nominal aerosol loading and atmospheric turbulence conditions for ranges up to 3 km while pointing vertically with 0.08 m/s precision.

Observations of Wind Profile of Marine Atmosphere Boundary Layer by Shipborne Coherent Doppler Lidar

Pulsed Coherent Doppler Lidar (CDL) system is so good as to prove the feasibility of the marine atmosphere boundary layer detection. A ship-mounted Coherent Doppler lidar was used to measure the wind profile and vertical velocity in the boundary layer over the Yellow sea in 2014. Furthermore, for the purpose of reducing the impact of vibration during movement and correcting the LOS velocity, the paper introduces the attitude correction algorithm and comparison results.

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.

Development and Operational Analysis of an All-Fiber Coherent Doppler Lidar System for Wind Sensing and Aerosol Profiling

A 1.5- $\mu\mbox{m}$ all-fiber coherent Doppler lidar system for wind sensing was developed and characterized. The system design and implementation is presented. The system employs a 20-kHz pulse repetition rate transmitter and samples return signals at 400 MHz. A field-programmable gate array is programmed to generate and accumulate real-time periodograms representing average power spectra of the Doppler shifted echo from a series of more than 60 range gates, where each range gate is 48 m. The accumulated periodograms are streamed to a host computer for subsequent processing to yield the line-of-sight wind velocity. Wind velocity estimates with a precision value of 0.08 m/s can be made under nominal aerosol loading and nominal atmospheric turbulence conditions for ranges up to 3 km. Wind velocities and aerosol profiles are obtained while scanning the pointing direction of the lidar, thus producing both horizontal and vertical representations of the wind vectors.

Diode-end-pumped continuously tunable single frequency Tm, Ho:LLF laser at 206 μm

We report on a laser diode-end-pumped continuously tunable single frequency Tm, Ho:LLF laser near room temperature. For transmission of 5%, the maximum single frequency output power of 221 mW at 2064.4 nm was obtained by using two uncoated etalons. The single frequency Tm, Ho:LLF laser operated on the fundamental transverse mode with an M2 factor of 1.13, and the output frequency could be tuned continuously near 1.5 GHz by angle tuning only of the 1 mm thick etalon. Furthermore, the influence of output coupler transmission on the laser performance was also investigated. The single frequency laser can be used as a seed laser for coherent Doppler lidar and differential absorption lidar systems.

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.