rus
Issue #6/2014
М. Andreev, D. Vasiliev, M. Penkin, S. Smolentsev, A. Boreysho, D. Klochkov, M. Konyaev, A. Orlov, A. Chugreev
Coherent Doppler Lidars For Wind Monitoring
Coherent Doppler Lidars For Wind Monitoring
Coherent Doppler lidars operation modes for continuous monitoring wind conditions at low and medium attitudes, their technical and metrological characteristics are provided.
Теги: air service safety doppler lidar heterodyne detection wind lidar wind-shear безопасность авиаперевозок ветровой лидар гетеродинное детектирование допплеровский лидар сдвиг ветра
W
ind lidar (LIDAR – LIght Detection and Ranging) is intended for measuring wind speed and direction within a few kilometers range, as well as detecting hazardous weather conditions in a clean atmosphere. Lidar monitoring of wind situations is used for aviation, wind power plants, and etc. safety assistance. Doppler lidar, as well as Doppler radar, determines the speed of a moving object by measuring the frequency shift of back-scattered electromagnetic radiation. The short-wave meteorological radar allows to measure wind parameters when it rains, when it is snowing, under cloud or dense atmospheric aerosol. At high air transparency there are present some dangerous meteorological conditions, such as microflaws, gusts and atmospheric turbulence, their visualization is possible by the lidars sensing. An important task is detection of the aircraft trail near the flight strip [1,2].
At 2–3km attitudes there always is an Earthe’s boundary natural atmospheric aerosol layer with 0.1–10µm particles, the sizes of which correspond to optical wavelengths. According to the Mi scattering theory this is responsible for more efficient laser beam reflection in clean atmosphere in comparison with radio waves.
To date in the world there are only few enterprises that produce commercial available wind lidars. The most successful products are lidars series Windcube (Leosphere, France), Zephir (Qinetiq, UK), Galion (SgurrEnergy, UK) and WindTracer (CTI Lockheed Martin, USA). In Russia the national developer and the only serial manufacturer of wind lidars is Laser Systems Ltd. (St. Petersburg).
Its development started in 2008, and in five years a wind lidar for monitoring wind situation at small attitudes in order to provide safety at aircraft take-offs and landings received the Interstate Aviation Committee (IAC) certificate. Laser Systems’ Lidars PLV300 and PLV2000 operate in the eye-safe 1.55-µm wavelength.
PRINCIPLES OF OPERATION OF CW AND PULSED COHERENT DOPPLER LIDARS
Under laser beam reflection at aerosol particles carried by the wind a Doppler shift of the lightwave frequency is present [3]. A difference between the laser radiation frequency and the received signal corresponds to the radial component of the wind speed:
,
Where Vr is the radial component of wind speed, i. e. projection of the wind speed instantaneous vector V = {Vx, Vy, Vz} on the sensing direction; – directly measured Doppler shift of laser radiation frequency, λ – probe radiation wavelength.
At 1.55µm wavelength the frequency shift corresponding to 1 m/s radial speed is nearly equal to 1.3MHz. The measurement of the frequencies difference of sensing and back reflected optical radiation in lidars PLV300 and PLV2000 is carried out by heterodyne detection. By optical mixing the accepted radiation and reference light frequencies the photoelectric detector registers an optical component that corresponds to the frequencies difference. The maximum Doppler peak of the Fourier-transformed signal corresponds to the wind speed in the studied air space (Fig. 1). In detail the algorithm of frequency spectrum peak finding in is described in [4].
The spatial resolution of these measurements is determined by the laser operating mode, the lidars optical arrangement and data processing algorithms. There are two types of lidars – CW and pulsed lidars, based in accordance on continuous and pulsed laser sources.
In the case of a Continuous Wave (CW) lidar the distance along the measurement trace is defined by the varifocal transmitting–receiving telescope laser beam focusing field. Spatial resolution (Fn) of the CW lidar can be calculated from half width of the Lorentz weighting function (along to the beam axis) that determines the focal point scattering efficiency:
, (1)
where f is the focus distance, r – distance along the beam axis, Zr – Rayleigh waist length.
Using of CW Doppler lidar with such a circuit and a 100 mm telescope for wind speed profiling at low altitudes (10–150 meters) allows accurate measurements with high range resolution (1–20 meters). The dependence of the half width of the Lorentz weighting function on focusing distance is quadratic so the space resolution decreases with height increase (Fig.2).
The pulsed lidar can perform measurements at high altitudes. The lidar’s spatial resolution (measurement trace discretization) is defined by the laser pulse duration and it is constant over all measurement altitudes (Fig.2). For carrying out measurements the pulse duration must not be less then 400ns. This value corresponds to the spatial resolution 60m. Using shorter pulses is unadvisable because this would decrease the accuracy of wind speed measurements: due to the Heisenberg uncertainty principle the shorter optical pulse, the broader is its frequency spectrum
With measurement distances less then 300m the CW lidar spatial resolution is preferable ensuring it is an essential instrument for detecting low attitude wind shear and (or) microburst winds in the glide path critical segments that are extremely dangerous whilst aircraft taking-off or landing. At attitudes more then 300m pulse lidar PLW2000 gains benefits due to constant spatial resolution.
PLW300 AND PLW2000 DESIGN
Laser systems Ltd. has developed a series of autonomous lidars [2] for continual wind monitoring (Fig.3). The specifications of lidars are listed in Table.
The block diagram of coherent Doppler lidars PLW300 and PLW2000 is shown on Fig.4. Balanced optical detector that is used in the detection scheme allows to select the difference frequency small signal that is then amplified and sampled be high speed ADC at 100MHz (PLW300) and 32 MHz (PLW2000). To get Doppler spectra a Fast Fourier Transform (FFT) of the signal is fulfilled by the Field Programmable Gate Array (FPGA) in real time.
When the sensing beam axis is fixed the lidar measures only the radial component of the wind speed vector. To obtain the whole information of wind speed and direction V = {Vz, Vx, Vy} it is necessary to carry out measurements at least in three different sensing beam directions.
PLW300 lidar uses conical (or VAD, velocity-azimuth-display) scan. The angle between zenith and beam axis is 22 degrees. Transmitting and receiving telescope has the variable point of focusing (Fig 6a). The vector of the wind is calculated from the measured line-of-sight velocities by the non-linear least squares fitting procedure [5].
Two-mirror scanner of PLW2000 can aim the beam in any direction of upper hemisphere, which allows different schemes of scanning:
Plan Position Indicator (full circle or sector) (PPI) – scan angle is varied continuously in azimuth with a fixed elevation angle;
Range Height Indicator (RHI) – elevation angle is scanned in any range at a fixed azimuth angle;
Vertical wind profiler (DBS, VAD) – measurements are made at several discrete azimuth angles (DBS, Doppler Beam Steering) or continuously scanned azimuth angle with 30-degree elevation angle.
Line-of-site (LOS) – azimuth and elevation angle are fixed during measurements.
CERTIFICATION TESTS OF PLW300
Certification tests of PLW300 have been carried out at the location of WMM-310 "Typhoon" (Fig.6b) weather station. The station is equipped with the MK-15 ultrasonic anemometers on the meteorological mast. The sensors were installed at the levels 8, 25, 73, 121, 217, and 301 m.
Tests have been carried out few times over the year at the different meteorological conditions. Maximal absolute errors of the wind speed and directionmeasurementsfor PLW300 did not exceedthe required limits (±10% for speed measurements, and ±10° for the accuracy of direction measurements) [6]. The resultof taken measurements bythe lidarand the ultrasonic anemometersare shown on Fig. 7 and 8. On the basis of testing PLW300 was certified by theInterstate Aviation Committee (IAC) as the equipment for aviation safety (Certificate number 544).
SUMMARY
The eye-safe spectral range wind lidarsprovide the information on the direction and the speed of the wind and visualize the uniformities of wind fields. The PLW300 lidar allows to detectgust fronts and wind shifts at low altitudes (up to 300 meters) with the high spatial resolution. The purpose of the pulsed wind lidar PLW2000 is the remote measurements of wind speed and direction at distances up to 2 kilometers and more as well as 3D scans of wind fields.
ind lidar (LIDAR – LIght Detection and Ranging) is intended for measuring wind speed and direction within a few kilometers range, as well as detecting hazardous weather conditions in a clean atmosphere. Lidar monitoring of wind situations is used for aviation, wind power plants, and etc. safety assistance. Doppler lidar, as well as Doppler radar, determines the speed of a moving object by measuring the frequency shift of back-scattered electromagnetic radiation. The short-wave meteorological radar allows to measure wind parameters when it rains, when it is snowing, under cloud or dense atmospheric aerosol. At high air transparency there are present some dangerous meteorological conditions, such as microflaws, gusts and atmospheric turbulence, their visualization is possible by the lidars sensing. An important task is detection of the aircraft trail near the flight strip [1,2].
At 2–3km attitudes there always is an Earthe’s boundary natural atmospheric aerosol layer with 0.1–10µm particles, the sizes of which correspond to optical wavelengths. According to the Mi scattering theory this is responsible for more efficient laser beam reflection in clean atmosphere in comparison with radio waves.
To date in the world there are only few enterprises that produce commercial available wind lidars. The most successful products are lidars series Windcube (Leosphere, France), Zephir (Qinetiq, UK), Galion (SgurrEnergy, UK) and WindTracer (CTI Lockheed Martin, USA). In Russia the national developer and the only serial manufacturer of wind lidars is Laser Systems Ltd. (St. Petersburg).
Its development started in 2008, and in five years a wind lidar for monitoring wind situation at small attitudes in order to provide safety at aircraft take-offs and landings received the Interstate Aviation Committee (IAC) certificate. Laser Systems’ Lidars PLV300 and PLV2000 operate in the eye-safe 1.55-µm wavelength.
PRINCIPLES OF OPERATION OF CW AND PULSED COHERENT DOPPLER LIDARS
Under laser beam reflection at aerosol particles carried by the wind a Doppler shift of the lightwave frequency is present [3]. A difference between the laser radiation frequency and the received signal corresponds to the radial component of the wind speed:
,
Where Vr is the radial component of wind speed, i. e. projection of the wind speed instantaneous vector V = {Vx, Vy, Vz} on the sensing direction; – directly measured Doppler shift of laser radiation frequency, λ – probe radiation wavelength.
At 1.55µm wavelength the frequency shift corresponding to 1 m/s radial speed is nearly equal to 1.3MHz. The measurement of the frequencies difference of sensing and back reflected optical radiation in lidars PLV300 and PLV2000 is carried out by heterodyne detection. By optical mixing the accepted radiation and reference light frequencies the photoelectric detector registers an optical component that corresponds to the frequencies difference. The maximum Doppler peak of the Fourier-transformed signal corresponds to the wind speed in the studied air space (Fig. 1). In detail the algorithm of frequency spectrum peak finding in is described in [4].
The spatial resolution of these measurements is determined by the laser operating mode, the lidars optical arrangement and data processing algorithms. There are two types of lidars – CW and pulsed lidars, based in accordance on continuous and pulsed laser sources.
In the case of a Continuous Wave (CW) lidar the distance along the measurement trace is defined by the varifocal transmitting–receiving telescope laser beam focusing field. Spatial resolution (Fn) of the CW lidar can be calculated from half width of the Lorentz weighting function (along to the beam axis) that determines the focal point scattering efficiency:
, (1)
where f is the focus distance, r – distance along the beam axis, Zr – Rayleigh waist length.
Using of CW Doppler lidar with such a circuit and a 100 mm telescope for wind speed profiling at low altitudes (10–150 meters) allows accurate measurements with high range resolution (1–20 meters). The dependence of the half width of the Lorentz weighting function on focusing distance is quadratic so the space resolution decreases with height increase (Fig.2).
The pulsed lidar can perform measurements at high altitudes. The lidar’s spatial resolution (measurement trace discretization) is defined by the laser pulse duration and it is constant over all measurement altitudes (Fig.2). For carrying out measurements the pulse duration must not be less then 400ns. This value corresponds to the spatial resolution 60m. Using shorter pulses is unadvisable because this would decrease the accuracy of wind speed measurements: due to the Heisenberg uncertainty principle the shorter optical pulse, the broader is its frequency spectrum
With measurement distances less then 300m the CW lidar spatial resolution is preferable ensuring it is an essential instrument for detecting low attitude wind shear and (or) microburst winds in the glide path critical segments that are extremely dangerous whilst aircraft taking-off or landing. At attitudes more then 300m pulse lidar PLW2000 gains benefits due to constant spatial resolution.
PLW300 AND PLW2000 DESIGN
Laser systems Ltd. has developed a series of autonomous lidars [2] for continual wind monitoring (Fig.3). The specifications of lidars are listed in Table.
The block diagram of coherent Doppler lidars PLW300 and PLW2000 is shown on Fig.4. Balanced optical detector that is used in the detection scheme allows to select the difference frequency small signal that is then amplified and sampled be high speed ADC at 100MHz (PLW300) and 32 MHz (PLW2000). To get Doppler spectra a Fast Fourier Transform (FFT) of the signal is fulfilled by the Field Programmable Gate Array (FPGA) in real time.
When the sensing beam axis is fixed the lidar measures only the radial component of the wind speed vector. To obtain the whole information of wind speed and direction V = {Vz, Vx, Vy} it is necessary to carry out measurements at least in three different sensing beam directions.
PLW300 lidar uses conical (or VAD, velocity-azimuth-display) scan. The angle between zenith and beam axis is 22 degrees. Transmitting and receiving telescope has the variable point of focusing (Fig 6a). The vector of the wind is calculated from the measured line-of-sight velocities by the non-linear least squares fitting procedure [5].
Two-mirror scanner of PLW2000 can aim the beam in any direction of upper hemisphere, which allows different schemes of scanning:
Plan Position Indicator (full circle or sector) (PPI) – scan angle is varied continuously in azimuth with a fixed elevation angle;
Range Height Indicator (RHI) – elevation angle is scanned in any range at a fixed azimuth angle;
Vertical wind profiler (DBS, VAD) – measurements are made at several discrete azimuth angles (DBS, Doppler Beam Steering) or continuously scanned azimuth angle with 30-degree elevation angle.
Line-of-site (LOS) – azimuth and elevation angle are fixed during measurements.
CERTIFICATION TESTS OF PLW300
Certification tests of PLW300 have been carried out at the location of WMM-310 "Typhoon" (Fig.6b) weather station. The station is equipped with the MK-15 ultrasonic anemometers on the meteorological mast. The sensors were installed at the levels 8, 25, 73, 121, 217, and 301 m.
Tests have been carried out few times over the year at the different meteorological conditions. Maximal absolute errors of the wind speed and directionmeasurementsfor PLW300 did not exceedthe required limits (±10% for speed measurements, and ±10° for the accuracy of direction measurements) [6]. The resultof taken measurements bythe lidarand the ultrasonic anemometersare shown on Fig. 7 and 8. On the basis of testing PLW300 was certified by theInterstate Aviation Committee (IAC) as the equipment for aviation safety (Certificate number 544).
SUMMARY
The eye-safe spectral range wind lidarsprovide the information on the direction and the speed of the wind and visualize the uniformities of wind fields. The PLW300 lidar allows to detectgust fronts and wind shifts at low altitudes (up to 300 meters) with the high spatial resolution. The purpose of the pulsed wind lidar PLW2000 is the remote measurements of wind speed and direction at distances up to 2 kilometers and more as well as 3D scans of wind fields.
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