ECE 583 Lectures 23 LIDAR Continued Wind Sensing

ECE 583 Lectures 23 LIDAR Continued Wind Sensing 1 2 Types of LIDAR •DIAL •Raman LIDAR •Wind LIDAR •HSRL LIDAR •Spaceborne LIDAR 3 Doppler win...
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ECE 583 Lectures 23 LIDAR Continued Wind Sensing

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Types of LIDAR •DIAL •Raman LIDAR •Wind LIDAR •HSRL LIDAR •Spaceborne LIDAR

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Doppler winds

The Doppler effect provides a way of measuring the motions of scatterers –used as ‘tracers’ of the wind, and may vary from molecules sub-micron aerosol, to cloud and precipitation, to insects to refractive index fluctuations of many meters in size depending on the lidar or radar wavelengths.

O’

θ k’x’-ω’t’

kx-ωt ν′ = ν v 0

when O’ moves toward source

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Velocity-azimuth-display VAD)

v r = v h cos(φ − φo ) cos ε + v f sin ε at max and min scan positions v u = v h cos ε + v f sin ε v d = − v h cos ε + v f sin ε so vu − vd vh = 2 cos ε

vu + vd vf = 2 sin ε 5

Doppler Lidar • Power received by a Doppler lidar is dependent on the same things as an atmospheric backscatter lidar • Need to distinguish Doppler changes in the frequency of the returned light versus that of the outgoing light • There are two fundamentally different approaches to Doppler lidar – -Coherent, or heterodyne, Doppler Lidar which uses particulate return signals -Incoherent, or direct, Doppler lidar which primarily uses molecular return signals 6

NOAA Working Group on Space-Based Lidar Winds

15th Coherent Laser Radar Conference

http://space.hsv.usra.edu/workshops.html

Toulouse, France

June 22 - 26, 2009

Visit the conference web pages

In about 2013 ESA Aeolus will be the first space borne wind lidar

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(~ 4 pm mid-visible)

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Doppler Lidar Measurement principle

Detect DetectDoppler Dopplershift shiftof of backscatter backscattersignal signalas asfunction functionof of time timeafter afterpulse pulseemission emission 9

Ways to Evaluate Spectral Shifts • Coherently (Heterodyning) – Mix return signal with light that has the frequency of the outgoing signal and look for beat signals

• Incoherently – Fringe Imaging: Images the fringes from an interferometer allowing the spectrum to be plotted – Edge Technique: Using a narrow frequency filter to measure changes in transmission due to frequency changes

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Doppler Lidar Receivers • Coherent or heterodyne detection •Developed in the 1970’s with CO2 gas laser technology at ~9.6 um by NOAA and Coherent Technologies Inc. •Solid state 1.06 and 2.1 um systems were developed in the 1990’s at Stanford but 1.06 um systems are very non-eyesafe.

• Direct or non-coherent detection • Initially developed by Italians and first practical measurement were by the French and a Un. Of Michigan group in the 1980’s – later Michigan Aerospace. •Proposed currently for eyesafe operation at 355 nm using molecular or aerosol backscattered signal by two competing spectral methods -Fringe imaging approach -Edge filter technique 11

What Is Coherent Lidar? •

• •



Coherent (heterodyne) detection of weak signal with a strong, stable reference laser (local oscillator) increases SNR to approach theoretical best performance and rejects background light Frequency of beat signal is proportional to the target velocity - truly a direct measurement of velocity Translation of optical frequency to radio frequency allows signal processing with mature and flexible electronics and software, and reduces 1/f noise Extremely narrow bandpass filter using electronics or software rejects even more noise Reference – Menzies and Hardesty, 1989

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Coherent Doppler Lidar + High photon efficiency + Insensitive to solar background light

• Requires aerosol backscatter (no molecular version) The wide frequency width of the molecular signal results in molecular scattering being a noise component, rather than signal for coherent lidar detection.

" Simplified heterodyne receiver. The incoming signal is mixed with a very stable local oscillator (LO) ... -60 -70

Amplitude (db)

• Measured signal is RF ‘beat’ frequency of atmospheric signal and local oscillator

-80 -90 -100 -110 -120 -130 -140 0

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Frequency (Hz)

Unlike Doppler Radar, a wind profile may be measured from a single pulse return.

" … to produce a ‘beat’ frequency proportional to Doppler shift 13

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[about 5 uradians for a .5m aperture]

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Examples of Operating Doppler Lidar Instruments: Coherent 1

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Examples of Coherent Doppler Wind Lidar Data NASA/MSFC

NOAA/ETL

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Due to Lack of Sufficient Aerosol Cross Section, Coherent Lidar Does Not Operate Reliably in the Upper Atmosphere

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Direct Detection Doppler Lidar • Measured signal is proportional to intensity • High resolution optical filter used to measure Doppler shift • Draws on technology used with other space lidars (MOLA, GLAS, VCL, Picasso) • Well developed solid state lasers • Large aperture ‘light bucket ‘ telescopes • Photon counting detectors

• Shot averaging to increase S/N • Utilizes aerosol or molecular backscatter • Molecular provides clear air winds in free troposphere/over oceans

• 2 primary implementations ‘Double Edge’ and ‘Fringe Imaging’ 23

What comes back to a lidar receiver from the atmosphere really? Without wind Doppler Broadened Backscatter Spectrum

With wind Doppler Shifted and Broadened Backscatter Spectrum

Aerosol (λ−2)

Red shift

Blue shift

Molecular (λ−4)

Frequency

Frequency

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Fringe Imaging Doppler Receiver Concept

Incoming signal

1. Incoming light is imaged through the FP etalon onto a CCD array 2. Doppler frequency shift is proportional to the change in the radius of the etalon fringe*

Dreturn ∝ λreturn Dout ∝ λout ΔλDop = λout-λreturn

Fabry Perot etalon

Imaging Detector (CCD)

•Several methods have been proposed to map the circular fringes to the rectangular CCD One is a circle-to-line converter, a half cone shaped reflector.

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Fabry-Perot Etalon as Filter

For transmission we have a maximum when:

2nd cos(θ ) = mλ 26

Double Edge Measurement Concept 1. Incoming light is collimated, split into 2 channels and sent through the FP etalon. The light in each channel is focussed to a photon counting detector giving signals I1 and I2. 2. The Doppler frequency shift is proportional to the change in the ratio of the measured signals I1/I2 which varies as the laser wavelength moves up and down on the steep edge of the filters. λ

Fabry Perot etalon

Incoming signal

I2(λ)

λout ∝ ( I1/I2)out λreturn ∝ ( I1/I2)return ΔλDop = λout-λreturn

I2(λ)

27 Aerosol Channel at 1064 nm Molecular Channel at 355 nm

How Does the Edge Technique Work? • Steep slope of a narrow frequency filter is used to convert changes in frequency into changes in intensity. • Outgoing laser pulse is sampled and used to determine what the outgoing frequency is. • Return signal is then compared to the outgoing frequency to determine the Doppler shift.

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Schematic of an Edge Detection Receiver I FC (ν ) I N (ν ) = = C cal * F (ν ) I EM Beam Splitter

Pin from telescope

Narrow Frequency Filter TBS

C

RBS

TN(ν)·TFPIpk

TBPF

P FPI DET G, φ

TBPFo Po Co

EM DET Go, φo

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Change in Intensity Through a Narrow Filter due to Frequency Shift (Edge Technique) Δν

Doppler

=

I C

cal

N

ΔIN (ν + Δ ν ) − I N (ν ) = ⋅ m (ν , ν + Δ ν Doppler ) C cal ⋅ m

•(Where m is the average slope over the interval ν to ν+Δν) Normalized Transmission

1.0

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IN(ν)

.50

ΔIN

IN (ν +Δ ν)

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R E F

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S I G

ΔνDoppler 50

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Relative Frequency in MHz 30

Limitations on the Edge Technique • Simplest form uses sharp aerosol return to measure the Doppler shift. – Broad molecular return acts as a systematic offset to the velocity measurement – Offset varies as the amount of aerosols to molecules varies.

• Systems using the broader molecular return or both the aerosol and molecular return are far more complicated and costly to develop. 31

GLOW- Goddard Lidar Observatory for Winds

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Direct Detection Doppler Lidar Development Roadmap TWiLiTE Airborne Doppler Lidar 20

WB57

ER2

Altitude (km)

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10 TWiLiTE configured for WB57 3’ Pallet (ESTO IIP04)

GLOW mobile lidar 5

(SMD,NMP,IPO, ARO, ROSES07-WLS)

TWiLiTE configured for ER2 Q-Bay (ESTO IIP04)

TWiLiTE configured for DC8 Nadir Port 7 (ROSES07-AITT)

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Tropospheric Wind Lidar Technology Experiment (TWiLiTE) Instrument Incubator Program •The TWiLiTE instrument is a compact, rugged direct detection scanning Doppler lidar designed to measure wind profiles in clear air from 18 km to the surface. • TWiLiTE operates autonomously on NASA research aircraft (ER-2, DC-8, WB-57). • Initial engineering flight tests on the NASA ER-2 in February, 2009 demonstrated autonomous operation of all major systems. • TWiLiTE will mount in the DC-8 cargo bay using either the fore or aft nadir port.

TWiLiTE system configured for ER-2 QBay

TWiLiTE ER-2 Integration 34 February, 2009 34

TWiLiTE Instrument Parameters Wavelength Telescope/Scanner Area Laser Linewidth (FWHH) Laser Energy/Pulse (8 W) Etalon FSR Etalon FWHH Edge Channel Separation Locking Channel Separation Interference filter BW (FWHH) PMT Quantum Efficiency

354.7 nm 0.08 m2 150 MHz 40 mJ @ 200 pps 16.65 GHz 2.84 GHz 6.64 GHz 4.74 GHz 120 pm 25%

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ER-2 Engineering Flights Feb 17-27, 2009

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Examples of Operating Doppler Lidar Instruments: Fringe Imaging

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GroundWinds NH • Direct-detection Doppler lidar – – – –

Operates at 532 nm 3.5 W @ 10 Hz 0.5 meter aperture High-resolution molecular and aerosol channels – CCD detectors – 0.7-18 km range ASL 38

GroundWinds Hawaii • Direct-detection Doppler lidar – – – –

Operates at 355 nm 3.5 W @ 10 Hz 0.5 meter aperture High-resolution molecular and aerosol channels – CCD detectors – 4.0-21 km range ASL 39

Preliminary GWHI Validation Results Wind Speed

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the the ADM-Aeolus ADM-Aeolus

Mission Mission

sense sensethe thewind windaround aroundthe the globe globe Martin MartinEndemann Endemann Aeolus AeolusSystem System&&Instrument InstrumentManager Manager

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ISSUES FOR SPACEBORNE DOPPLER LIDAR •Winds must be made throughout the troposphere and lower stratosphere. •The range to the target is over 400 km. •The ~7000m velocity of the space craft must be compensated and corrected for. •The transmitted laser pulses must be eye safe on the ground.

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ESA Approach:

Keep it simple: Single Line-of-Sight wind profiles ƒ

Initial space borne wind lidar concepts proposed to measure two components of the wind vector. This requires conical scanner or two telescopes: • very complex design, • need for pointing correction mechanisms, • problem to subtract satellite velocity from wind vectors.

ƒ

A study to define best scan strategy (Lorenc, 1992) resulted in understanding that improvement of Numerical Weather Prediction is nearly independent on direction of wind components measured, i.e. same improvement for two single direction vectors as for one 2-D vector (mainly the number of boundary conditions for NWP models counts)

¾¾

The Themeasurement measurementof ofaasingle singlecomponent component(LOS) (LOS)wind windprofiles profileswill will simplify wind lidar instrument design significantly simplify wind lidar instrument design significantly

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Future global wind observations with ALADIN on the Atmospheric Dynamics Mission ADM-Aeolus

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ADM-Aeolus Implementation Atmospheric LAser Doppler INstrument ALADIN ƒ First Doppler lidar in space ƒ Operating in ultraviolett @ 355 nm to measure wind from molecular Rayleigh and aerosol/cloud Mie backscatter ƒ Line-of-Sight LOS wind profiles in troposphere to lower stratosphere with vertical resolution from 250 m - 2 km, averaged over 50 km every 200 km ƒ Requirement on HLOS: accuracy 1 m/s (0-2 km) and 2 m/s (2-16 km), bias 0.4 m/s, slope error 0.7 % ƒ LOS is pointing 35 ° from nadir orthogonal to the ground track velocity, yaw steering applied ƒ First High Spectral Resolution Lidar HSRL in space to obtain aerosol/cloud optical properties 45

Principle of Wind Measurement with ALADIN Atmospheric LAser Doppler INstrument ALADIN ƒ Direct-Detection Doppler Lidar at 355 nm with 2 spectrometers to analyse backscatter signal from molecules (Rayleigh) and aerosol/clouds (Mie) ƒ Double edge technique for spectrally broad molecular return, e.g. NASA GLOW instrument (Gentry et al. 2000), but sequential implementation Principle of spectrometer für molecular signal ƒ Fizeau spectrometer for spectrally small aerosol/cloud return ƒ ALADIN is a High-Spectral Resolution Lidar HSRL with 3 channels: 2 for molecular signal, 1 for aerosol/cloud signal => retrieval of profiles of aerosol/cloud optical properties possible ƒ backscatter coefficient ƒ extinction coefficient ƒ lidar ratio

principle of spectrometer für aerosol signal

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ADM-Aeolus Coverage and Data Availability ƒ 3200 wind profiles per day:

about factor 3 more than radiosondes ƒ 3 hour data availability after observation; 30 minutes data availability for parts of orbit ƒ X-Band data-downlink to Svalbard, processing center up to Level 1B in Tromsö (Norway), up to Level 2C at ECMWF ƒ launch planned for September 2008 ƒ mission lifetime 3 years: observations from 2009-2011

50 km Observations during 6 hour period Overview paper about ADM-Aeolus:Stoffelen et al. 2005, Bull. Am. Met. Soc.

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Aeolus Orbit and Measurement Track Aeolus measures HLOS-wind profiles averaged over 50 km observations (corresponding 7 s flight time). The observations are 200 km apart (corresponding 28 s flight time). Picture shows the measurement track over 150 s duration.

Aeolus is in a dusk-dawn sun-synchronous orbit of about 400 km altitude with a 7-day repeat cycle (109 orbits). 48

Data Receiving Station Svalbard

Single Singledata datareception receptionstation stationin in Svalbard Svalbardallows allowsdownlink downlinkevery everyorbit; orbit; Aeolus Aeolusallows allowsaddition additionof offill-in fill-instations stations to reduce data latency to 30 min to reduce data latency to 30 minfor for regional data. regional data.

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Negative numbers = better (smaller forecast uncertainty)

Single LOS Impact Analysis: major improvement predicted

U-Wind U-WindAccuracy Accuracy250 250hPa, hPa,12 12hhForecast Forecast 31 days October 2000, mean of 10 assimilation 31 days October 2000, mean of 10 assimilationensembles ensembles from: from:Erik ErikAndersson, Andersson,David DavidTan Tan(ECMWF) (ECMWF)

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Aeolus Satellite Budgets by CDR 2005 mass: 1100 kg dry +116-266 kg fuel power: 1.4 kW avg. (solar array 2.5 kW) mass instrument: 470 kg power instrument: 840 W avg. (laser 510 W) Doppler Lidar Instrument ALADIN Nd:YAG laser in burst mode operation (125 mJ - 150 mJ @ 355 nm, 100 Hz) 1.5 m Cassegrain telescope Dual-Channel-Receiver with ACCD (Accumulating CCD Detector) Pointing and Orbit Control GPS, Star-Tracker, Inertial Measurement Unit, Yaw steering to compensate for earth rotation Launcher Rockot (SS-19 ICBM), Dnepr (SS-18 ICBM) or Vega (ESA) 51

ALADIN Atmospheric Laser Doppler Instrument ALADIN is the only payload of Aeolus. Its size is dominated by the large afocal telescope of 1.5 m diameter. It uses diode pumped Nd:YAG laser to generate UV-light pulses (355 nm) emitted to the atmosphere. Two transmitter laser assemblies (blue) and the receiver (yellow) are on the structure below the telescope. A large radiator (mounted on the satellite bus) is coupled with heat pipes to the transmitter lasers. Star trackers are mounted on ALADIN structure to give best possible pointing reference. Total mass is 450 kg, 830 W power need.

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Aeolus Structure Model Acoustic and Shaker Test 2005

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ALADIN optical layout Transmitter laser assembly: Reference Laser Head with stabilized tunable MISER lasers seeding the Power Laser Head with low power oscillator, two amplifiers and tripling stage two redundant laser assemblies in ALADIN

Mie receiver: Fizeau interferometer, thermally stable design, Outputs collimated single accumulation CCD

Transmit/receive telescope: 1.5 m diameter, SiC lightweight structure (mass about 50 kg), thermally focused Transmit/receive optics: high stability optical design, polarizer as T/R switch, 1 focus for chopper location, 1 focus as field stop, background filter (1 nm equivalent bandwidth + prism for broad-band rejection Rayleigh receiver: Double edge etalons, sequentially illuminated, Outputs focused on single accumulation CCD 54

ALADIN receiver optics The backscattered light and optics is analysed by two interferometers to obtain the Doppler shifts from aerosol (Mie-) and molecular (Rayleigh-) scattering of the atmosphere: A high-resolution multichannel analyser for the aerosol return (narrow line width), and a double channel balanced receiver for the molecular return (large line width). 55

Detector: Accumulation CCD

The Thelight lightisisanalysed analysedwith withtwo twoCCD CCDsensor sensor(16x16 (16x16pixels), pixels),with withon-chip on-chip accumulation (15 or 50 laser pulses) accumulation (15 or 50 laser pulses) Quantum Quantumefficiency efficiencyof ofthe theCCDs CCDsexceeds exceeds80 80%. %.Due Dueto tothe theon-chip on-chip accumulation accumulationfeature, feature,they theyreach reachshot-noise shot-noiselimited limiteddetection detectionsensitivity. sensitivity.

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ALADIN transmitter laser (TXA) A A diode diode pumped pumped Nd:YAG Nd:YAG laser laser is is generating generating single single frequency frequency pulses pulses at at 355 355 nm nm wavelength wavelength with with 150 150 mJ mJ energy energy at at 100 100 Hz Hz repetition repetition rate. rate. ItIt is is operated operated in in burst burst mode mode of of 12 12 ss on on (5 (5 ss warm warm up, up, 77 ss measurement), measurement), and and 16 16 ss off off to to increase increase life life time time and and reduce reduce power power consumption. consumption. For For single single mode mode operation, operation, the the laser laser is is injection injection seeded seeded with with output output from from aa cw cw MISER MISER laser laser in in the the Reference Reference Laser Laser Head Head (RLH) (RLH) which which is is coupled coupled via via single-mode single-mode fibres fibres to to the the power power laser laser head. head. The The laser laser is is conductively conductively cooled cooled via via heat heat pipes pipes mounted mounted on on thermal thermal interface interface plates. plates.

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ALADIN transmitter laser - PLH Power PowerLaser LaserHead Headisis aaclosed closedbox boxsupported supportedat atthe thethermal thermal interface plate. interface plate. Active Activecomponents componentsare aremounted mountedon on this thermal interface plate, while this thermal interface plate, while passive passivecomponents componentsare arelocated locatedon onthe the optical bench. optical bench.

Power Laser Head outside

Mass: 31 kg Power: 470 W

inside 58

ALADIN telescope primary FM Lightweight SiC structure

M1 FM mirror

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ALADIN Laser and Optical Receiver

Power Laser Head EngineeringQualification Model EQM during tests in Sep. 2005: first UV laser output achieved

Optics from Pre-Development Model PDM; now part of ALADIN Airborne Demonstrator

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ALADIN OSTM

PLH-1 PLH-2

RLH-1

OBA

RLH-2

Completed ALADIN optics module (OSTM) with Power Laser Heads (PLH), Refernce Laser Heads (RLH) and Optical Bench Assembly (OBA)

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ALADIN Rayleigh receiver (double edge etalon)

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ADM-Aeolus Pre-Launch Campaigns in 2006 and 2007 Ground Campaign at Meteorological Observatory of German Weather Service DWD in Lindenberg (close to Berlin) for fall 2006 with ALADIN Airborne Demonstrator, 2-µm Doppler Lidar, 482 MHz windprofiler radar and other instruments

2 airborne campaigns with ALADIN Airborne Demonstrator and 2-µm Doppler lidar on-board DLR Falcon aircraft in 2007

Fig. Volker Lehmann (DWD) 63

NOAA Working Group on Space-Based Lidar Winds

NOAA – NASA Concept for Space Doppler Wind Lidar

http://space.hsv.usra.edu/workshops.html

Measure full two-vector winds by scanning or multiple receivers Have two lidar instruments: 1. A 355 nm direct dection system for upper atmosphere winds. 2. A 2.1 um coherent system for boundary layer winds.

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Types of LIDAR •DIAL •Raman LIDAR •Wind LIDAR •Spaceborne LIDAR •HSRL LIDAR

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