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ECE 583Lectures 23

LIDAR ContinuedWind Sensing

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3

•DIAL

•Raman LIDAR

•Wind LIDAR

•HSRL LIDAR

•Spaceborne LIDAR

Types of LIDAR

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Doppler winds The Doppler effect provides a way of measuring the motions ofscatterers –used as ‘tracers’ of the wind, and may vary from moleculessub-micron aerosol, to cloud and precipitation, to insects to refractiveindex fluctuations of many meters in size depending on the lidaror radar wavelengths.

kx-ωt k’x’-ω’t’θ

v

λϑ=νΔ

λ=ν−ν′=νΔ

<<−

+ν=ν′

/cosvor

/vc v

c/vc/v

D

D

2211

sign convention varies; 0>νΔ D

when O’ moves toward source

O’

5

εε

εεεε

εεφφ

sin2vv v

cos2vvv

sosinv cosvv

sinv cosvvpositionsscan min andmax at

sinv cos)cos(vv

duf

duh

fhd

fhu

f

hr

+=

−=

+−=+=

+−= o

Velocity-azimuth-display VAD)

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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

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NOAA Working Group on Space-Based Lidar

Winds

15th Coherent Laser Radar Conference

June 22 - 26, 2009

Toulouse, France

Visit the conference web pages

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

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

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(~ 4 pm mid-visible)

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Doppler LidarMeasurement principle

Detect Doppler shift of backscatter signal as function of time after pulse emission

Detect Doppler shift of backscatter signal as function of time after pulse emission

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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

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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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-140

-130

-120

-110

-100

-90

-80

-70

-60

0 1 108 2 108 3 108 4 108 5 108

Am

plitu

de (

db)

Frequency (Hz)

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

Coherent Doppler Lidar

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

+ High photon efficiency

+ Insensitive to solar background light

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

• Requires aerosol backscatter (no molecular version)

The wide frequency width of the molecularsignal results in molecular scattering being a noise component, rather than signal for coherent lidar detection.

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

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15

16

17

18

19

[about 5 uradians for a .5m aperture]

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Examples of Operating Doppler Lidar Instruments: Coherent

4 2 0 2 42

1

0

1

2

4 2 0 2 41

0.5

0

0.5

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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• 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’

Direct Detection Doppler Lidar

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Doppler Shifted and BroadenedBackscatter Spectrum

Frequency

What comes back to a lidar receiver from the atmosphere really?

Molecular (λ−4)

Aerosol (λ−2)

Doppler Broadened Backscatter Spectrum

Frequency

Without wind With wind

Red shift Blue shift

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1. Incoming light is imaged through theFP etalon onto a CCD array

2. Doppler frequency shift is proportionalto the change in the radius of the etalon fringe*

Fringe Imaging Doppler Receiver Concept

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

Dreturn ∝ λreturn

Imaging Detector (CCD) In

com

ing

sign

al Fabry Perotetalon

Dout ∝ λout

ΔλDop = λout-λreturn

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Fabry-Perot Etalon as Filter

λθ mnd =)cos(2

For transmission we have a maximum when:

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Double Edge Measurement Concept

Aerosol Channel at 1064 nm Molecular Channel at 355 nm

Inco

min

g si

gnal

Fabry Perotetalon λout ∝ ( I1/I2)out

λreturn ∝ ( I1/I2)return

ΔλDop = λout-λreturn

1. Incoming light is collimated, split into 2 channels and sent through theFP etalon. The light in each channel is focussed to a photon countingdetector giving signals I1 and I2.

2. The Doppler frequency shift is proportional to the change in the ratioof the measured signals I1/I2 which varies as the laser wavelengthmoves up and down on the steep edge of the filters.

I2(λ)

λ

I2(λ)

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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

FPI DETG, φ

EM DETGo, φo

Narrow Frequency Filter

BeamSplitter

TBPFo

TBPFTN(ν)·TFPIpk

Pin from telescope

TBS

RBS

C

Co

P

Po

)(*)()( ννν FCI

II calEM

FCN ==

30

ΔIN

REF

SIG

ΔνDoppler

1.0

.50

.25

.75

25 50 75 100

Nor

mal

ized

Tra

nsm

issi

on

Relative Frequency in MHz

IN(ν)

IN (ν +Δ ν)

mCI

mCII

cal

N

Dopplercal

NNDoppler ⋅

Δ=

Δ+⋅−Δ+

=Δ),(

)()(ννν

νννν

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

Change in Intensity Through a Narrow Filter due to Frequency Shift

(Edge Technique)

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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.

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GLOW- Goddard Lidar Observatory for Winds

3333

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15

10

5

0

DC8

WB57 ER2

Alti

tude

(km

)

GLOW mobile lidar

TWiLiTE configured for DC8 Nadir Port 7

(ROSES07-AITT)

TWiLiTE configured for WB57 3’ Pallet

(ESTO IIP04)

TWiLiTE configured for ER2 Q-Bay(ESTO IIP04)(SMD,NMP,IPO, ARO,

ROSES07-WLS)

TWiLiTE Airborne Doppler Lidar

Direct Detection Doppler Lidar Development Roadmap

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•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.

Tropospheric Wind Lidar Technology Experiment(TWiLiTE) Instrument Incubator Program

TWiLiTE system configured for ER-2 QBay

TWiLiTE ER-2 IntegrationFebruary, 2009

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Wavelength 354.7 nmTelescope/Scanner Area 0.08 m2

Laser Linewidth (FWHH) 150 MHzLaser Energy/Pulse (8 W) 40 mJ @ 200 ppsEtalon FSR 16.65 GHzEtalon FWHH 2.84 GHzEdge Channel Separation 6.64 GHzLocking Channel Separation 4.74 GHzInterference filter BW (FWHH) 120 pmPMT Quantum Efficiency 25%

TWiLiTE Instrument Parameters

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ER-2 Engineering FlightsFeb 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

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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

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Preliminary GWHI Validation ResultsWind Speed

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the ADM-Aeolus Mission

the ADM-Aeolus Missionsense the wind around the

globesense the wind around the

globe

Martin EndemannAeolus System & Instrument Manager

Martin EndemannAeolus System & Instrument 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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Keep it simple:Single Line-of-Sight wind profiles

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.

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)

The measurement of a single component (LOS) wind profiles will simplify wind lidar instrument design significantly

The measurement of a single component (LOS) wind profiles will simplify wind lidar instrument design significantly

ESA Approach:

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Future global wind observations with ALADIN on the

Atmospheric Dynamics Mission ADM-Aeolus

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Atmospheric LAser Doppler INstrumentALADIN

First Doppler lidar in spaceOperating in ultraviolett @ 355 nm to measure wind from molecular Rayleigh and aerosol/cloud Mie backscatterLine-of-Sight LOS wind profiles in troposphere to lower stratosphere with vertical resolution from 250 m - 2 km, averaged over 50 km every 200 kmRequirement 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 %LOS is pointing 35 ° from nadir orthogonal to the ground track velocity, yaw steering appliedFirst High Spectral Resolution Lidar HSRL in space to obtain aerosol/cloud optical properties

ADM-Aeolus Implementation

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Principle of Wind Measurement with ALADINAtmospheric LAser Doppler INstrument

ALADINDirect-Detection Doppler Lidar at 355 nm with 2 spectrometers to analyse backscatter signal from molecules (Rayleigh) and aerosol/clouds (Mie)Double edge technique for spectrally broad molecular return, e.g. NASA GLOW instrument (Gentry et al. 2000), but sequential implementation Fizeau spectrometer for spectrally small aerosol/cloud returnALADIN 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

backscatter coefficientextinction coefficientlidar ratio

Principle of spectrometer für molecular signal

principle of spectrometer für aerosol signal

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ADM-Aeolus Coverage and Data Availability

3200 wind profiles per day: about factor 3 more than radiosondes

3 hour data availability afterobservation; 30 minutes data availabilityfor parts of orbit

X-Band data-downlink to Svalbard, processing center up to Level 1B in Tromsö (Norway), up to Level 2C at ECMWF

launch planned for September 2008

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

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.

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Data Receiving StationSvalbard

Single data reception station in Svalbard allows downlink every orbit;Aeolus allows addition of fill-in stations to reduce data latency to 30 min for regional data.

Single data reception station in Svalbard allows downlink every orbit;Aeolus allows addition of fill-in stations to reduce data latency to 30 min for regional data.

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U-Wind Accuracy 250 hPa, 12 h Forecast31 days October 2000, mean of 10 assimilation ensembles

from: Erik Andersson, David Tan (ECMWF)

U-Wind Accuracy 250 hPa, 12 h Forecast31 days October 2000, mean of 10 assimilation ensembles

from: Erik Andersson, David Tan (ECMWF)

Single LOS Impact Analysis:major improvement predicted

Neg

ativ

e nu

mbe

rs =

bet

ter

(sm

alle

r for

ecas

t unc

erta

inty

)

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Aeolus Satellite

Budgets by CDR 2005mass: 1100 kg dry +116-266 kg fuelpower: 1.4 kW avg. (solar array 2.5 kW)mass instrument: 470 kgpower 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 telescopeDual-Channel-Receiver with ACCD (Accumulating CCD Detector)

Pointing and Orbit ControlGPS, Star-Tracker, Inertial Measurement Unit, Yaw steering to compensate for earth rotation

LauncherRockot (SS-19 ICBM), Dnepr (SS-18 ICBM) or Vega (ESA)

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ALADIN is the only payload of Aeolus. Its size is dominated by the large afocaltelescope 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.

ALADIN Atmospheric Laser Doppler Instrument

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Aeolus Structure Model Acoustic and Shaker Test 2005

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ALADIN optical layoutTransmit/receive telescope:1.5 m diameter, SiC lightweight structure (mass about 50 kg), thermally focusedTransmit/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 rejectionRayleigh receiver: Double edge etalons, sequentially illuminated, Outputs focused on single accumulation CCD

Mie receiver: Fizeau interferometer,

thermally stable design, Outputs collimated single

accumulation CCD

Transmitter laser assembly:Reference Laser Headwith stabilized tunable

MISER lasersseeding the

Power Laser Headwith low power oscillator,

two amplifiers and tripling stage

two redundant laser assemblies in ALADIN

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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 multi-channel analyser for the aerosol return (narrow line width), and a double channel balanced receiver for the molecular return (large line width).

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Detector:Accumulation CCD

The light is analysed with two CCD sensor (16x16 pixels), with on-chip accumulation (15 or 50 laser pulses)Quantum efficiency of the CCDs exceeds 80 %. Due to the on-chip accumulation feature, they reach shot-noise limited detection sensitivity.

The light is analysed with two CCD sensor (16x16 pixels), with on-chip accumulation (15 or 50 laser pulses)Quantum efficiency of the CCDs exceeds 80 %. Due to the on-chip accumulation feature, they reach shot-noise limited detection sensitivity.

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ALADIN transmitter laser (TXA)A diode pumped Nd:YAG laser is generating single frequency pulses at 355 nm wavelength with 150 mJenergy at 100 Hz repetition rate.

It is operated in burst mode of 12 s on (5 s warm up, 7 s measurement), and 16 s off to increase life time and reduce power consumption.

For single mode operation, the laser is injection seeded with output from a cw MISER laser in the Reference Laser Head (RLH) which is coupled via single-mode fibres to the power laser head.

The laser is conductively cooled via heat pipes mounted on thermal interface plates.

A diode pumped Nd:YAG laser is generating single frequency pulses at 355 nm wavelength with 150 mJenergy at 100 Hz repetition rate.

It is operated in burst mode of 12 s on (5 s warm up, 7 s measurement), and 16 s off to increase life time and reduce power consumption.

For single mode operation, the laser is injection seeded with output from a cw MISER laser in the Reference Laser Head (RLH) which is coupled via single-mode fibres to the power laser head.

The laser is conductively cooled via heat pipes mounted on thermal interface plates.

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ALADIN transmitter laser - PLH

inside

Power Laser Head outside

Mass: 31 kg

Power: 470 W

Power Laser Head is a closed box supported at the thermal interface plate.Active components are mounted on this thermal interface plate, while passive components are located on the optical bench.

Power Laser Head is a closed box supported at the thermal interface plate.Active components are mounted on this thermal interface plate, while passive components are located on the optical bench.

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M1 FM mirror

ALADIN telescope primary FMLightweight SiC structure

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

ALADIN Laser and Optical Receiver

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

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ALADIN OSTM

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

PLH-1

RLH-2RLH-1 OBA

PLH-2

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ALADIN Rayleigh receiver(double edge etalon)

63Fig. Volker Lehmann (DWD)

ADM-Aeolus Pre-Launch Campaigns in 2006 and 2007

Ground Campaign at Meteorological Observatoryof German Weather Service DWD in Lindenberg(close to Berlin) for fall 2006 with ALADINAirborne 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

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NOAA – NASA Concept for Space Doppler Wind Lidar

Measure full two-vector winds by scanning or multiple receivers

Have two lidar instruments:

1. A 355 nm direct dectionsystem for upper atmosphere winds.

2. A 2.1 um coherent system for boundary layer winds.

NOAA Working Group on Space-Based Lidar

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

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•DIAL

•Raman LIDAR

•Wind LIDAR

•Spaceborne LIDAR

•HSRL LIDAR

Types of LIDAR