B. Gentry/GSFC GTWS 2/26/01

Doppler Wind LidarMeasurement Principles

Bruce GentryNASA / Goddard Space Flight Center

based on a presentation made to the Global Tropospheric Wind Sounder Workshop

Greenbelt, MDFebruary 26, 2001

B. Gentry/GSFC GTWS 2/26/01

Doppler Lidar Measurement Concept

DOPPLER RECEIVER - Multiple possibilities• Coherent ‘heterodyne’ (e.g. SPARCLE/MSFC) • Direct detection “Double Edge” (e.g. Zephyr/GSFC)• Direct detection “Fringe Imaging” (e.g. Michigan Aerospace Corp.)

Molecular ()

Aerosol ()

Backscattered Spectrum

Frequency

DOP

R=2ct

R=2ct

Doppler Lidar Profiling Geometry

x=vs/LR

R= Range to sample volume (km)c= speed of light (km/s)t = time of flight of pulse (s)R=Range resolution (km)t= integration interval (s)= Nadir angle (deg)

z0= Orbital altitude (km)z= Sample altitude (km)z=Vertical resolution (km)

z=z0-R/cos

z=R/cos

z0

vs= Spacecraft velocity (km/s)LR=Laser rep rate(Hz)x=Laser spot separation (km)

… N

B. Gentry/GSFC GTWS 2/26/01

400 km

A Satellite DWL Coverage Scheme with 4 Lines-of Sight (2 fore, 2 aft)

Swath width =566 km

ForeAft

28

3 k

m

B. Gentry/GSFC GTWS 2/26/01

Doppler Lidar Receivers

• Coherent or heterodyne detection• Proposed for eyesafe operation at 9.6 microns and and 2 microns using aerosol backscattered signal

• Direct or non-coherent detection• Proposed for eyesafe operation at 355 nm using molecularor aerosol backscattered signal• Fringe imaging approach • Edge filter technique

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

Courtesy M. Kavaya, MSFC

B. Gentry/GSFC GTWS 2/26/01

-140

-130

-120

-110

-100

-90

-80

-70

-60

0 1 108 2 108 3 108 4 108 5 108

Amplitude (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)

B. Gentry/GSFC GTWS 2/26/01

Examples of Coherent Doppler Wind Lidar Data

NASA/MSFC

NOAA/ETL

B. Gentry/GSFC GTWS 2/26/01

• 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

B. Gentry/GSFC GTWS 2/26/01

1. Incoming light is imaged through the FP etalon onto a CCD array2. Doppler frequency shift is proportional to 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 CCD

Dreturn return

Imaging Detector (CCD) In

com

ing

sign

al

Fabry Perot etalon

Dout out

Dop = out-return

B. Gentry/GSFC GTWS 2/26/01

Double Edge Measurement Concept

Aerosol Channel at 1064 nm Molecular Channel at 355 nm

Inco

min

g si

gnal

Fabry Perot etalon out ( I1/I2)out

return ( I1/I2)return

Dop = out-return

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.

I2()

I2()

B. Gentry/GSFC GTWS 2/26/01

• Demonstrates system level performance for validation of instrument models and verification of algorithms • Field testbed for demonstration of new component technologies • Provides unique capability to profile tropospheric winds

GLOW- Goddard Lidar Observatory for Winds