b. gentry/gsfcgtws 2/26/01 doppler wind lidar measurement principles bruce gentry nasa / goddard...
TRANSCRIPT
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