B. Gentry/GSFCGTWS 2/26/01 Doppler Wind Lidar Measurement Principles Bruce Gentry NASA / Goddard Space Flight Center based on a presentation made to the Global Tropospheric Wind Sounder Workshop Greenbelt, MD February 26, 2001

B. Gentry/GSFCGTWS 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  DO P

  R=2c  t R=2ct Doppler Lidar Profiling Geometry  x=v s /L R 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) z 0 = Orbital altitude (km) z= Sample altitude (km)  z=Vertical resolution (km) z=z 0 -R/cos   z=  R/cos  z0z0 v s = Spacecraft velocity (km/s) L R =Laser rep rate(Hz)  x=Laser spot separation (km) … N

B. Gentry/GSFCGTWS 2/26/ km A Satellite DWL Coverage Scheme with 4 Lines-of Sight (2 fore, 2 aft) Swath width =566 km Fore Aft 283 km

B. Gentry/GSFCGTWS 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 molecular or 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/GSFCGTWS 2/26/01  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/GSFCGTWS 2/26/01 Examples of Coherent Doppler Wind Lidar Data NASA/MSFC NOAA/ETL

B. Gentry/GSFCGTWS 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/GSFCGTWS 2/26/01 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* Fringe Imaging Doppler Receiver Concept * Several methods have been proposed to map the circular fringes to the rectangular CCD D return  return Imaging Detector (CCD) Incoming signal Fabry Perot etalon D out  out  Dop = out - return

B. Gentry/GSFCGTWS 2/26/01 Double Edge Measurement Concept Aerosol Channel at 1064 nm Molecular Channel at 355 nm Incoming signal Fabry Perot etalon out  ( I 1 /I 2 ) out return  ( I 1 /I 2 ) 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 I 1 and I The Doppler frequency shift is proportional to the change in the ratio of the measured signals I 1 /I 2 which varies as the laser wavelength moves up and down on the steep edge of the filters. I 2 ( ) I 2 ( )

B. Gentry/GSFCGTWS 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