Doppler Wind Lidar

1
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
2
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.)

3
Doppler Lidar Profiling Geometry
z0 Orbital altitude (km) z Sample altitude
(km) ?zVertical resolution (km)
R Range to sample volume (km) c speed of light
(km/s) t time of flight of pulse (s) ?RRange
resolution (km) ?t integration interval (s) ?
Nadir angle (deg)
vs Spacecraft velocity (km/s) LRLaser rep
rate(Hz) ?xLaser spot separation (km)
4
A Satellite DWL Coverage Scheme with 4 Lines-of
Sight (2 fore, 2 aft)
Fore Aft
283 km
400 km
Swath width 566 km
5
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

6
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
7
Coherent Doppler Lidar

• 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)

? Simplified heterodyne receiver. The incoming
signal is mixed with a very stable local
oscillator (LO) ...
? to produce a beat frequency proportional
to Doppler shift
8
Examples of Coherent Doppler Wind Lidar Data
NASA/MSFC
NOAA/ETL
9
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

10
Fringe Imaging Doppler Receiver Concept
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
??Dop ?out-?return
Several methods have been proposed to map the
circular fringes to the rectangular CCD
11
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.
?
?out ? ( I1/I2)out
?return ? ( I1/I2)return
Incoming signal
??Dop ?out-?return
Aerosol Channel at 1064 nm
Molecular Channel at 355 nm
12
GLOW- Goddard Lidar Observatory for Winds

• 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