Coherent Doppler Lidar Measurement of River Surface Velocity

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• Coherent Doppler Lidar Measurement of River
  Surface Velocity
• Michael J. Kavaya
• NASA/LaRC
• to
• Working Group on Space-Based Lidar Winds
• Oxnard, CA
• Feb. 7-9, 2001

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• Authors
• Steven C. Johnson, MSFC
• Thomas J. Papetti, UAH/CAO
• Philip A. Kromis, CSC
• Michael J. Kavaya, LaRC
• J. Rothermel, MSFC
• D. Bowdle UAH,
• F. Amzajerdian, UAH/CAO (soon LaRC)
• Acknowledgements
• Tim Miller, MSFC
• Dave Emmitt Chris OHandley, SWA
• P. Capizzo, Raytheon

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• Why Investigate Doppler
• Lidar Measurement Of Water
• Velocity?
• NASAs Hydrological Cycle Program
• USGS Desire For New River Discharge
  Instrumentation
• Potential Of Ocean Returns To Aid Calibration Of
• Global Doppler Lidar Wind Measurement
• See
• NASA Post-2002 Land Surface Hydrology Mission
  Component for Surface Water Monitoring
  HYDRA-SAT, C. Vorosmarty et al, April 12-14,
  1999.
• First Meeting Report of the Working Group on
  Future Space-based Hydrology Missions, Aug. 3-4,
  2000

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• Lidar Hardware
• 2.02-Micron TmYAG
• Pulsed, 6 Hz
• 50 mJ, 400 ns, 10 cm
• Flashlamp pumped
• Procured from CTI (8/93)
• Loaned to and flown by Air Force/CTI on C-141
  (6/95)

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• Field Deployments To
• Tennessee River
• Sheffield, AL bluff overlook ( 50 m)
• Downstream from Wilson dam ( 3 miles)
• Deployments 12/17/99, 2/24/00, 11/14/00

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• Geometry

Lidar
Depression Angle
Land
Lidar Height Above Target
Min. R Or Greater
Normal Angle
River
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• Constraints
• Range to water must be greater than minimum range
  of lidar
• Too large a depression angle will let lidar
  strike bluff and/or have insufficient range to
  water
• Too small a depression angle will cause a large
  normal angle
• Too large a normal angle will have very small
  water backscatter
• Too small a normal angle will not intercept much
  water velocity
• Desire a wind range gate before the water hence
  even larger target ranges

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bEQ 10-6
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• Chronology
• 12/17/99, 2/24/00 deployments
• Depression angles from horizontal as large as 5
  deg.
• Ambiguous data
• Engineering effort to reduce lidar minimum range
  to allow greater depression angles to raise water
  signal
• Minimum range successfully lowered from 350 m to
  120 m
• 11/14/00 depression angles as large as 18 deg.

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Methods for shortening minimum range
Backscatter reduction by optics surface quality
improvement and various layout modifications
Several such modifications were made, but orders
of magnitude of backscatter reduction are
necessary to significantly shorten minimum range,
due to exponential nature of pulse tail Pulse
tail suppression (the method chosen) Tail
suppression produces a direct reduction of
minimum range to the point at which the tail is
suppressed without significant loss of outgoing
pulse energy
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Method of tail suppression
Intra-resonator Acousto-Optic Loss Modulator
(AOM) Convenient AOM already existed in
transmitter for Q-switching function Effective
Multiple passes through modulator during one
pulse duration produce rapid and complete
suppression
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Transmitter Q-switch location
Modulator moved 10 mm in this direction to reduce
modulation delay
Laser Oscillator
Direction of acoustic propagation
Lamps CrTmYAG rod
¼-wave plate
Output coupler
PZT
Etalon
Output
Brewster plate
¼-wave plate
Q-switch AOM 50 MHz
PZT
R100
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Power of spurious backscatter pulse
Pulse with tail
Pulse with tail suppressed
0.02
Power (normalized to peak)
0.01
0
- 1000
- 500
0
500
1000
1500
2000
- 1500
Time (ns)
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Spurious backscatter pulse (97 MHz heterodyne IF)
0.10
Pulse with tail
Pulse with tail suppressed
0.05
Voltage (normalized to peak)
0
0
0.05
Note significant tail 2 ms past peak
0.10
1500
1000
500
0
500
1000
1500
2000
j
ns
Time (ns)
Time (ns)
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• Chronology (cont.)
• 12/17/99, 2/24/00 deployments
• Depression angles from horizontal as large as 5
  deg.
• Ambiguous data
• Engineering effort to reduce lidar minimum range
  to allow greater depression angles to raise water
  signal
• Minimum range successfully lowered from 350 m to
  120 m
• 11/14/00 depression angles as large as 18 deg.
• Better results but not definitive. Where water
  signal is noticeable, the velocity is near zero.
  Difficult to obtain air velocity range gate
  before water.

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Example of a River Measurement?
7.4 m/s
Signal Amplitude
Velocity
Away
Possible River Return
RF Switch
Outgoing Pulse
Return from Air
Air Velocity
River Surface?
Range
Toward
Range
600 m
Nov. 14, 2000 Run 6 20-pulse integration 10o
depression angle Upwind and downstream
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• Comments
• Water backscatter varies greatly with normal
  angle up to ?20? deg.
• Function will depend on water purity, waves,
  surface wind
• 400 km, 30 deg. space mission will hit ocean at
  32 deg. 833 km, 45 deg. space
  mission will hit ocean at 53 deg.
• Further reducing minimum range and/or flying
  lidar on aircraft will still have problem of near
  zero air and water velocities. Where can we find
  large air and water velocity?
• How does surface wind affect water velocity?
• Controllable water target (range, angle, flow,
  purity) may greatly help sort out effects. Plan
  to build.
• Plan further analytical study to define the
  effects of water spray above river surface, and
  river surface waves and ripples on signal
  amplitude and flow velocity estimate.