1. 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

2. Authors Acknowledgements 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 O’Handley, SWA
P. Capizzo, Raytheon

3. Why Investigate Doppler Lidar Measurement Of Water Velocity?
NASA’s 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

4. Lidar Hardware 2.02-Micron Tm:YAG 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)

6. 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

8. Geometry Lidar Land River Lidar Height Above Target Min. R Or Greater
Depression Angle
Land
Lidar Height Above Target
Min. R
Or Greater
Normal Angle
River

9. 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

10. bEQ ~ 10-6

11. 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.

12. 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

13. 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

14. Transmitter Q-switch location
Modulator moved 10 mm in this direction to reduce modulation delay
Laser Oscillator
Direction of acoustic propagation
Lamps & Cr:Tm:YAG rod
¼-wave plate
Output coupler
PZT
Etalon
Output
¼-wave plate
Brewster plate
Q-switch AOM 50 MHz
PZT
R=100%

15. Power of spurious backscatter pulse Power (normalized to peak)
Pulse with tail
Pulse with tail suppressed
0.02
Power (normalized to peak)
0.01
- 1500
- 1000
- 500
500
1000
1500
2000
Time (ns)

16. Spurious backscatter pulse (97 MHz heterodyne IF)
0.10
Pulse with tail
Pulse with tail suppressed
0.05
Voltage (normalized to peak)
0.05
Note significant tail 2 ms past peak
0.10
1500
1000
500
500
1000
1500
2000 j
Time (ns) ns
Time (ns)

17. 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.

18. Example of a River Measurement?
7.4 m/s
Signal Amplitude
Away
Velocity
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

19. 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.; 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.