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Coherent Doppler Lidar Measurement of River Surface Velocity

Michael J. KavayaNASA/LaRC

to

Working Group on Space-Based Lidar WindsOxnard, CA

Feb. 7-9, 2001

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AuthorsSteven C. Johnson, MSFC

Thomas J. Papetti, UAH/CAOPhilip A. Kromis, CSC

Michael J. Kavaya, LaRCJ. Rothermel, MSFC

D. Bowdle UAH,F. Amzajerdian, UAH/CAO (soon LaRC)

AcknowledgementsTim Miller, MSFC

Dave Emmitt & Chris O’Handley, SWAP. Capizzo, Raytheon

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Why Investigate DopplerLidar 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

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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)

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Field Deployments ToTennessee 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

Land

Lidar

River

Depression Angle

Normal Angle

Lidar Height Above TargetMin. ROr Greater

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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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EQ ~ 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

PZT R=100%

Brewster plate

Etalon

Q-switch AOM 50 MHz

¼-wave plate

¼-wave plate

Output coupler PZT

Direction of acoustic propagation

Modulator moved 10 mm in this direction to reduce modulation delay

Output

Lamps & Cr:Tm:YAG

rod

Laser Oscillator

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- 1500 - 1000 - 500 0 500 1000 1500 2000

0.01

0.02

Pulse with tail

Pulse with tail suppressed

Time (ns)

Pow

er (

norm

aliz

ed to

pea

k)Power of spurious backscatter pulse

0

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0.05

0

0.05

Time (ns)

jns

0

1500 1000 500 0 500 1000 1500 2000

Time (ns)

Pulse with tail suppressed

Pulse with tail

Spurious backscatter pulse (97 MHz heterodyne IF)

Vol

tage

(nor

mal

ized

to p

eak) 0.10

0.10

Note significant tail 2 s past peak

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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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0 50 100 150 200 250 300 350 4000

500

1000

1500

2000

2500

0 50 100 150 200 250 300 350 400-60

-40

-20

0

20

40

60

Return from Air

River Surface?

Air Velocity

Possible River Return

Range

Signal Amplitude Velocity

Range

Example of a River Measurement?

Nov. 14, 2000; Run 620-pulse integration

~10o depression angleUpwind and downstream

Away

Toward600 m

OutgoingPulse

RF Switch

7.4 m/s

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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.