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Extraction of the surface velocity of rivers with

SAR-ATI

H. Runge1, S. Suchandt1, R. Horn2, T. Eiglsperger3

German Aerospace Center (DLR)

1Remote Sensing Technology Institute

2Microwaves & Radar Institute

3University Erlangen-Nuremberg

7th CNES/DLR Workshop on Information

Extraction and Scene Understanding for

Meter Resolution Images

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Why do we need measurement of river surface currents with remote sensing?

The surface velocity is one important parameter to measure the river run-off

River discharge is the only parameter in the water cycle that can be measured integrated over a large area.

River discharge is a sensitive parameter for climate changes.

With the rapid growth of world population the fresh water resources are of vital interest.

Gauging stations can not be maintained in all parts of the world.

From some countries run-off data are not made available.

In case of flooding better run-off predictions are necessary.

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d

A1

vs, az, Flight Track

Ground Track

x, vx

y, vy

Rground

R0 zh

A2

Moving Target

s

yATI v

vd

sin2

Interferometric phase:

SAR Along-Track Interferometry

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The „Train Off the Track Effect“

][sin

0 mv

vRaz

s

y

A moving target with across-track motion appears displaced in azimuth direction in the SAR image.

The signal from the water surface will appear displaced & superimposed on clutter at the river bank in the focused SAR image.

az

vy

rg

az

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Over smooth water the microwaves are reflected away from the sensor

Higher return over rough surface

Considerable Surface Roughness is Required

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ESAR Aircraft ESAR Aircraft

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ESAR ATI System Parameter

E-SAR ATI system

frequency band: X (9.6 GHz)

range bandwidth:

100 MHz

pulse repetition frequency:

1000 Hz / channel

antenna separation:

0.87119 m

sensor velocity: 88 m/s

incidence angle: 20 – 60 deg

SLC resolution: 1.50 x 0.09 m

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Gauge „Puppling“

Drainage Basin of the River Isar

site „Lenggries“

site „Kochel“

DLR test sites where in-situ measurements have been performed by the Bavarian Hydrological Office

Munich

DLR

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Puppling Test Site I

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ESAR Data of Puppling Test Site

Amplitude Coherence ATI Phase

az

slant range

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SAR Amplitude Image of the Puppling Site

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ATI Phase Analysis

Pixel with non-zero phase are back projected to their “original” position.

Only pixel which are then positioned over the river will be considered.

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Result from ATI Analysis

Result for ATI current velocity:

Pixel analyzed: 61

Min/Max: 2.27 / 0.56 m/s

Mean: 1.22 m/s

Result from in-situ measurement:

Mean: 1,70 m/s

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Result from ATI Analysis

Result for the current velocity from ATI method:

Pixel analyzed: 61

Min/Max: 2.27 / 0.56 m/s

Mean: 1.22 m/s

Result from in-situ measurement:

Mean: 1,70 m/s

Due to the superposition with stationary clutter the ATI measurements are biased to lower velocities. The error is 28%.

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Simulation of Azimuth Displacement for E-SAR Case

2.5

2.0

1.51.0

0.5

river surface velocity [m/s]

river flow direction

70 m

azimuth displacement

rg

az

The azimuth displacement method is not affected by clutter!

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Original SAR image

The two SAR channels are used for clutter cancellation

Remaining amplitude after complex subtraction of the two channels (DPCA)

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SAR Amplitude Image of the Puppling Site

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Scene after Clutter Cancellation

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Result for the current velocity from the displacement method plus DPCA clutter suppression:

Mean displacement: 86m

Corresponding velocity : 1.90 m/s

Result from in-situ measurement:

Mean: 1,70 m/s

The difference to the reference measurement has been reduced to 12%!

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Wolfratshausen 2006 Data Takes

1

3

2

4

06trafic0210x1_t01

06trafic0211x1_t01

06trafic0209x1_t01 06trafic0208x1_t01

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Isar 12/05/06 - Ground Truth Measurements

1-4:Sections where ground truth data where acquired with floaters.

Main river stream with greatest surface roughness

Section

Ref. Measurement

[m/s]

1 2.43

2 2.37

3 1.90

4 2.04

N

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Amplitude image of DT0210x1

Results from 24.11.06: corrected DPCA modeling

Illumination

flightdirectio

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Clutter reduction by DPCA filtering

Results from 24.11.06: corrected DPCA modeling

Illumination

flightdirectio

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Displaced river surface (red line) versus actual flow (blue

line)

Results from 21.03.07

Illumination

flightdirectio

n

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Surface velocity from displacement

Section

Ref. Measurement

[m/s]

1 2.43

2 2.37

3 1.90

4 2.04

Results from 21.03.07

Illumination

flightdirectio

n

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Results from 21.03.07

Section

Ref. Measurement

[m/s]

1 2.43

2 2.37

3 1.90

4 2.04

Illumination

flightdirectio

n

Surface velocity from displacement

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Illumination

Results from 21.03.07

Illumination

flightdirectio

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Surface velocity from displacement

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Results from 18.10.06

Illumination

flightdirectio

n

Amplitude image of DT0211x1

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Results from 18.10.06

Illumination

flightdirectio

n

Clutter reduction by DPCA filtering

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Illumination

flightdirectio

n

Displaced river surface (red line) versus actual flow (blue line)

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Section

Ref. Measurement

[m/s]

1 2.43

2 2.37

3 1.90

4 2.04

Illumination

flightdirectio

n

Surface velocity from displacement

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Results from 21.03.07, daz reference = area with greatest roughness; slightly corrected DPCA modeling

Section

Ref. Measurement

[m/s]

1 2.43

2 2.37

3 1.90

4 2.04

Illumination

flightdirectio

n

Surface velocity from displacement

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Results from 21.03.07, daz reference = area with greatest roughness; slightly corrected DPCA modeling

Illumination

flightdirectio

n

Surface velocity from displacement

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Conclusions

Due to strong clutter on the river bank the ATI measurements are biased to lower velocities

Clutter cancellation led to a SAR velocity measurement which differ in the order of only 10 to 15% from the in-situ measurements

A 3-channel SAR will allow to do both DPCA and ATI.

With such a system no auxillary information concerning the actual flow direction is required.