SWOT

Near Nadir Ka-band SAR Interferometry:SWOT Airborne Experiment

Xiaoqing Wu, JPL, California Institute of Technology, USA

Scott Hensley, JPL, California Institute of Technology, USA

Ernesto Rodriguez, JPL, California Institute of Technology, USA

Delwyn Moller, Remote Sensing Solutions, Barnstable, USA

Ronald Muellerschoen, JPL, California Institute of Technology, USA

Thierry Michel, JPL, California Institute of Technology, USA

IGARSS 2011 SWOT Session , July 27, 2011

SWOT

Background

• SWOT (Surface Water Ocean Topography) is a planned NASA and CNES joint mission for monitoring Earth’s surface water.

• The major instrument of SWOT is KaRIn (a single pass Ka-band Radar Interferometrer)

• To measure sea surface heights and terrestrial water heights with a total 120 km width swath from both left and right sides

• To cover +90 % of Earth’s surface

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SWOT

KaRIn system and Ka-band airborne system

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System parameters KaRIn Airborne system

Platform height 970 km ~12 km

Carrier frequency 35.7 GHz 35.7 GHz

Signal bandwidth 200 MHz 80 MHz

Peak transmit power 1.5 kW 35 W

PRF (per channel and side) 4420 Hz ~800 Hz

Antenna boresight in elevation 2.7o 31o

Antenna azimuth beamwidth 0.12o 0.9o

Baseline length (physical) 10 m 25 cm

Baseline angle 0 45o

Roll angle to achieve near nadir geometry for airborne system NA 6o ~ 9o

Ground range swath (one side) 10 – 70 km 2 – 6 km

SWOT

Verification and validation through airborne experiments

• Characterize power return of water surface with near nadir geometry

• Test and Verify SWOT ground processing algorithms

• Evaluate and predict performances of KaRIn system

Need to overcome issues of airborne systems

• multi-path due to reflection from antenna fairings

• possible interferometric phase drift due to lack of calibration signals

• Baseline calibration – particularly baseline orientation angle

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SWOT

Magnitude pattern after flattening

Antenna data derived interferogram after flattening

Antenna mount

Phase residual from multi-path

SWOT

Greenland Summit height map

70 k

m

7 km

SWOT

Dependency of cross track ripples on along track (Greenland)A

long

trac

k 22

0 km

Phase drift as a function of along-track estimated from comparison with ATM height

Near rangeFar range Absolute phase

SWOT Residual Baseline orientation angle estimation and correction

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Height map wrapped in 10 m After residual roll correction

7 km

SWOT

Near nadir test site: Van Hook Arm

9

4.5 km

6

km

SWOT Water and land power comparison and application for water land classification

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Water surface

Land

Pow

er (

db)

classification image (left) based on magnitude image and cross range dependent threshold. White: water surface; black: land surface

SWOTHeight measurement vs DEM

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Height map wrapped in 50 m SRTM DEM wrapped in 50 m

5.4 km

33 k

m

SWOT Height measurement accuracy

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Height difference with SRTM DEM

-10 m +10 m

SWOT

Summary• Demonstrated techniques to correct phase residuals in

airborne system from:– Multi-paths due to antennas– unknown interferometric phase drift – unknown residual baseline orientation angle

• Characterized the near nadir water surface reflectivity

• Height measurements from airborne system match very well with SRTM DEM within 10 meters for land areas.

• The water surface height measurement accuracy of about 25 cm at near range is achieved.

• Some techniques will be verified again with AirSWOT airborne system in near future and incorporated into SWOT ground data processing system.

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