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 2

SWOT KaRIn system and Ka-band airborne system 3 System parametersKaRInAirborne system Platform height970 km~12 km Carrier frequency35.7 GHz Signal bandwidth200 MHz80 MHz Peak transmit power1.5 kW35 W PRF (per channel and side)4420 Hz~800 Hz Antenna boresight in elevation2.7 o 31 o Antenna azimuth beamwidth0.12 o 0.9 o Baseline length (physical)10 m25 cm Baseline angle045 o Roll angle to achieve near nadir geometry for airborne system NA6 o ~ 9 o Ground range swath (one side)10 – 70 km2 – 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 4

SWOT Magnitude pattern after flattening Antenna data derived interferogram after flattening Antenna mount Phase residual from multi-path

SWOT Greenland Summit height map 70 km 7 km

SWOT Dependency of cross track ripples on along track (Greenland) Along track 220 km Phase drift as a function of along-track estimated from comparison with ATM height Near range Far range Absolute phase

SWOT Residual Baseline orientation angle estimation and correction 8 Height map wrapped in 10 mAfter residual roll correction 7 km

SWOT Near nadir test site: Van Hook Arm km 6 km

SWOT Water and land power comparison and application for water land classification 10 Water surface Land Power (db) classification image (left) based on magnitude image and cross range dependent threshold. White: water surface; black: land surface

SWOT Height measurement vs DEM 11 Height map wrapped in 50 m SRTM DEM wrapped in 50 m 5.4 km 33 km

SWOT Height measurement accuracy 12 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. 13