(2) Norut, Tromsø, Norway (harald.johnsen@norut.no) Improved measurement of sea surface velocity from synthetic aperture radar Morten Wergeland Hansen (1), Harald Johnsen (2), Geir Engen (2), and Jan Even Øie Nilsen (1) (1) Nansen Environmental and Remote Sensing Center, Bergen, Norway (morten.hansen@nersc.no), (2) Norut, Tromsø, Norway (harald.johnsen@norut.no)

Content Introduction SAR Doppler centroid shift Errors and calibration Surface velocity and inter-comparison Wind fields Geostrophic current from the ESA GlobCurrent project Summary and conclusion

Measuring ocean currents from space Mean zonal current from ASAR Doppler centroid anomaly Geostrophic current from CNES‐CLS09 MDT (Rio et al., 2011; GRACE geoid, altmetry, drifters) Radar altimeters Inversion of geostrophic current from Sea Surface Height (SSH) measurements Requires balance between the Coriolis and pressure forces due to large scale surface slopes relative to the geoid Feature tracking in repeated image acquisitions of ocean color, SST, and surface roughness SAR Along-Track Interferometry (ATI), requires a second receiving antenna (or a split antenna) Doppler centroid shift anomaly 2008 – 2011 Goal: Improve and extend to full Envisat period (2002-2012) 1993-2008

Doppler centroid shift anomaly Power Time Doppler centroid Frequency Time Target at rest

Doppler centroid shift anomaly Power Time Doppler centroid Frequency Target moving Chapron et al. (2003, 2005) Time Target at rest

1-2 Hz accuracy requirement for surface current retrieval (in a single image) (VD is horizontal range Doppler velocity) 𝚫f=5 Hz Doppler uncertainty translates to 30 cm/s at about 27 degrees incidence angle Need Doppler accuracy about 1-2 Hz for 10 cm/s accuracy in current velocity for a single scene… 5 cm/s current accuracy hardly attainable unless 𝚫f<1Hz

SAR Doppler centroid shift includes various contributions: Doppler calibration SAR Doppler centroid shift includes various contributions: Geophysical (wind, waves and current) Geometric (estimated from satellite orbit and attitude) Electronic (antenna mis-pointing) Residual error Non-geophysical terms must be precisely estimated and removed Required accuracy about 1-2 Hz

Errors caused by attitude variations Top: Latitude as function of time Bottom: Mean cross-track DCA as function of time, HH polarization Note difference between ascending and descending pass… Time

Antenna mis-pointing 2010-01-01 – ascending pass, HH pol Range

Antenna mis-pointing 2010-01-01 – descending pass, HH pol Range

Correction of errors from antenna pattern and attitude DC anomaly Antenna pattern corrected Attitude corrected 1st January 2010, 08:50 UTC Scenes with no land cannot easily be corrected for attitude offsets…

New vs old product Grid: 4-9 km x 8 km Grid: 1 km pixels (range x azimuth) Grid: 1 km pixels

Imaged surface properties Wind Backscatter from resonant Bragg waves Velocities align with the wind direction Long waves Specular reflection Breaking Tilt and hydrodynamic modulations Current Affects the wave steepness Direction is arbitrary

Corrected Doppler and NRCS Geophysical Doppler shift [Hz] Normalized Radar Cross Section [dB]

Collocations NCEP forecast wind ASAR geophysical Doppler shift

Radarsat-2 ScanSAR Wind forecast from NCEP 2016-01-27 18:00

Ship observations in the English Channel Ascending pass composite – mean wind from west

Gulf Stream Mosaic of SAR Doppler in January 2010 (-60:60 Hz) January 2010, mean zonal component of the geostrophic current (GlobCurrent; altimetry)

Summary Future SAR missions should aim towards 1-2 Hz accuracy in the geophysical Doppler shift The new Doppler products are provided at much higher resolution than before (1x1 km^2 vs 5x8 km^2 grids) Examples of well calibrated data shows The combined effects of wind and surface current Ships in the English channel The new Doppler processing system will be used in producing new long-term timeseries of sea surface current from SAR to allow Improved Mean Dynamic Topography Better monitoring of the variability of sea surface currents, particularly in the climatically important Nordic Seas region