1. (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
(2) Norut, Tromsø, Norway

2. 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

3. 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 ( )

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

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

6. 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

7. 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

8. 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

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

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

11. 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…

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

13. 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

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

15. Collocations
NCEP forecast wind
ASAR geophysical Doppler shift

16. Radarsat-2 ScanSAR
Wind forecast from NCEP :00

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

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

19. 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