On the Effect of Surface Motion in SAR Altimeter Observations of the Open Ocean

Alejandro Egido (NOAA / GST Inc., United States)

CoAuthors

Chris Ray (St Mary, USA)

Event: 2019 Ocean Surface Topography Science Team Meeting

Session: Instrument Processing: Measurement and Retracking

Presentation type: Type Oral

The effects of surface waves motion on synthetic aperture radar (SAR) images of the ocean has been an intense topic of study for a number of decades, [Hasselmann, et al., 1985], [Alpers and Bruening, 1986]. As the SAR locates targets on the azimuth dimension based on their Doppler history, the main effect of the surface motion is a misplacement of targets within the image. In the case of a distributed target as the ocean surface, this displacement originates a degradation of the image. One of these effects is a loss of azimuth resolution, induced by a) the vertical acceleration of surface scatterers, and b) the loss of coherence due to the vertical motion of the different scatterers within the resolution cell. At all effects, this can be interpreted as a widening of the azimuth point target response (PTR) of the instrument. Despite the fact that the azimuth smearing can be very significant, i.e. in some cases more than 50% of the azimuth resolution, this effect has been largely overlooked in SAR altimetry.

For SAR systems with a moderate resolution, as is the case of delay/Doppler altimetry, [Raney, 1998], waves of intermediate wavelengths are the ones that play a more significant role, and in this case, it is the finite surface coherence that induces the degradation of azimuth resolution, [Alpers and Bruening, 1986].

This can be readily observed in the delay/Doppler data, when computing the autocorrelation function of the speckle noise for adjacent Doppler beams. The consequence of this is a decrease of the multilooking efficiency, which results in an increase in the measurement error of geophysical parameters. In addition, the widening of the along-track point target response impacts the estimation of geophysical parameters, particularly significant wave height (SWH). We believe this effect could be the root cause of the previously observed discrepancies in SWH between the delay/Doppler and the pseudo-low resolution mode data, [Raynal, et al, 2018].

Through the processing and analysis of Sentinel-3 SAR data and simulations of realistic ocean surfaces, we evaluate the surface motion effects on the final delay/Doppler multilooked waveforms and develop the theory to update the SAR waveform model to better account for the sea state dependent surface wave's motion.



[Hasselmann, et al., 1985], Hasselmann, K., Raney, R. K., Plant, W. J., Alpers, W., Shuchman, R. A., Lyzenga, D. R., Rufenach, C. L., and Tucker, M. J. ( 1985), Theory of synthetic aperture radar ocean imaging: A MARSEN view, J. Geophys. Res., 90( C3), 4659– 4686, doi:10.1029/JC090iC03p04659.

[Alpers and Bruening, 1986] W. R. Alpers and C. Bruening, "On the Relative Importance of Motion-Related Contributions to the SAR Imaging Mechanism of Ocean Surface Waves," in IEEE Transactions on Geoscience and Remote Sensing, vol. GE-24, no. 6, pp. 873-885, Nov. 1986.

[Raney, 1998] R. K. Raney, "The delay/Doppler radar altimeter," in IEEE Transactions on Geoscience and Remote Sensing, vol. 36, no. 5, pp. 1578-1588, Sep 1998.

[Raynal, et al, 2018] M. Raynal, T. Moreau, N. Tran, S. LabroueF. Boy, P. Féménias, F. Borde, Assessment of the SARM processing sensitivity to swell, 2018 Ocean Surface Topography Science team meeting, Ponta Delgada, PT

 

Oral presentation show times:

Room Start Date End Date
The Forum Tue, Oct 22 2019,09:30 Tue, Oct 22 2019,09:45
Alejandro Egido
NOAA / GST Inc.
United States
alejandro.egido@noaa.gov