Chapter 5
Direct Surface Current Field Imaging
from Space by Along-Track InSAR
and Conventional SAR
Roland Romeiser, Johnny Johannessen, Bertrand Chapron, Fabrice Collard,
Vladimir Kudryavtsev, Hartmut Runge, and Steffen Suchandt
5.1 Introduction
Since the SEASAT mission in 1978 (Fu and Holt, 1982), spaceborne synthetic
aperture radars (SARs) have acquired millions of high-resolution images of ocean
scenes, which have been used for applications such as wave and wind retrievals,
oil pollution monitoring, ship detection, sea ice monitoring, and the interpretation
of signatures of surface current gradients over oceanic fronts, internal waves, and
shallow-water bathymetry. Unfortunately, despite the fact that a SAR is a Doppler
radar, conventional SAR images do not provide direct information on target velocities, since the Doppler information in the raw data is normally utilised to obtain the
highest possible spatial resolution in flight (azimuth) direction. In a process called
aperture synthesis, targets are mapped to azimuthal locations in the image where
their contribution to the spectrum of the received signal during the SAR overpass
appears at a Doppler frequency of 0. This implies the assumption that targets have a
radial (line-of-sight) velocity of 0. Targets with a nonzero radial velocity will appear
shifted in azimuth direction, and it is sometimes possible to retrieve their velocity
from the visible displacement (e.g. between train and track or between ship and
wake), but this is not possible for distributed targets such as the ocean surface.
Within the last decade, considerable progress has been made in the development of two techniques that do permit a direct retrieval of line-of-sight surface
current fields from SAR data. One technique, called along-track interferometry
(ATI), requires a second antenna. The other technique is based on Doppler centroid estimates from conventional SAR raw data at a reduced spatial resolution.
Both techniques have been demonstrated in several experiments, and they are
available for immediate use with existing satellites. The direct imaging of surface
currents at relatively high spatial resolutions is particularly attractive for applications for which radar altimetry (e.g. Wunsch and Stammer, 1998) does not work and
R. Romeiser (B)
Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami,
FL 33149-1031, USA
e-mail: rromeiser@rsmas.miami.edu
73
V. Barale et al. (eds.), Oceanography from Space,
DOI 10.1007/978-90-481-8681-5_5, C
Springer Science+Business Media B.V. 2010
Direct Surface Current Field Imaging
from Space by Along-Track InSAR
and Conventional SAR
Roland Romeiser, Johnny Johannessen, Bertrand Chapron, Fabrice Collard,
Vladimir Kudryavtsev, Hartmut Runge, and Steffen Suchandt
5.1 Introduction
Since the SEASAT mission in 1978 (Fu and Holt, 1982), spaceborne synthetic
aperture radars (SARs) have acquired millions of high-resolution images of ocean
scenes, which have been used for applications such as wave and wind retrievals,
oil pollution monitoring, ship detection, sea ice monitoring, and the interpretation
of signatures of surface current gradients over oceanic fronts, internal waves, and
shallow-water bathymetry. Unfortunately, despite the fact that a SAR is a Doppler
radar, conventional SAR images do not provide direct information on target velocities, since the Doppler information in the raw data is normally utilised to obtain the
highest possible spatial resolution in flight (azimuth) direction. In a process called
aperture synthesis, targets are mapped to azimuthal locations in the image where
their contribution to the spectrum of the received signal during the SAR overpass
appears at a Doppler frequency of 0. This implies the assumption that targets have a
radial (line-of-sight) velocity of 0. Targets with a nonzero radial velocity will appear
shifted in azimuth direction, and it is sometimes possible to retrieve their velocity
from the visible displacement (e.g. between train and track or between ship and
wake), but this is not possible for distributed targets such as the ocean surface.
Within the last decade, considerable progress has been made in the development of two techniques that do permit a direct retrieval of line-of-sight surface
current fields from SAR data. One technique, called along-track interferometry
(ATI), requires a second antenna. The other technique is based on Doppler centroid estimates from conventional SAR raw data at a reduced spatial resolution.
Both techniques have been demonstrated in several experiments, and they are
available for immediate use with existing satellites. The direct imaging of surface
currents at relatively high spatial resolutions is particularly attractive for applications for which radar altimetry (e.g. Wunsch and Stammer, 1998) does not work and
R. Romeiser (B)
Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami,
FL 33149-1031, USA
e-mail: rromeiser@rsmas.miami.edu
73
V. Barale et al. (eds.), Oceanography from Space,
DOI 10.1007/978-90-481-8681-5_5, C
Springer Science+Business Media B.V. 2010