the incidence angle of the ERS SAR is oblique (23) to the local mean angle of
the ocean surface, there is almost no direct
specular reflection except at very high sea states.
It is therefore assumed that at first approximation Bragg resonance is the primary
mechanism for backscattering radar pulses.
The Bragg equation defines the ocean wavelengths for Bragg scattering as a function
of radar wavelength and incidence angle:
The short Bragg-scale waves are formed in response to wind stress. If the sea
surface is rippled by a light breeze with no long waves present, the radar
backscatter is due to the component of the wave spectrum which resonates with
the radar wavelength.
The threshold windspeed value for the C-band waves is estimated to be at about
3.25 m/s at 10 meters above the surface. The Bragg resonant wave has its crest
nominally at right angles to the range direction.
For surface waves with crests at an angle to
the radar line-of-sight (see this figure )
the Bragg scattering criterion is
is the wavelength of the surface waves propagating at angle
to the radar line- of sight.
The SAR directly images the spatial distribution of the Bragg-scale waves. The
spatial distribution may be effected by longer gravity waves, through tilt modulation,
hydrodynamic modulation and velocity bunching.
Moreover, variable wind speed, changes in stratification in the atmospheric
boundary layer, and variable currents associated with upper ocean circulation
features such as fronts, eddies, internal waves and bottom topography effect
the Bragg waves.
Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,