GUIDE WAVE ANALYSIS AND FORECASTING - WMO

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the SAR image. Our problem is to go the other way: to
derive the directional wave spectrum given the SAR
image. Research has shown that if a global wave model
is run to provide an initial estimate of the wave spectrum,
then the difference between the observed SAR
spectrum and that estimated from the model spectrum
can be used to correct the model spectrum and hence to
improve the wave model output (Hasselmann et al.,
1991).
A practical problem with SAR, which tends to
inhibit its use, is the vast quantity of data it produces
(108 bits per second) and the consequent cost of processing
and acquiring the data. Moreover, these data cannot
be stored on board the satellite, so SAR data can only be
obtained when the satellite is in sight of a ground receiving
station. An exception to this is the ERS-1 which
obtains a small SAR image, with a footprint of 5 x 5 km,
every 200 km. These “wave vignettes” can be stored on
board and transmitted later to ground.
8.5.6 Microwave radiometry
As well as reflecting incident radio waves, the sea radiates
thermal radio noise as a function of its temperature
and emissivity; this radiation can be detected by a
microwave radiometer (analogous to an astronomer’s
radio telescope). The emissivity varies with surface
roughness, amount of foam and, to a small extent, with
salinity. Thus the signal received at the antenna is mainly
GUIDE TO WAVE ANALYSIS AND FORECASTING
Secondary
component
Ocean
wavelength
(metres)
Figure 8.6(a) (left) —
SEASAT SAR image of the
wave field between the
Islands of Foula and
Shetland (ESA photograph,
processed at the Royal
Aircraft Establishment,
United Kingdom)
Figure 8.6(b) (below) —
Examples of directional
wave-energy spectrum
derived by digital processing
of SEASAT synthetic aperture
radar data. The five
levels of greyness indicate
spectral amplitude, while the
distance from the centre
represents wavenumber
(2π/λ). The circles are identified
in wavelength. A
200 m swell system is
shown coming from ESE
and broader spread 100 m
waves coming from ENE.
The analysis has an 180°
ambiguity (after Beale,
1981)
Spacecraft
velocity vector
Primary component
Image energy
density (linear scale)