9.3 Remote Sensing Techniques
317
1. Variation in the local angle of incidence associated with variation in the
facet orientation and position.
2. Variation in the energy of Bragg scattering, caused by interactions between the short ripples and longer waves.
3. Oscillation of the Doppler shift of the return signal which results in variations of the apparent facet density in the SAR image plane.
A full discussion of the extraction processes and comparison with the WAM
model can be found in the book by Komen et al. (1994).
9.3.3 Tides Observed by Satellites
Satellite altimeter data are extensively used to improve ocean tide models (see
Chap. 5). The current methodologies for this improvement can be categorized
in three groups: hydrodynamic modelling, empirical methods which use only
data, and assimilation methods which use both data and hydrodynamic modelling. The first and second methods, and the resulting tidal pattern of the
world oceans have been described in Chap. 5. Here we consider an application
of satellite altimeter measurements to improve the empirical tide models. In
particular, Cartwright and Ray (1990) used one year of altimetry data from
GEOSAT to derive estimates of the diurnal and semi diurnal oceanic tides for
latitudes between 58°N and 60 0 S. Global maps for M2 and 52 constituents
compare well with ground truth at 66 open ocean sites.
The TOPEX/POSEIDON mission provided another opportunity to reduce the
error of tidal prediction when comparing with ground-truth data. Ma et al.
(1994) used one year of TOPEX/POSEIDON altimeter measurements to correct
the Cartwright and Ray (1990) model. The corrections were determined on a
3° x 3° grid. Comparison of tide gauge data and improved model predictions
showed that the root-mean-square differences for M2 constituents reduced from
3.9 cm to 2.7 cm. The corresponding reductions for 52 and Kl constituents are
from 2.7 cm to 1.7 cm, and from 2.0 cm to 1.7 cm, respectively.
A combination of hydrodynamic modelling and TOPEX/POSEIDON altimeter
measurements was used by Le Provost et al. (1995, 1998b) to calculate tides in
the world oceans (for more details see Sect. 5.3.3). The increased accuracy of
tidal prediction is required not only for better representation of tides but also
for the study of general ocean circulation.
9.3.4 Ocean Circulation Observed by Satellites
The importance of accurate sea surface height measurements for large-scale
ocean circulation results from the fact that the sea surface topography, relative
to the geoid, represents the pressure field of the geostrophic part of the general circulation. However, sea surface height is influenced not only by surface
geostrophic currents, but also by surface waves, tides and atmospheric pressure variations. Furthermore, the altimeter measurement of sea surface height
317
1. Variation in the local angle of incidence associated with variation in the
facet orientation and position.
2. Variation in the energy of Bragg scattering, caused by interactions between the short ripples and longer waves.
3. Oscillation of the Doppler shift of the return signal which results in variations of the apparent facet density in the SAR image plane.
A full discussion of the extraction processes and comparison with the WAM
model can be found in the book by Komen et al. (1994).
9.3.3 Tides Observed by Satellites
Satellite altimeter data are extensively used to improve ocean tide models (see
Chap. 5). The current methodologies for this improvement can be categorized
in three groups: hydrodynamic modelling, empirical methods which use only
data, and assimilation methods which use both data and hydrodynamic modelling. The first and second methods, and the resulting tidal pattern of the
world oceans have been described in Chap. 5. Here we consider an application
of satellite altimeter measurements to improve the empirical tide models. In
particular, Cartwright and Ray (1990) used one year of altimetry data from
GEOSAT to derive estimates of the diurnal and semi diurnal oceanic tides for
latitudes between 58°N and 60 0 S. Global maps for M2 and 52 constituents
compare well with ground truth at 66 open ocean sites.
The TOPEX/POSEIDON mission provided another opportunity to reduce the
error of tidal prediction when comparing with ground-truth data. Ma et al.
(1994) used one year of TOPEX/POSEIDON altimeter measurements to correct
the Cartwright and Ray (1990) model. The corrections were determined on a
3° x 3° grid. Comparison of tide gauge data and improved model predictions
showed that the root-mean-square differences for M2 constituents reduced from
3.9 cm to 2.7 cm. The corresponding reductions for 52 and Kl constituents are
from 2.7 cm to 1.7 cm, and from 2.0 cm to 1.7 cm, respectively.
A combination of hydrodynamic modelling and TOPEX/POSEIDON altimeter
measurements was used by Le Provost et al. (1995, 1998b) to calculate tides in
the world oceans (for more details see Sect. 5.3.3). The increased accuracy of
tidal prediction is required not only for better representation of tides but also
for the study of general ocean circulation.
9.3.4 Ocean Circulation Observed by Satellites
The importance of accurate sea surface height measurements for large-scale
ocean circulation results from the fact that the sea surface topography, relative
to the geoid, represents the pressure field of the geostrophic part of the general circulation. However, sea surface height is influenced not only by surface
geostrophic currents, but also by surface waves, tides and atmospheric pressure variations. Furthermore, the altimeter measurement of sea surface height