This Special Study Group was established to study various geodetic and oceanographic inversion methods and data assimilation techniques. Through a deeper understanding of such techniques new ideas may be brought in order to enhance the use of satellite altimetry.
1) The estimation of the marine gravity field has been highly improved with data from the geodetic missions of Geosat and ERS-1. However, most processing schemes leave parts of the medium and long wavelength parts of the gravity field unsolved. The SSG should study
a) The influence of ocean variability and on the data type (sea surface heights, slopes, or curvature data) on the recovery of the gravity field,
b) The use of TOPEX/POSEIDON altimetry as reference frame for GEOSAT and ERS-1 data,
c) The use a Global Circulation Models for elimination of the sea surface topography,
d) Procedures for processing altimeter data in a global gravity field mapping.
2) The inversion of altimetry into marine geoid and sea surface topography has been improved along with the increased accuracies of the altimeter data and the geopotential models. However, in many regions the gravity models are not adequately accurate. The SSG should study
a) The a-priori spectrum for the topography (homogeneous and isotropic),
b) The use of hydrodynamic flow mechanisms (geostrophy, friction, viscosity),
c) The use of hydrodynamic constraints (mass, salt, and heat balance),
d) The use of other data sources (ship gravimetry, hydrography, AVHRR/ATSR surface temperature, etc.).
3) The mapping of the ocean tides has been vastly improved in the deep ocean through the TOPEX/POSEIDON mission. However, in shelf regions major inconsistencies between the various models exist. The SSG should study
a) The trade-off between hydrodynamics and altimetry and the role of errors in the bathymetry,
b) Interpolation/extrapolation of ocean tides using empirical methods, assimilation techniques, or inversion techniques,
c) The smoothness and resolution of the ocean tides,
d) The use of other data sources (tide gauges, loadings, GPS, SAR).
O.Ba. Andersen (Denmark)
M. Brovelli (Italy)
R. Coleman (Australia)
G.D. Egbert (U.S.A.)
G. Evensen (Norway)
O. Francis (Belgium)
Y. Fukuda (Japan)
H. van Gysen (South Africa)
R.H.N. Haagmans (The Netherlands)
W. Keller (Germany)
P.J. van Leeuwen (The Netherlands)
F. Lyard (England)
P.-Y. Le Traon (France)
R.S. Nerem (U.S.A.)
N. Pavlis (U.S.A).
R. Ray (U.S.A.)
D. Stammer (U.S.A.)
P.L. Woodworth (England)
Changyou Zhang (U.S.A.)
The field of satellite altimetry has been extremely active through the past four year period and important progress has been achieved in the area subject to this study group. Study group members were present at 2-3 meetings each year to present results.
The activities of the space agencies have been very important for the activities of this study group. Two satellites have been launched (ERS-2 and GFO) and the TOPEX/POSEIDON extended mission was initiated. Especially, the satellite mission associated science working teams (SWT) and the workshops that have been held for the SWTs have played an important role for the scientific work. The TOPEX/POSEIDON SWT has meet annually. Also, the annual EGS meetings have had session on altimetry jointly between geodesists and oceanographers.
The progress within the specific tasks of this study group are as follows:
1) The estimation of the marine gravity field has been highly improved with data from the geodetic missions of GEOSAT and ERS-1-2. The procedures for global processing of altimetry in a determination of the marine gravity field has been improved significantly. Especially, the improvement in the modelling of the covariance function has been important. In estimates of the mean sea surface the use of TOPEX/POSEIDON was very valuable to define the reference frame.
2) The inversion of altimetry into marine geoid and sea surface topography has been improved along with the increased accuracies of the altimeter data and the geopotential models. Further improvements have been obtained by introducing dynamic constraints in the determination the sea surface topography. Through the period there has been put focus on long term changes in the sea surface topography to study Sea Level Rise and Global Change. Especially, issues related to calibration of the altimeter and atmospheric models used for corrections have been discussed.
3) The mapping of the ocean tides has been vastly improved in the deep ocean through the TOPEX/POSEIDON mission. Through the period the ocean tide modelling at shallow seas has been vastly improved. The improvements have been achieved by combining more data and by improving the estimation techniques. Furthermore, more tidal constituents have been included. The role of errors in the bathymetry models have been demonstrated to cause a trade-off between hydrodynamics and altimetry.
Conclusions and recommendations
Though the level of activities within the field of satellite altimetry is very high and many interesting results for geodesists are found, it became evident though the period that the role of the study group was unimportant. The members put much more effort into the collaboration that was supported by the space agencies. Hence, it is not recommended that SSG-4.168 is continued.
Additional information on the activities within the field
of satellite altimetry and information about publications may be found
at http://topex-www.jpl.nasa.gov and http://www.esrin.esa.it