The last eight years have seen enormous progress in the application of the Geosat altimetry data to the study of oceanographic problems, albeit with a great deal of effort in treating the various errors in the data. Significant advancement has been made in the knowledge of the temporal variabilities of the western boundary currents, the mesoscale eddies, the tropical circulation, and the Antarctic Circumpolar Current. The multiyear, global data set has generated a great deal of progress in developing methods of data assimilation, setting the stage for using altimetry data as a fundamental constraint for global ocean modeling and description.
The launch of TOPEX/POSEIDON has provided scientists with the first truly global ocean observing system, producing a ``snap shot'' of the global ocean circulation with an unprecedented accuracy every 10 days. Coupling with two other concurrent developments---the World Ocean Circulation Experiment (WOCE) and the maturing technology of global ocean modeling---TOPEX/POSEIDON is creating enormous opportunities for making significant advancement in the understanding of global ocean dynamics. The high accuracy of TOPEX/POSEIDON would help calibrate and improve other less accurate missions such as ERS-1 and 2 and enhance their value for large-scale circulation studies. To achieve the ultimate goal of understanding the ocean's role in climate, the observation provided by TOPEX/POSEIDON must be sustained with consistent quality. Plans are in place to extend the mission from three to at least six years as long as the flight hardware is functioning. A series of follow-on missions to extend the TOPEX/POSEIDON data stream to the indefinite future is being studied jointly by NASA, NOAA, and the French CNES (Centre National d'Etudes Spatiales). The prospect of having a combination of various altimeters with at least one of them having the TOPEX/POSEIDON accuracy holds great promise for ocean circulation research in the next decade. This series of altimetry missions will become a key element of a global ocean observing system for unravelling the ocean's role in climate change.
Acknowledgments. The bibliography in altimetry research complied by Dr. C. R. Rapley was a valuable source of information for the review. The authors are indebted to Dudley Chelton for his thorough and constructive review of the paper. The research described in the paper was carried out in part (LLF) by the Jet Propulsion Laboratory, California Institute of Technology, under contract with National Aeronautics and Space Administration. Support from the TOPEX/POSEIDON Project funded under the NASA TOPEX/POSEIDON Announcement of Opportunity is acknowledged.