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28 - 29 March 2001
Report Edited by Dudley B. Chelton
The 9-year record of TOPEX/POSEIDON (T/P) altimeter data has demonstrated the utility of highly accurate ocean topography measurements for a wide range of scientific and operational applications. While the 10-day exact-repeat sampling pattern of the T/P orbit minimizes temporal aliasing of mesoscale variability at the measurement locations along the satellite ground track, the corresponding 2.8-degree longitudinal spacing of ground tracks restricts the spatial scales of sea surface height variability that can be resolved by the T/P sampling pattern. In the figure to the right, the upper right quadrant defined by the intersecting dashed lines defines the approximate domain of space and time scales that can be resolved in SSH fields constructed from T/P data. There are many physical processes that occur on spatial scales that are not fully resolved by the T/P dataset.
In recognition of the importance of these shorter-scale phenomena, NASA established a High-Resolution Ocean Topography Science Working Group (HOT SWG) to review the scientific and operational rationales for high-resolution measurements of ocean topography, and to review the status of existing and developing technologies for measuring ocean topography with higher resolution than has heretofore been possible.
One approach to obtaining high-resolution measurements of ocean topography is from a constellation of conventional nadir-looking satellites flying in formation with coordinated orbits. The false-color image below shows mesoscale features in the sea surface temperature field associated with the Gulf Stream off the east coast of the United States. The diagonal lines represent the sampling pattern for measurements along five evenly spaced, parallel tracks in the 66-degree inclination and 9.9156-day exact repeat period of the TOPEX/POSEIDON orbit. The heavy lines indicate the sampling pattern of a single altimeter in the TOPEX/POSEIDON orbit. Two new technologies have been proposed for low-cost, low-risk multi-satellite altimetry. One concept, called WITTEX, is based on the delay-Doppler technique, which provides measurements with high accuracy and small footprint size from a low-power, dual-frequency altimeter. The other concept, called AltiKa, is based on measurements at a high Ka-band frequency of about 35 GHz at which ionospheric effects are negligible, thus providing measurements with small footprint size from a low-cost single-frequency altimeter.
Another approach to obtaining high-resolution measurements of ocean topography is by radar interfometry across a swath of width 200 km centered on the satellite ground track, as shown schematically in the figure labeled Radar Interfometry to the right. While the interferometric technique for swath altimetry is still under development, the simulations that have been conducted to date reveal the exciting potential for global high-resolution measurements of ocean topography.
A third approach to obtaining high-resolution measurements of ocean topography is based on analysis of the reflections of the signals transmitted by the satellite constellation of the Global Positioning System (GPS). Although the technique of altimetry from GPS reflections is less mature than the other technologies, the potential for obtaining global high-resolution measurements of ocean topography is appealing. With continued technology development and improved understanding of this technique, GPS altimetry may yield altimetric measurements with the accuracy required for oceanographic applications.
The HOT SWG has recently completed its work. The results are summarized in a 224-page report. An electronic copy of the full report, as well as electronic copies of the Executive Summary and of the Overview can be downloaded in Adobe PDF format by clicking the thumbnails below. The PDF files of the overview and the full report include bookmarks which are helpful for navigating through the PDF file. If you are using version 5.0 of Adobe Acrobat Reader, click the bookmark tab on the left side of the window. If you are using Version 4.0, click the "Window" drop-down menu and then click "Show Bookmarks" to display the bookmarks on the left side of the window. If you are using version 3.0 or lower, you may have difficulties viewing the file.