Dataset Descriptions

• TOPEX/Poseidon Sea Surface Height Residuals
•  Along-track grid (6.2 km spacing)  Sep 23, 1992- Aug 11, 2002
•  Uniform grid (1° x 1° x 5-day averages)  Nov 1992- Mar 3, 2001


• TOPEX/Poseidon Significant Wave Heights
  • Along-track grid (6.2 km spacing)   Sep 23, 1992- Jan 03, 2001

Altimetry Background

Importance of Sea Level Monitoring
      Sea level and its horizontal slope and time changes are the surface expression of ocean processes occurring over large spatial and temporal scales.  Large-scale currents (such as the Kuroshio, extending up to kilometers in depth) and planetary Rossby waves (thousands of km in horizontal extent) carry the memories of past air-sea exchanges that can affect subsequent weather at great distances from the source of the original air-sea exchange.  Sea level also provides evidence of local heating by solar radiation, the perennial tides, and the addition of water to the ocean by melting ice caps or glaciers. 

Development of Altimetry
       Altimeters emit a sequence of short pulses at microwave frequencies and then measure the return times to ascertain the instrument-surface distance.  The development of satellite altimeters for the active sensing of ocean surface topography has been one of the primary objectives of NASA's Ocean Processes Program since 1970 and of NASA's Earth Science Enterprise (formerly Mission to Planet Earth) since 1990.  Over the past 10 years, altimetric measurements of sea level has become an indispensable research tool.  The Figure below traces the history and future of altimetry missions.  Skylab (1974) demonstrated that satellite altimetry was possible, Geos-3 (1974-77) made measurements good enough for geoid studies, Seasat (1978-78) showed the potential for ocean studies, and Geosat (1985-89) and ERS-1 (1991-1996) produced routine ocean products without the long wavelength components, due to orbit and media errors. 
      It now may be feasible to monitor the rise of the global sea level and its potential acceleration, to an accuracy of 1 mm/yr or better.  The current altimetry missions, ERS-2 (1996-) and especially TOPEX/Poseidon (1992-) added precise, large-scale measurements, allowing retrieval of both fast and interannual sea level variations.  This is made possible through high accuracy (3 cm for TOPEX/Poseidon, Fu et al., 1994), the ability to map the global ocean with a temporal sampling of a few days, and the prospect of a long-term time series decades into the future.  The stringent accuracy requirement necessitates additional improvements to present and future altimeter data, to accurately link missions from different times, and optimally combine measurements from contemporaneous missions that display systematic differences. Coastal work requires high spatial resolution, which can only be obtained by combining data from different contemporary satellites, which demands that their sea level not have systematic differences, leading to similar needs. 

Altimeter Spacecrafts: 1990's and Beyond

• extend the usefulness of altimetric sea level from several satellite missions as close to the coastlines as possible, including littoral areas and semi-enclosed seas  
• combine sea level from the different missions in a consistent manner  
• generate simple along-track sea level residual products  
• generate grids, 1 degree or finer in latitude and longitude and 5 days in time in deep water, and every 0.5 degree or finer in shallow waters  
• assess the errors of all the data products  
• use a data structure for the products that facilitates both extracting along track data in small regions for browsing and adding newly processed data.