A First Comparison of Simultaneous Sea Level Measurements from Envisat, GFO, Jason-1, and TOPEX/Poseidon

The multiple altimeter missions have not only advanced our knowledge of ocean circulation, ice sheet topography, and global climate, but also improved the accuracy of altimetric measurements by cross-calibration and validation. In this paper, one year's simultaneous maps of sea level anomaly (MSLA) data obtained from four altimeters, Envisat, Geosat Follow-On (GFO), Jason-1, and TOPEX/Poseidon (T/P), have been compiled for a preliminary comparison. First, the discrepancy in global geographical distribution of each product relative to the merged MSLA field is analyzed and its signal retrieval capability is discussed. Second, the space/time variability of each discrepancy in the Atlantic Ocean, Indian Ocean, Pacific Ocean, Northern Hemisphere, Southern Hemisphere, and global ocean is studied. Third, each discrepancy as a function of latitude, longitude, and merged MSLA is presented. The results show that Jason-1 is the best single-mission for mapping large scale sea level variation, while T/P in its new orbit presents the poorest estimation of SLA due to the short period (from cycle 369 to 403) used to determine the mean profile. A clear understanding of each product discrepancy is necessary for a meaningful combination or merging of multi-altimeter data, optimal product selection, as well as for their assimilation into numerical models.