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Centimeter-Level Cross Calibration of TOPEX and Jason Using Retracking

Ernesto Rodriguez(1) , Philip Callahan(1) , and Kelley Case(1)

(1) Jet Propulsion Laboratory, Mail Stop 300-319, Pasadena, CA 91109-8099, United States


The use of altimetry data for new challenging applications, such as determining sea level rise or more accurate sea surface slopes for bathymetry determination, has made improving the accuracy of altimetry heights of paramount importance. As an outstanding example, we note that the TOPEX and Jason altimeters suffer from relative sea state biases which may be related to the different processing of the waveform data.

In the past, the use of altimeter waveform retracking has been limited by computer resources. This has led to the use of sub-optimal, but fast, retrackers which often had to trade-off height biases against noise. With the advent of faster computers, it is possible to attempt a truly optimal retracking algorithm. To this end, we introduce a Maximum a Posteriori (MAP) retracking algorithm which significantly improves the covariance of the retrieved parameters. We will present the theoretical basis of this algorithm and compare against other algorithms currently in use.

We have performed waveform retracking of TOPEX and Jason-1 data during their colinear phase in 2002 using the MAP retracker in order to understand and remove instrumental effects that may affect the cross calibration. Because the satellites were on the same track only about 70 seconds apart, environmental effects are identical, so one can more directly compare the instrumental response. By using the same algorithms on the two altimeters we also eliminate this source of difference in the comparison. We will present findings on variations in the instruments in a variety of conditions, e.g., wave heights, to produce geographically consistent results. These results will allow the cross calibration of TOPEX and Jason-1 to the one centimeter level which is needed for continuing global sea level studies.



                 Last modified: 07.10.03