Determination of the Relative Bias Between JASON and TOPEX Using Tide Gauge Data

Low-frequency climate changes, such as the determination of global mean sea level, require time series on the order of decades or longer. However, no single altimeter satellite can ever hope to accumulate the necessary uninterrupted time series. Therefore, it becomes increasingly important to calibrate the measurements from various satellite altimeter missions relative to one another in order to integrate the measurements into a comprehensive data set. In previous studies, a relative bias has been estimated between Geosat and TOPEX using data from the WOCE tide gauge network. However, the true accuracy of the method is unclear given the large time gap and no other external verification. During the TOPEX/Poseidon-Jason Calibration/Validation Mission, 210 days of data are available to test the relative bias computed from tide gauges with a more accurate calculation computed from direct Jason-TOPEX SSH residuals. This provides a unique opportunity to validate the technique and assess its accuracy. Here, we discuss the technique of relative bias calculation with tide gauges, and examine the results when applied to Jason and TOPEX. Results suggest an accuracy of a few millimeters. Also, results are presented for the relative bias calculation between TOPEX Side A and TOPEX Side B. The Side A/Side B tests include the application of different sea state bias models and their effects on bias determination. A comprehensive investigation is presented concerning the effects of altimeter data gaps and overlaps on these relative bias estimates. Additionally, data from the new TOPEX Tandem Mission are examined in order to quantify the effect of mean sea surface (MSS) error on relative bias estimation. During the Tandem Mission, TOPEX will be flying along a new groundtrack which has not been included in current MSS models.