Error Analysis of Empirical Sea State Bias Models for Pulse-limited and SAR Altimetry

Abstract:

The sea state bias (SSB) is an altimeter range delay caused by ocean waves, and remains the largest error in the sea level error budget for pulse-limited altimetry, and has not yet been established for SAR mode altimetry. Understanding the level of error within SSB models becomes increasingly important with the upcoming Sentinel-6A mission as we transition from pulse-limited to SAR mode altimetry while also maintaining the 28-year legacy of altimeter-derived sea level measurements.

Empirical approaches using in-flight observations of sea surface height are typically employed to determine models for the SSB. These observations contain signals such as dynamic ocean topography, orbit and altimeter measurement errors, as well as ionosphere, troposphere, and geophysical modeling errors, which all contribute to errors in SSB estimates. In an effort to improve sea level estimation and maintain climate record continuity, we investigate the relative contribution of these error sources to provide an error assessment of pulse-limited and SAR mode SSB models.

In order to evaluate errors in the SSB estimates, we have examined the correlation between sea level correction uncertainties and model variables, the propagation of error in altimeter measurements, geophysical model errors, retracker uncertainty, and instrumentation error. Due to the differing altimetric design between pulse-limited and SAR mode, further analysis has been given to investigate across-track mispointing effects, and swell and wave effects. These evaluations are conducted using a simple interpolation approach to SSB modeling, which enables the model to be used as a tool to examine mismodeled sea level corrections and altimetric errors as a function of sea state variables.