Removing Intra-1-Hz Covariant Error to Improve Altimetric Profiles of σ0 and Sea Surface Height

Waveform retracking is the process by which a simple mathematical model is fitted to altimeter returns. Over the ocean, the waveform location, the amplitude, and the shape can be fitted by models with 3-5 free parameters, which may, in turn, be linked to geophysical properties of the surface of interest-principally sea surface height (SSH), wave height, and normalized backscatter strength (sigma(0), related to wind speed). However, random multiplicative noise, which is due to the summation of power from multiple differently orientated surfaces, produces errors in the estimation of these model parameters. Examination of the correlations among parameters estimated for each waveform leads to simple empirical corrections that reduce the waveform-to-waveform noise in geophysical estimates, resulting in smoother (and more realistic) along-track profiles of sigma(0) and SSH. These adjustments are fundamentally dependent upon the waveform model and retracker implemented, but when applied show improved agreement between near-simultaneous measurements from different altimeter missions. The effectiveness of these empirical adjustments is documented fully for MLE-4 retracking of the Jason-3 altimeter, with a reduction in the 1-s variance of sigma(0) by 97%. However, the ideas are applicable and beneficial for data from other altimeters, with small improvements in sigma(0) for MLE-3 and for AltiKa at Ka-band, while reductions in range variance of similar to 40% are noted for most retrackers evaluated.