  ESA's Envisat mission carried a RA-2 radar altimeter since its launch in 2002
to sense sea state and especially measure sea surface height (SSH). The onboard
processing combined multiple echoes incoherently to reduce Speckle noise and
benefit from data compression. In fact, according to past literature the
amplitudes were generally expected to be independent. Nevertheless, samples of
complex data time series of individual echoes (IE) were down-linked and
archived since 2004 for research studies. In this note we demonstrate that
there is sufficient inter-pulse coherence for Doppler processing and we suggest
that the archived data can be re-processed into improved SSH. This is of
particular interest in challenging domains (e.g., coastal zone) where coherent
processing can mitigate errors from ocean surface backscatter inhomogeneity and
nearby land backscatter. A new method called zero-Doppler to process IEs is
thus proposed and discussed.
