Coherent ranging with Envisat radar altimeter: a new perspective in analyzing altimeter data using Doppler Processing

TL;DR

This paper reveals that ESA's Envisat radar altimeter data contains inter-pulse coherence, enabling Doppler processing to improve sea surface height measurements, especially in challenging coastal zones.

Contribution

It introduces the novel zero-Doppler processing method for archived Envisat data, leveraging inter-pulse coherence for enhanced altimeter accuracy.

Findings

Inter-pulse coherence exists in Envisat data.

Zero-Doppler processing improves SSH accuracy.

Potential benefits in coastal and complex terrains.

Abstract

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.