Retrieval of ocean wave spectra from ERS-SAR: a validation study

The only instrument which supplies observations of ocean wave spectra with global coverage is the synthetic aperture radar (SAR) mounted on satellites. The most extensive data set has been provided by ERS-1 and ERS-2, which have collected over 4 million “wave mode” SAR imagettes since 1991. A closed relation that calculates the SAR spectrum from a given wave spectrum was proposed by Hasselmann and Hasselmann (1991). To retrieve the ocean wave spectrum from a measured SAR spectrum, this relation must be inverted. This inversion is non-trivial due to the nonlinearity of the forward relation, and requires first-guess information. The choice of first-guess information is dependent on what is available, and the weighting of SAR and first-guess information is to some extent arbitrary. Hence, various inversion strategies can be formulated. In this study, the authors validate the results of two such strategies against independent buoy data