IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 49, NO. 1, JANUARY 2011 155 Ocean Wave Integral Parameter Measurements Using Envisat ASAR Wave Mode Data Xiao-Ming Li, Susanne Lehner, and Thomas Bruns Abstract—An empirical algorithm to retrieve integral ocean wave parameters such as significant wave height (SWH), mean wave period, and wave height of waves with period larger than 12 s (H 12 ) from synthetic aperture radar (SAR) images over sea surface is presented. The algorithm is an extension to the Envisat Advanced SAR (ASAR) wave mode data based on the CWAVE approach developed for ERS-2 SAR wave mode data and is thus called CWAVE_ENV (CWAVE for Envisat). Calibrated ASAR images are used as the only source of input without needing prior information from an ocean wave model (WAM) as the standard algorithms used in weather centers. This algorithm makes SAR an independent instrument measuring integrated wave parame- ters like SWH and mean wave period to altimeter quality. A global data set of 25 000 pairs of ASAR wave mode images and collocated reanalysis WAM results from the European Centre for Medium-Range Weather Forecasts (ECMWF) is used to tune CWAVE_ENV model coefficients. Validation conducted by com- paring the retrieved SWH to in situ buoy measurements shows a scatter index of 0.24 and 0.16 when compared to the ECMWF reanalysis WAM. Two case studies are presented to evaluate the performance of the CWAVE_ENV algorithm for high sea state. A North Atlantic storm during which SWH is above 18 m as observed by SAR and Radar Altimeter simultaneously is analyzed. For an extreme swell case that occurred in the Indian Ocean, the potential of using SWH measurements from ASAR wave mode data derived by the CWAVE_ENV algorithm is demonstrated. Index Terms—Empirical algorithm, integral wave parameter, synthetic aperture radar (SAR), wave mode data. I. I NTRODUCTION O CEAN waves are the ocean’s most obvious surface fea- ture, which interact with atmosphere, ocean currents, bottom topography, and with one another. For many reasons, an understanding of their statistical properties is required, such as marine transportation, global sea state statistics, and its changes, as well as ocean wave parameters in specific locations for harbor and ocean engineering, ship design, and coastal protection. Ocean waves are traditionally measured in situ at one point, as by moored buoys, which are normally located near to coast, giving very limited spatial coverage. Satellite remote sensing, particularly active microwave sensors, e.g., synthetic Manuscript received December 21, 2009; revised April 23, 2010; accepted May 23, 2010. Date of publication July 12, 2010; date of current version December 27, 2010. X.-M. Li and S. Lehner are with the Remote Sensing Technology Insti- tute, German Aerospace Center (DLR), 82234 Wessling, Germany (e-mail: Xiao.Li@dlr.de; Susanne.Lehner@dlr.de). T. Bruns is with Seeschifffahrtsberatung, German Weather Service (DWD), 20359 Hamburg, Germany (e-mail: Thomas.Bruns@dwd.de). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TGRS.2010.2052364 aperture radar (SAR) and radar altimeter (RA), offers alternate approaches to observe ocean surface waves on a global scale. SAR is a unique sensor that can provide 2-D ocean surface information with high spatial resolution, independent of cloud cover and daytime. The L-band SAR sensor onboard SEASAT launched in 1978 provided a first realization of global ocean surface measure- ments from space (see [1]). From 1991 until now, the ERS-1, ERS-2, and Envisat missions launched by the European Space Agency (ESA) have operationally provided continuous SAR ocean wave measurements. On these platforms, SAR and RA are onboard jointly. In principle, two completely independent surface wave measurements from space are available. While the altimeter measurements are acquired at nadir, the SAR measurements are taken at about 300 km away looking to the right. Therefore, these double tracks provide simultaneous sea state measurements, which is particularly useful for extreme sea state validation in storms with strong gradient in wave height field. Both measurements can be used jointly for wave climate analysis, reducing the limitation of spatial sampling. Aside from the assimilation of RA measurements at nadir track, SAR can provide another source of observation as an additional quality control for data assimilation in numerical wave models (WAMs). The measurement of significant wave height (SWH) by al- timeters is well established, and the retrieved accuracy is com- parable to that of in situ buoy measurements, e.g., [2] and [3]. In addition to SWH, mean wave period is another important sea state parameter. Unlike the approach for SWH measurements, retrieval of wave period from RA is still under development. Several empirical models, e.g., [4]–[6], have been proposed to obtain wave period measurements from altimeter data. Following an overview of sea state measurements from RA data, the current algorithms to derive the 2-D ocean wave spectra from SAR are briefly summarized. A new method to derive integral wave parameters from Advanced SAR (ASAR) images is presented. A. Ocean Wave Measurements From SAR Nonlinear Retrieval Approach: The mechanisms of SAR imaging sea surface gravity waves generally consist of the lin- ear approaches of tilt and hydrodynamic modulation, as well as the nonlinear distortion induced by the radial wave motions [7]. This leads, among other effects, to image smearing and to a loss of information beyond the so-called azimuth cutoff wavelength [8]. For ERS and Envisat SAR, this corresponds typically to wavelengths shorter than about 200 m in the along-track 0196-2892/$26.00 © 2010 IEEE