IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 7, NO. 4, OCTOBER 2010 621 Sea-State Determination Using GNSS-R Data J. F. Marchan-Hernandez, E. Valencia, N. Rodriguez-Alvarez, I. Ramos-Perez, X. Bosch-Lluis, A. Camps, Francisco Eugenio, and Javier Marcello Abstract—Global Navigation Satellite Systems (GNSS) signals can be used to infer geophysical data related to the surface where they scatter. When dealing with the sea surface, its state influences the GNSS scattered signals and, therefore, the GNSS reflectom- etry (GNSS-R) observables. The aim of the Advanced L-band Emissivity and Reflectivity Observations of the Sea Surface 2008 field experiment was to gather experimental data to study the relationship of the GNSS-R delay-Doppler maps (DDMs) and the sea state. This work describes the field campaign and the main results obtained, where among them is the use of the DDM volume as a roughness descriptor weakly affected by the GPS satellite geometry. Index Terms—Delay-Doppler map (DDM), Global Navigation Satellite Systems reflectometry (GNSS-R), sea state. I. I NTRODUCTION S EA SURFACE salinity is a key parameter in both clima- tology and oceanography fields, and its retrieval can be accomplished by means of L-band radiometry [1]. The sea state introduces a change in the observed brightness temperatures that must be corrected for. This can be achieved using active illumination systems, as planned for the National Aeronautics and Space Administration’s Aquarius salinity mission, or by means of reflected signals from Global Navigation Satellite Systems reflectometry (GNSS-R). The use of GNSS-R has been studied through the last 15 years, which yielded promising results: from altimetry ap- plications [2], [3] to soil moisture determination [4], ice char- acterization [5], or sea-state retrieval [6], [7]. For the GNSS-R sea-state determination approach to perform the roughness- induced radiometric correction, the power and mass require- ments are significantly lowered since there is no receiver, and a small antenna can be used. Moreover, the bistatic geometry Manuscript received October 2, 2009; revised December 29, 2009. Date of publication April 1, 2010; date of current version October 13, 2010. This work, conducted as part of the award “Passive Advanced Unit (PAU): A Hybrid L-band Radiometer, GNSS-Reflectometer and IR-Radiometer for Passive Re- mote Sensing of the Ocean” made under the European Heads of Research Councils and European Science Foundation European Young Investigator (EURYI) Awards scheme in 2004, was supported by funds from the Partici- pating Organizations of EURYI and the EC Sixth Framework Program and was also funded by Research Projects ESP2007-65667-C04 and AYA2008-05906- C02-01/ESP. J. F. Marchan-Hernandez, E. Valencia, N. Rodriguez-Alvarez, I. Ramos-Perez, X. Bosch-Lluis, and A. Camps are with the Remote Sensing Laboratory, Departament de Teoria del Senyal i Comunicacions, and the Aeronautics and Space Research Center (CRAE), Institut d’Estudis Espacials de Catalunya, Universitat Politècnica de Catalunya, 08034 Barcelona, Spain (e-mail: jfmarchan@tsc.upc.edu). F. Eugenio and J. Marcello are with the Departamento de Señales y Sistemas, Universidad de Las Palmas de Gran Canaria, 35017 Las Palmas de Gran Canaria, Spain. 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/LGRS.2010.2043213 ensures a strong signal return in the specular direction, in opposition to the weak return obtained with monostatic radar off-nadir configurations. Additionally, the GNSS-R approach is less sensitive to power calibration since both direct (op- portunity transmitter–receiver link) and scattered signals are processed. The existence of several deployed or planned GNSS constellations (GPS, GLONASS, Galileo, Compass, IRNSS, ...) increases the availability of temporal and spatial colocated GNSS-R and radiometric measurements. In [8], a model of the reflected waveform is fitted to the measured waveform using the wind speed (WS) as the tuning parameter. Another approach calls for using the so-called interferometric complex field, resulting from the coherent processing of both direct and reflected signals, to obtain a direct estimate of the significant wave height (SWH) [9]. These two approaches require the use of both electromagnetic and sea surface models. To avoid modeling errors in the brightness temperature correction for salinity retrieval, a direct relationship between the sea state and a GNSS-R observable was put forward in [10]. The proposed GNSS-R descriptor is the volume of the normalized delay- Doppler map [(DDM); maximum amplitude equal to one] above a threshold to eliminate the noise dependence. Such a descriptor is related to the extent of the glistening zone (the area from which scattered signals are observed), which depends on the sea state. To test this assumption, the Advanced L-band Emissivity and Reflectivity Observations of the Sea Surface 2008 (ALBATROSS 2008) campaign was conducted in the north coast of the Gran Canaria island (Mirador del Balcón, La Aldea de San Nicolás). II. CAMPAIGN DESCRIPTION The campaign aimed to acquire an extensive data set of collocated GNSS-R and L-band radiometric data over the sea surface under different sea-state conditions using ground-based sensors. The three main requirements of the test site were: 1) its geometry, the highest possible height over the sea surface, the better, to cover the largest possible range of delays; 2) sea- state variability; and 3) its orientation to north, to minimize Sun and galaxy contamination in the radiometric measurements. The selected site (Mirador del Balcón) is a scenic viewpoint located in the steep northwest coast of the Gran Canaria island (Canary Islands, Spain). It is located about 400 m over the mean sea level, with a slope of ∼45 ◦ down to the sea. The area is driven by strong and moist north-component winds (trade winds) mainly in the summer, with minimum influence of the land. The Passive Advanced Unit (PAU)/GNSS-R [11] and the L-band Automatic Radiometer [12] instruments were located at the viewpoint aiming toward the sea (Fig. 1). Oceanographic buoys gathering ground truth data were moored near the observation site within the field of view of the instruments: Two salinity buoys from Universidad de las 1545-598X/$26.00 © 2010 IEEE