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Volkov D.L.,University of California at Los Angeles | Volkov D.L.,Jet Propulsion Laboratory | Pujol M.-I.,Collecte Localisation Satellites
Journal of Geophysical Research: Oceans | Year: 2012

Satellite altimetry provides high-quality sea surface height data that have been successfully used to study the variability of sea level and surface geostrophic circulation at different spatial and temporal scales. However, the high-latitude regions have traditionally been avoided due to the persistent sea ice cover. Most of the validation studies have focused on the areas below the polar circles. In this paper we examine the quality and performance of a gridded satellite altimetry product in the Nordic, Barents, and Kara seas. The altimetric sea level in coastal areas is validated using available tide gauge records. We show that at most locations in the Nordic seas the altimetry and tide gauge measurements are in a good agreement in terms of the root-mean square differences and the amplitudes and phases of the seasonal cycle. The agreement deteriorates in the shallow areas of the Barents and Kara seas subject to the seasonal presence of sea ice, and where the altimetry data are contaminated by the residual aliasing of unresolved high-frequency signals. The comparison of linear trends at the locations of tide gauges reveals discrepancies that need to be taken into account when interpreting long-term changes of sea level in the region. Away from the coast the altimetry data are compared to drifter trajectories, corrected for Ekman currents. The drifter trajectories are found consistent with the mesoscale variability of the altimetric sea level. This study provides the first comprehensive validation of a gridded satellite altimetry data product in the high-latitude seas. Copyright 2012 by the American Geophysical Union.

Bringer A.,University of Toulon | Guerin C.-A.,University of Toulon | Chapron B.,French Research Institute for Exploitation of the Sea | Mouche A.A.,Collecte Localisation Satellites
IEEE Transactions on Geoscience and Remote Sensing | Year: 2012

The simulation and interpretation of microwave sea radar return in the near-nadir region are still issues in view of the limitations of the geometrical optics approximation and the multiscale and non-Gaussian nature of the surface. We show that an unambiguous and fully consistent physical approach can be reached in the framework of the physical optics. The model is developed on the basis of various satellite and airborne C-, Ku-, and Ka-band measurements using different reference surface roughness spectra. As found, the introduction of a peakedness correction based upon the excess kurtosis of slopes is necessary to obtain consistent analysis across the microwave frequency range. The model yields accurate simulations for the omnidirectional near-nadir normalized radar cross section in different frequency bands, provided the spectrum satisfies some a priori constraints on the distribution of the total and filtered slopes. © 2012 IEEE.

Delpey M.T.,Service Hydrographique et Oceanographique de la Marine | Ardhuin F.,Service Hydrographique et Oceanographique de la Marine | Collard F.,French Research Institute for Exploitation of the Sea | Chapron B.,Collecte Localisation Satellites
Journal of Geophysical Research: Oceans | Year: 2010

The space-time structure of long-period ocean swell fields is investigated, with particular attention given to features in the direction orthogonal to the propagation direction. This study combines space-borne synthetic aperture radar (SAR) data with numerical model hindcasts and time series recorded by in situ instruments. In each data set the swell field is defined by a common storm source. The correlation of swell height time series is very high along a single great circle path with a time shift given by the deep water dispersion relation of the dominant swells. This correlation is also high for locations situated on different great circles in entire ocean basins. Given the Earth radius R, we define the distance from the source R and the transversal angle β so α that and would be equal the colatitude and longitude for a storm centered on the North Pole. Outside of land influence, the swell height field at time t, Hss(β, α, t) is well approximated by a function Hss,0(t - R/Cg) (α sin (α) times another function r2 (β), where Cg is a representative group speed. Here r2 (β) derived from SAR data is very broad, with a width at half the maximum that is larger than 70, and varies significantly from storm to storm. Land shadows introduce further modifications so that in general r2 is a function of and . This separation of variables and the smoothness of the Hss field, allows the estimation of the full field of Hss from sparse measurements, such as wave mode SAR data, combined with one time series, such as that provided by a single buoy. A first crude estimation of a synthetic Hss field based on this principle already shows that swell hindcasts and forecasts can be improved by assimilating such synthetic observations. © 2010 by the American Geophysical Union.

Yaya P.,Collecte Localisation Satellites | Tourain C.,French National Center for Space Studies
Advances in Space Research | Year: 2010

Among the factors which may disrupt the DORIS measurements quality, the ground antennas environment is of high importance. For a set of 15 selected DORIS beacon, the differences between the effective and theoretical power received on-board the satellites (SPOT-5 and Envisat) have been analyzed in terms of spatial direction around the antenna. Such antenna maps have also been established regarding the Doppler residuals of the least-square precise orbit adjustment. Thanks to 360 views from the antennas and aerial views of the sites, the impact of the signal obstructions (trees, roofs, antennas . . .) on power attenuation and Doppler residuals is discussed. Depending on the nature of the obstructed object, the attenuation level can reach more than 5 dB, and the residual RMS of the orbit adjustment may be doubled from the nominal value, reaching 1 mm/s locally. The nature of the ground at the foot of the antennas has been correlated to DORIS signal quality at high elevation: reflections on flat surfaces (e.g. roofs) affect the signal more significantly than reflections on natural ground (e.g. soil). In particular, a modeling of the multipath phenomenon affecting Fairbanks site has been established and fits remarkably with the observations. Finally, an evaluation of the direct impact of obstructing objects on the orbit has also been performed. The example of a scaffolding at Kauai site displays a few millimeters error in the along-track position of the satellite. © 2010 COSPAR. Published by Elsevier Ltd. All rights reserved.

Ardhuin F.,French Research Institute for Exploitation of the Sea | Tournadre J.,French Research Institute for Exploitation of the Sea | Queffeulou P.,French Research Institute for Exploitation of the Sea | Girard-Ardhuin F.,French Research Institute for Exploitation of the Sea | Collard F.,Collecte Localisation Satellites
Ocean Modelling | Year: 2011

The variability of small-size iceberg distributions is revealed from a novel analysis of satellite altimeter data. A strong annual cycle is modulated by pulse-like events confined to single ocean basins, with dense iceberg populations in the South Atlantic in 2004-2005, and in the South Pacific in 2008. Anomalies in sea surface temperatures of the order of 1. °C may be related to the iceberg distribution. Icebergs also appear very strongly associated with anomalies in the heights of ocean waves. A preliminary parameterization of wave blocking by icebergs significantly reduces wave model errors in the region south of 45° South, and has a perceptible influence on all the west coasts of the Southern hemisphere. © 2011 Elsevier Ltd.

Lopez R.,Collecte Localisation Satellites | Lopez R.,CNRS Laboratory of Analysis and Architectures of Systems | Malarde J.-P.,Collecte Localisation Satellites | Royer F.,Collecte Localisation Satellites | Gaspar P.,Collecte Localisation Satellites
IEEE Transactions on Geoscience and Remote Sensing | Year: 2014

The Argos service was launched in 1978 to serve environmental applications, including oceanography, wildlife tracking, fishing vessel monitoring, and maritime safety. The system allows for worldwide near-real-time positioning and data collection of platform terminal transmitters (PTTs). The positioning of the PTTs is achieved by exploiting the Doppler shift in the carrier frequency of the transmitter as recorded by satelliteborne Argos receivers. Until March 15, 2011, a classical nonlinear least squares estimation technique was systematically used to estimate Argos positions. Since then, a second positioning algorithm using a multiple-model Kalman filter was implemented in the operational Argos positioning software. This paper presents this new algorithm and analyzes its performance using a large data set obtained from over 200 mobiles carrying both an Argos transmitter and a GPS receiver used as ground truth. The results show that the new algorithm significantly improves the positioning accuracy, particularly in difficult conditions (for class-A and class-B locations, in the Argos terminology). Moreover, the new algorithm enables the retrieval of a larger number of estimated positions and the systematic estimation of the location error. © 2013 IEEE.

Loyer S.,Collecte Localisation Satellites | Perosanz F.,French National Center for Space Studies | Mercier F.,French National Center for Space Studies | Capdeville H.,Collecte Localisation Satellites | Marty J.-C.,French National Center for Space Studies
Journal of Geodesy | Year: 2012

CNES (Centre National d'Etudes Spatiales) and CLS (Collecte Localisation Satellites) became an International GNSS Service (IGS) Analysis Center (AC) the 20th of May 2010. Since 2009, we are using the integer ambiguity fixing at the zero-difference level strategy in our software package (GINS/Dynamo) as an alternative to classical differential approaches. This method played a key role among all the improvements in the GPS processing we made during this period. This paper provides to the users the theoretical background, the strategies and the models used to compute the products (GPS orbits and clocks, weekly station coordinate estimates and Earth orientation parameters) that are submitted weekly to the IGS. The practical realization of the two-step, ambiguity-fixing scheme (wide-lane and narrow-lane) is described in detail. The ambiguity fixing improved our orbit overlaps from 6 to 3 cm WRMS in the tangential and normal directions. Since 2008, our products have been also regularly compared to the IGS final solutions by the IGS Analysis Center Coordinator. The joint effects of ambiguity fixing and dynamical model changes (satellite solar radiation pressure and albedo force) improved the consistency with IGS orbits from 35 to 18 mm 3D-WRMS. Our innovative strategy also gives additional powerful properties to the GPS satellite phase clock solutions. Single receiver (zero-difference) ambiguity resolution becomes possible. An overview of the applications is given. © 2012 Springer-Verlag.

Husson R.,Collecte Localisation Satellites | Ardhuin F.,French Research Institute for Exploitation of the Sea | Collard F.,Collecte Localisation Satellites | Chapron B.,French Research Institute for Exploitation of the Sea | Balanche A.,French Research Institute for Exploitation of the Sea
Geophysical Research Letters | Year: 2012

Swells radiating across ocean basins are fingerprints of the large ocean storms that generated them, which are otherwise poorly observed. Here we analyze the signature of one swell event in the seismic noise recorded all around the Pacific and we show that it is a natural complement to the global coverage provided by the Synthetic Aperture Radar wave mode data from ENVISAT. In particular the seismic stations are much more sensitive to low frequency and amplitude signals than buoys and SAR, capturing swell forerunners a couple of days before they can be detected from space or in situ data. This information helps detect in the SAR measurements the presence of very long swell, with periods of 22s in our case example, that were otherwise excluded. © 2012. American Geophysical Union. All Rights Reserved.

Legeais J.-F.,Collecte Localisation Satellites | Ablain M.,Collecte Localisation Satellites | Thao S.,Collecte Localisation Satellites
Ocean Science | Year: 2014

The assessment of long-term errors in altimeter sea level measurements is essential for studies related to the mean sea level (MSL) evolution. One of the main contributors to the long-term sea level uncertainties is the correction of the altimeter range from the wet troposphere path delay, which is provided by onboard microwave radiometers for the main altimeter missions. The wet troposphere correction (WTC) derived from the operational European Centre for Medium-Range Weather Forecast (ECMWF) atmospheric model is usually used as a reference for comparison with the radiometer WTC. However, due to several improvements in the processing, this model is not homogenous over the altimetry period (from 1993 onwards), preventing the detection of errors in the radiometer WTC, especially in the first altimetry decade. In this study, we determine the quality of WTC provided by the operational ECMWF atmospheric model in comparison with the fields derived from the ERA-Interim (ECMWF) and the National Centers for Environmental Predictions/National Center for Atmospheric Research (NCEP/NCAR) reanalyses. Separating our analyses on several temporal and spatial scales, we demonstrate that ERA-Interim provides the best modeled WTC for the altimeter sea level at climate scales. This allows us to better evaluate the radiometer WTC errors, especially for the first altimetry decade (1993-2002), and thus to improve the altimeter MSL error budget. This work also demonstrates the relevance of the interactions between the "altimetry" and "atmosphere" communities, since the expertise of each is of benefit to the other. © 2014 Author(s).

Halimi A.,CNRS Toulouse Institute in Information Technology | Mailhes C.,CNRS Toulouse Institute in Information Technology | Tourneret J.-Y.,CNRS Toulouse Institute in Information Technology | Thibaut P.,Collecte Localisation Satellites | Boy F.,French National Center for Space Studies
IEEE Transactions on Geoscience and Remote Sensing | Year: 2013

Much attention has been recently devoted to the analysis of coastal altimetric waveforms. When approaching the coast, altimetric waveforms are sometimes corrupted by peaks caused by high reflective areas inside the illuminated land surfaces or by the modification of the sea state close to the shoreline. This paper introduces a new parametric model for these peaky altimetric waveforms. This model assumes that the received altimetric waveform is the sum of a Brown echo and an asymmetric Gaussian peak. The asymmetric Gaussian peak is parameterized by a location, an amplitude, a width, and an asymmetry coefficient. A maximum-likelihood estimator is studied to estimate the Brown plus peak model parameters. The Cramér-Rao lower bounds of the model parameters are then derived providing minimum variances for any unbiased estimator, i.e., a reference in terms of estimation error. The performance of the proposed model and the resulting estimation strategy are evaluated via many simulations conducted on synthetic and real data. Results obtained in this paper show that the proposed model can be used to retrack efficiently standard oceanic Brown echoes as well as coastal echoes corrupted by symmetric or asymmetric Gaussian peaks. Thus, the Brown with Gaussian peak model is useful for analyzing altimetric measurements closer to the coast. Copyright © 1980-2012 IEEE.

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