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Tran N.,Collecte Localisation Satellite | Thibaut P.,Collecte Localisation Satellite | Poisson J.-C.,Collecte Localisation Satellite | Philipps S.,Collecte Localisation Satellite | And 2 more authors.
Marine Geodesy | Year: 2011

Calibration of the wind speed is an issue not only for the provision of an accurate metocean parameter but also for the stability of the whole altimetry system across several flying missions. Wind speed is one of the parameters used to derive the sea state bias correction and therefore affects the sea surface height measurement. The analysis performed in this paper benefits from an in-house reprocessing exercise of 1 year of Jason-2 data with both the 3- and 4-parameter Maximum Likelihood Estimator (MLE-3, and MLE-4) retracking algorithms. It took into account the new altimeter characterization file that will be used in the upcoming official reprocessing activity to generate version C of the Jason-2 Geophysical Data Record products. Updated comparison of MLE-3 and MLE-4 data confirms that the MLE-3 algorithm gives less high-frequency variations in σ 0. A calibration of the Jason-2 wind speed estimates has been performed based on a bias-adjustment of the radar backscatter data (+0.32 dB for MLE-3 data and +0.28 dB for MLE-4 ones) between Jason-2 and Jason-1. This allows us to re-assess the sea state bias differences between these two instruments from recomputed solutions and to show that there is no SSB difference between the two altimeters at cm-level. © Taylor & Francis Group, LLC.


Bruinsma S.L.,French National Center for Space Studies | Forste C.,Helmholtz Center Potsdam | Mulet S.,Collecte Localisation Satellite | Rio M.-H.,Collecte Localisation Satellite | And 2 more authors.
Marine Geodesy | Year: 2016

The impact of GOCE Satellite Gravity Gradiometer data on gravity field models was tested. All models were constructed with the same Laser Geodynamics Satellite (LAGEOS) and Gravity Recovery and Climate Experiment (GRACE) data, which were combined with one or two of the diagonal gravity gradient components for the entire GOCE mission (November 2009 to October 2013). The Stokes coefficients were estimated by solving large normal equation (NE) systems (i.e., the direct numerical approach). The models were evaluated through comparisons with the European Space Agency's (ESA) gravity field model DIR-R5, by GPS/Leveling, GOCE orbit determination, and geostrophic current evaluations. Among the single gradient models, only the model constructed with the vertical ZZ gradients gave good results that were in agreement with the formal errors. The model based only on XX gradients is the least accurate. The orbit results for all models are very close and confirm this finding. All models constructed with two diagonal gradient components are more accurate than the ZZ-only model due to doubling the amount of data and having two complementary observation directions. This translates also to a slower increase of model errors with spatial resolution. The different evaluation methods cannot unambiguously identify the most accurate two-component model. They do not always agree, emphasizing the importance of evaluating models using many different methods. The XZ gravity gradient gives a small positive contribution to model accuracy. © 2016 Taylor & Francis Group, LLC


Fournier S.,French Research Institute for Exploitation of the Sea | Reul N.,French Research Institute for Exploitation of the Sea | Chapron B.,French Research Institute for Exploitation of the Sea | Tenerelli J.,Collecte Localisation Satellite
European Space Agency, (Special Publication) ESA SP | Year: 2012

Analyzing SMOS first data acquired in 2010, we have unambiguously detected the Amazon Plume water in the measured SMOS brightness temperatures. The distinguished surface signatures correspond to very low salinity, below 32 psu, and associated brackish water from the river. SMOS data were recently reprocessed since January 2010 and now provide a one year time series of observations that can be used in conjunction with altimetry, SST and ocean color data to better characterize the Amazon and Orinoco plumes seasonal cycle evolution. We will present the results of a study using these new SMOS satellite SSS products in conjunction with altimetry currents, SST and ocean color data (Modis, Meris). © 2012 European Space Agency.


Dibarboure G.,Collecte Localisation Satellite | Boy F.,French National Center for Space Studies | Desjonqueres J.D.,French National Center for Space Studies | Labroue S.,Collecte Localisation Satellite | And 4 more authors.
Journal of Atmospheric and Oceanic Technology | Year: 2014

The observation of ocean scales smaller than 100km with low-resolution mode (LRM) altimetry products is degraded by the existence of a ''hump artifact'' visible on sea surface height (SSH) spectra. Through an analysis of simulations and actual data from multiple missions, this paper shows that the hump originates in a response to inhomogeneities in backscatter strength. Current retrackers cannot fit their Brown model properly because they were designed for a scene with homogeneous backscatter properties. The error is also smoothed along track because of the size and shape of the LRM disc-shaped footprint. Therefore, the hump is modulated by the altimeter design and altitude and by the retracker used. Because of the random nature of the phenomenon, a large majority of long topography segments (e.g., hundreds to thousands of kilometers) is affected. However, within these segments, a substantial fraction of the corruption is contained in small subsets of data (e.g., less than 10%). This paper shows that oceanography users interested in small-scale SSH signals can mitigate the hump corruption by using better editing and postprocessing algorithms on the 20-Hz rate of current products. Last, the thin stripe-shaped footprint of Cryosat-2's synthetic aperture radar mode (SARM) is not affected by the hump artifact, thus improving the observation of topography features ranging from 30 to 100 km. The differences between SARM and pseudo-LRM sigma0 can also be used to detect major hump events on pseudo-LRM data, which might be an asset to design/validate a new generation of algorithms aimed at reducing the hump artifact on the existing LRM record. © 2014 American Meteorological Society.


Abecassis M.,University of Hawaii at Manoa | Senina I.,Collecte Localisation Satellite | Lehodey P.,Collecte Localisation Satellite | Gaspar P.,Collecte Localisation Satellite | And 3 more authors.
PLoS ONE | Year: 2013

Habitat preferences for juvenile loggerhead turtles in the North Pacific were investigated with data from two several-year long tagging programs, using 224 satellite transmitters deployed on wild and captive-reared turtles. Animals ranged between 23 and 81 cm in straight carapace length. Tracks were used to investigate changes in temperature preferences and speed of the animals with size. Average sea surface temperatures along the tracks ranged from 18 to 23 °C. Bigger turtles generally experienced larger temperature ranges and were encountered in warmer surface waters. Seasonal differences between small and big turtles suggest that the larger ones dive deeper than the mixed layer and subsequently target warmer surface waters to rewarm. Average swimming speeds were under 1 km/h and increased with size for turtles bigger than 30 cm. However, when expressed in body lengths per second (bl s-1), smaller turtles showed much higher swimming speeds (>1 bl s-1) than bigger ones (0.5 bl s-1). Temperature and speed values at size estimated from the tracks were used to parameterize a habitat-based Eulerian model to predict areas of highest probability of presence in the North Pacific. The model-generated habitat index generally matched the tracks closely, capturing the north-south movements of tracked animals, but the model failed to replicate observed east-west movements, suggesting temperature and foraging preferences are not the only factors driving large-scale loggerhead movements. Model outputs could inform potential bycatch reduction strategies.


Thompson D.R.,Johns Hopkins University | Horstmann J.,Undersea Research Center | Mouche A.,Collecte Localisation Satellite | Winstead N.S.,Johns Hopkins University | And 2 more authors.
Journal of Geophysical Research: Oceans | Year: 2012

We discuss the status of our efforts to determine a suitable Geophysical Model Function (GMF) that relates X-band normalized radar cross section (NRCS) to the near surface wind vector over the ocean. Development of an X-band GMF has become particularly relevant due to the recent launches of several X-band satellite synthetic aperture radar (SAR) systems. We concentrate here on SAR data collected by the TerraSAR-X satellite launched by the German Space Agency in 2007. Inversion of TerraSAR-X NRCS imagery to wind speed is accomplished using both a simple physics-based GMF as well as an empirical GMF derived by interpolating more accurately tested C- and Ku-band GMFs to X-band. We compare the retrieved wind speeds from three TerraSAR-X scenes with in situ data when available and also with predictions from the Weather Research and Forecast (WRF) model. At vertical polarization, these comparisons show reasonable agreement with in situ data for both the physics-based and empirical GMFs. At horizontal polarization however, the NRCS from the physics-based GMF is generally too low, resulting in an under prediction results of the corresponding retrieved wind speeds. An unexpected result from the WRF comparisons is the similarity between the small-scale structure (on scales 5-10 km) observed in the SAR imagery and the corresponding WRF output. We believe that this similarity may allow TerraSAR-X imagery to provide a quantitative measure of the quality of the WRF boundary-layer parameterization schemes. Copyright 2012 by the American Geophysical Union.


Dibarboure G.,Collecte Localisation Satellite | Schaeffer P.,Collecte Localisation Satellite | Escudier P.,Collecte Localisation Satellite | Pujol M.-I.,Collecte Localisation Satellite | And 7 more authors.
Marine Geodesy | Year: 2012

The ageing of Jason-1, the risk of losing control of the satellite, and the collision risk with TOPEX/Poseidon (still in orbit and no longer maneuverable) initiated a reflection on a so-called "extension of life phase" (EoL) phase that would involve moving Jason-1 to a new orbit to mitigate collision risks while optimizing its science return. This paper describes three practical consequences of any such EoL phase: 1) the ability to build an unprecedented low inclination and high precision geodetic dataset, 2) the loss of coordination with Jason-2 and the associated mesoscale (and sea state) sampling degradation, and 3) the increased topography height error budget stemming from the use of a gridded mean sea surface in place of the classical repeat track analysis that operational systems have been using and improving for almost two decades.More than 17,000 potential orbits were analyzed to identify desirable altitude ranges that could host a Jason-1 EoL phase. The objective was to minimize the sampling degradation of ocean observations (primary objective of Jason-1) while securing a good geodetic EoL dataset (secondary objective of Jason-1). After a first automated screening and scoring process, the final orbit candidates are analyzed through an end-to-end Observing System Simulation Experiment (OSSE) protocol, assessing the multimission observational capability of the EoL phase in a DUACS/AVISO-like system.All EoL orbits are shown to be largely inferior to the interleaved orbit as far as oceanography is concerned. Yet some EoL options are shown to be more desirable than others because their sampling patterns blend well with Jason-2. Good geodetic orbit options could provide a unique bathymetry-oriented dataset and help improve gridded mean sea surfaces (MSS), while repetitive options with a short cycle could cancel some additional EoL errors if a conservative repeat track strategy is preferred. © 2012 Copyright Taylor and Francis Group, LLC.


Halimi A.,National Polytechnic Institute of Toulouse | Mailhes C.,National Polytechnic Institute of Toulouse | Tourneret J.-Y.,National Polytechnic Institute of Toulouse | Boy F.,French National Center for Space Studies | Moreau T.,Collecte Localisation Satellite
IEEE Transactions on Geoscience and Remote Sensing | Year: 2015

Delay/Doppler altimetry (DDA) aims at reducing the measurement noise and increasing the along-track resolution in comparison with conventional pulse-limited altimetry. In a previous paper, we have proposed a semi-analytical model for DDA, which considers some simplifications as the absence of mispointing antenna. This paper first proposes a new analytical expression for the flat surface impulse response (FSIR), considering antenna mispointing angles, a circular antenna pattern, no vertical speed effect, and uniform scattering. The 2-D delay/Doppler map is then obtained by a numerical computation of the convolution between the proposed analytical function, the probability density function of the heights of the specular scatterers, and the time/frequency point target response of the radar. The approximations used to obtain the semi-analytical model are analyzed, and the associated errors are quantified by analytical bounds for these errors. The second contribution of this paper concerns the estimation of the parameters associated with the multilook semi-analytical model. Two estimation strategies based on the least squares procedure are proposed. The proposed model and algorithms are validated on both synthetic and real waveforms. The obtained results are very promising and show the accuracy of this generalized model with respect to the previous model assuming zero antenna mispointing. © 2014 IEEE.

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