Ramonville-Saint-Agne, France
Ramonville-Saint-Agne, France

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Schaeffer P.,Collecte Localisation Satellite CLS | Faugere Y.,Collecte Localisation Satellite CLS | Legeais J.F.,Collecte Localisation Satellite CLS | Ollivier A.,Collecte Localisation Satellite CLS | And 2 more authors.
Marine Geodesy | Year: 2012

This article focuses on the determination and the validation of the new Mean Sea Surface from the CNES/CLS. This new model merges multiple satellite altimeters over 16 years of observations. Particular attention was paid on the analysis of the oceanic variability. Then a novel method was applied to remove this variability and to take into account different kinds of errors that affect the data. Several types of validations were performed to estimate the quality of the shortest topographic structures and to quantify the accuracy of the mean oceanic content at a larger scale. © 2012 Copyright Taylor and Francis Group, LLC.


Zawadzki L.,Collecte Localisation Satellite CLS | Ablain M.,Collecte Localisation Satellite CLS
Ocean Science | Year: 2016

The current mean sea level (MSL) continuous record, essential to understanding the climate evolution, is computed with the altimetric measurements of the TOPEX/Poseidon mission, succeeded by Jason-1 and later Jason-2. The accurate continuity of the record is ensured by the conservation of the "historical" TOPEX orbit as well as by calibration phases between the successive missions which enable a rigorous computation of their relative biases. In order to extend the current MSL record, Jason-3 will be the natural successor of Jason-2: on the same orbit with a calibration phase. Shortly after Jason-3, another altimetric climate-oriented mission, Sentinel-3a, will be launched on a different orbit. In this paper, simulated altimetric sea level data are used to study the sensitivity of the MSL continuous record to the change of the "historical" orbit for the new Sentinel-3a orbit. By estimating the impact of the absence of calibration phase on the MSL continuous record trend accuracy at the global and regional scales and the impact of the orbit change on the long-term continuity of this MSL record, this study shows that linking Sentinel-3a data instead of Jason-3 to the MSL continuous record would not meet climate user requirements regarding the MSL trend accuracy. © 2016 Author(s).


Iovan C.,IRD Montpellier | Ampou E.,IRD Montpellier | Ampou E.,INDESO Center | Andrefouet S.,IRD Montpellier | And 2 more authors.
2015 8th International Workshop on the Analysis of Multitemporal Remote Sensing Images, Multi-Temp 2015 | Year: 2015

This paper presents a framework for change detection of coral reef habitats in muti-temporal and multi-spectral satellite imagery acquired by multiple sensors. Our specific goal is to analyze the evolution coral reef habitats from images acquired over a period of twelve years. Our study area is located in the centre of the Coral Triangle, a hotspot of biodiversity in the Bunaken Island, Indonesia. In-situ data is used in a supervised classification approach to build classification models for eleven habitat types. Radiometric calibration is pairwise performed between each image in the data set and the reference one, and habitat models built on the reference image are used to classify the entire time series. Results obtained are discussed and the influential factors are put forward. © 2015 IEEE.


Halimi A.,Tesa | Mailhes C.,Tesa | Tourneret J.-Y.,Tesa | Boy F.,French National Center for Space Studies | Moreau T.,Collecte Localisation Satellite CLS
International Geoscience and Remote Sensing Symposium (IGARSS) | Year: 2014

This paper introduces a new model for delay/Doppler altimetry, taking into account the effect of antenna mispointing. After defining the proposed model, the effect of the antenna mispointing on the altimetric waveform is analyzed as a function of along-track and across-track angles. Two least squares approaches are investigated for estimating the parameters associated with the proposed model. The first algorithm estimates four parameters including the across-track mispointing (which affects the echo's shape). The second algorithm uses the mispointing angles provided by the star-trackers and estimates the three remaining parameters. The proposed model and algorithms are validated via simulations conducted on both synthetic and real data. © 2014 IEEE.


Thibaut P.,Collecte Localisation Satellite CLS | Poisson J.C.,Collecte Localisation Satellite CLS | Bronner E.,French National Center for Space Studies | Picot N.,French National Center for Space Studies
Marine Geodesy | Year: 2010

For all altimeter missions, precise estimates of geophysical parameters are obtained thanks to an algorithm called “retracking” that fits an analytical model to the measured waveforms. The Brown model provides a good representation of the return echo over deep ocean surfaces and is commonly used. Many different chains can be considered (and have already been tested) for this processing. An unweighted Least Square Estimate derived from a Maximum Likelihood Estimator (MLE) (Dumont 1985; Rodriguez 1988) is implemented in most altimeter ground processing approaches (TOPEX, Jason-1, Jason-2, and Envisat).The aim of this paper is to evaluate the performance of two retracking algorithms based on the same least square principle: The MLE3 algorithm estimates three parameters (range, significant wave height, and power) whereas the MLE4 estimates four parameters (the three previous ones and the slope of the waveform trailing edge). MLE3 was used on Jason-1 before star tracker problems occurred. The MLE4 algorithm has been used for Jason-1 Version B products and onward and for Jason-2 products from the start of the mission. Both algorithms are compared in the paper. Advantages and drawbacks of both algorithms are pointed out showing notable benefits provided by MLE4 especially for waveforms that do not conform to the Brown model. © 2010, Taylor & Francis Group, LLC.


Halimi A.,Tesa | Mailhes C.,Tesa | Tourneret J.-Y.,Tesa | Moreau T.,Collecte Localisation Satellite CLS | Boy F.,French National Center for Space Studies
European Signal Processing Conference | Year: 2014

Delay/Doppler radar altimetry is a new technology that has been receiving an increasing interest, especially since the launch of Cryosat-2 in 2010, the first altimeter using this technique. The Delay/Doppler technique aims at reducing the measurement noise and increasing the along-track resolution in comparison with conventional pulse limited altimetry. A new semi-analytical model with five parameters has been recently introduced for this new technology. However, two of these parameters are highly correlated resulting in bad estimation performance when estimating all parameters. This paper proposes a new strategy improving estimation performance for delay/Doppler altimetry. The proposed strategy exploits all the information contained in the delay/Doppler domain. A comparison with other classical algorithms (using the temporal samples only) allows to appreciate the gain in estimation performance obtained when using both temporal and Doppler data. © 2014 EURASIP.


Dibarboure G.,Collecte Localisation Satellite CLS | Ubelmann C.,Jet Propulsion Laboratory
Remote Sensing | Year: 2014

The proposed surface water and ocean topography (SWOT) mission aims at observing short scale ocean topography with an unprecedented resolution and accuracy. Its main proposed sensor is a radar interferometer, so a major source of topography error is the roll angle: the relative positions of SWOT's antennas must be known within a few micrometers. Because reaching SWOT's stringent requirements with onboard roll values is challenging, we carried out simulations as a contingency strategy (i.e., to be ready if roll is larger than anticipated) that could be used with ground-based data. We revisit the empirical calibration algorithms with additional solving methods (e.g., based on orbit sub-cycle) and more sophisticated performance assessments with spectral decompositions. We also explore the link between the performance of four calibration methods and the attributes of their respective calibration zones: size and geometry (e.g., crossover diamonds), temporal variability (e.g., how many days between overlapping SWOT images). In general, the so-called direct method (using a single SWOT image) yields better coverage and smaller calibrated roll residuals because the full extent of the swath can be used for calibration, but this method makes an extensive use of the external nadir constellation to separate roll from oceanic variability, and it is more prone to leakages from oceanic variability on roll (i.e., true topography signal is more likely to be corrupted if it is misinterpreted as roll) and inaccurate modeling of the true topography spectrum. For SWOT's baseline orbit (21 days repeat and 10.9 days sub-cycle), three other methods are found to be complementary with the direct method: swath crossovers, external nadir crossovers, and sub-cycle overlaps are shown to provide an additional calibration capability, albeit with complex latitude-varying coverage and performance. The main asset of using three or four methods concurrently is to minimize systematic leakages from oceanic variability or measurement errors, by maximizing overlap zones and by minimizing the temporal variability with one-day to three-day image differences. To that extent, SWOT's proposed "contingency orbit" is an attractive risk reduction asset: the one-day sub-cycle overlaps of adjoining swaths would provide a good, continuous, and self-sufficient (no need for external nadirs) calibration scheme. The benefit is however essentially located at mid to high-latitudes and it is substantial only for wavelengths longer than 100 km. © 2014 by the authors; licensee MDPI, Basel, Switzerland.


Tournadre J.,French Research Institute for Exploitation of the Sea | Poisson J.C.,Collecte Localisation Satellite CLS | Steunou N.,French National Center for Space Studies | Picard B.,Collecte Localisation Satellite CLS
Marine Geodesy | Year: 2015

The major drawback of Ka band, operating frequency of the AltiKa altimeter on board SARAL, is its sensitivity to atmospheric liquid water. Even light rain or heavy clouds can strongly attenuate the signal and distort the signal leading to erroneous geophysical parameters estimates. A good detection of the samples affected by atmospheric liquid water is crucial. As AltiKa operates at a single frequency, a new technique based on the detection by a Matching Pursuit algorithm of short scale variations of the slope of the echo waveform plateau has been developed and implemented prelaunch in the ground segment. As the parameterization of the detection algorithm was defined using Jason-1 data, the parameters were re-estimated during the cal-val phase, during which the algorithm was also updated. The measured sensor signal-to-noise ratio is significantly better than planned, the data loss due to attenuation by rain is significantly smaller than expected (<0.1%). For cycles 2 to 9, the flag detects about 9% of 1Hz data, 5.5% as rainy and 3.5 % as backscatter bloom (or sigma0 bloom). The results of the flagging process are compared to independent rain data from microwave radiometers to evaluate its performances in term of detection and false alarms. © 2015, Copyright © Taylor & Francis Group, LLC.


Halimi A.,Tesa | Mailhes C.,Tesa | Tourneret J.-Y.,Tesa | Thibaut P.,Collecte Localisation Satellite CLS
International Geoscience and Remote Sensing Symposium (IGARSS) | Year: 2011

Coastal altimetric waveforms may be corrupted by peaks. A simple parametric model was recently introduced to model peaky altimetric waveforms. This model assumes that the received altimetric waveform is the sum of a Brown echo and a Gaussian peak. This model has provided interesting results for symmetric peaks affecting altimetric signals. However, it is not appropriate for altimetric signals corrupted by asymmetric peaks. This paper introduces a Brown with asymmetric Gaussian peak model for altimetric waveforms. The parameters of this model are estimated by a maximum likelihood estimator. The performance of the proposed model and the resulting estimation strategy is evaluated via simulations conducted on synthetic and real data. © 2011 IEEE.


Tourneret J.-Y.,Tesa | Mailhes C.,Tesa | Severini J.,Tesa | Thibaut P.,Collecte Localisation Satellite CLS
International Geoscience and Remote Sensing Symposium (IGARSS) | Year: 2010

This paper addresses the problem of classifying altimetric signals according to their shapes. The proposed classifier is divided into three steps. A one-class support vector machine method is first used to isolate the large amount of Brown-like echoes from others signals which are considered as outliers. The second step extracts pertinent features from the the remaining echoes (which cannot be well described by the Brown model). These features are projected onto discriminant axes using linear discriminant analysis. The final step classifies the projected feature vectors using a standard Bayesian classifier. The proposed three step classification strategy is evaluated on supervised real altimetric echoes. © 2010 IEEE.

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