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For the past 25 years, polar-orbiting satellite radar altimeters have provided a valuable record of ice sheet elevation change and mass balance. One of the principle challenges associated with radar altimetry comes from the relatively... more
For the past 25 years, polar-orbiting satellite radar altimeters have provided a valuable record of ice sheet elevation change and mass balance. One of the principle challenges associated with radar altimetry comes from the relatively large ground footprint of conventional pulse-limited radars, which reduces their capacity to make measurements in areas of complex topographic terrain. In recent years, progress has been made towards improving ground resolution, through the implementation of Synthetic Aperture Radar (SAR), or Delay-Doppler, techniques. In 2010, the launch of CryoSat-2 heralded the start of a new era of SAR Interferometric (SARIn) altimetry. However, because the satellite operated in SARIn and LRM mode over the ice sheets, many of the non-interferometric SAR altimeter processing techniques have been optimized for water and sea ice surfaces only. The launch of Sentinel-3, which provides full non-interferometric SAR coverage of the ice sheets, therefore presents the oppor...
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Satellite radar altimetry, initially designed for studying ocean surface topography, demonstrated a strong potential for the continuous monitoring of ice sheets and land surfaces over the last 25 years. If radar altimetry is mostly used... more
Satellite radar altimetry, initially designed for studying ocean surface topography, demonstrated a strong potential for the continuous monitoring of ice sheets and land surfaces over the last 25 years. If radar altimetry is mostly used for its capacity to determine surface height, the backscattering coefficients provide information on the surface properties. Spatio-temporal variations of radar altimetry backscattering over land were related to the nature of the surface and its changes against time. This study presents the results of an along-track analysis of radar altimetry echoes over land at S, C, Ku and Ka bands using data from Jason-2, ERS-2, ENVISAT and SARAL on their nominal orbit. Temporal average and deviations are presented at global scale.
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In order to validate the satellite altimeter measurements over Antarctica, we analyze the Seasat scatterometer backscatter coefficient which has been measured at different incidence and azimuth angles and for the two polarizations. We... more
In order to validate the satellite altimeter measurements over Antarctica, we analyze the Seasat scatterometer backscatter coefficient which has been measured at different incidence and azimuth angles and for the two polarizations. We show that the signal at large incidence angles is a mixed signal of volume, backscattered by subsurface layering and surface scattering whose proportions depend on physical characteristics of the snow. Only the effect of the volume scattering by spherical grains on the altimetric signal can be estimated: It is of the order of a few percents of the total altimetric signal and the induced height error is lower than 25 ± 20 cm, if the echo is properly retracked. Other effects, such as internal density boundary are not estimated but a mean to detect the areas of great effect is given.
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During the past decade, sea ice freeboard height has been monitored with various satellite altimetric missions with the aim of producing long-term time series of ice thickness. To achieve this goal, it is essential to analyze potential... more
During the past decade, sea ice freeboard height has been monitored with various satellite altimetric missions with the aim of producing long-term time series of ice thickness. To achieve this goal, it is essential to analyze potential inter-mission biases and to produce freeboard height datasets as free of instrumental error as possible. In the present study, we compare Envisat and CryoSat-2 freeboard height during the common flight period (2010–2012). Our results show that Envisat freeboard height is always thinner (−14 cm in average) when compared to CryoSat-2 (3 cm in average). In addition, Envisat freeboard height displays an unrealistive negative growth from November to April (−2.4 to…
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Satellite radar altimetry, initially designed for studying ocean surface topography, demonstrated a strong potential for the continuous monitoring of ice sheets and land surfaces over the last 25 years. If radar altimetry is mostly used... more
Satellite radar altimetry, initially designed for studying ocean surface topography, demonstrated a strong potential for the continuous monitoring of ice sheets and land surfaces over the last 25 years. If radar altimetry is mostly used for its capacity to determine surface height, the backscattering coefficients provide information on the surface properties. Spatio-temporal variations of radar altimetry backscattering over land and ice sheets were related to the nature of the surface and its changes against time. This study presents the results of an along-track analysis of radar altimetry echoes over land, Antarctica and Greenland at Ku and S bands from June 2002 to July 2003 using the ERS-2 and ENVISAT datasets on their nominal orbit during the tandem phase of the two missions. Temporal average and deviations are presented at global scale for ascending and descending tracks for the two missions.
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During the OCEANOBS99 Conference, scientists argued that a multi-satellite altimetric system should be maintained beyond 2005. In 1999, proposals were formulated to complement the Jason series of high accuracy ocean altimetry missions. In... more
During the OCEANOBS99 Conference, scientists argued that a multi-satellite altimetric system should be maintained beyond 2005. In 1999, proposals were formulated to complement the Jason series of high accuracy ocean altimetry missions. In that frame, J. Verron et al. (1999) produced the “Altika” proposal which is presented in this paper. As a complement to the Jason series, the present authors
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The experience gained by analysing the Seasat altimetric observation of the ocean dynamic topography is getting more valuable as almost 10 years of nearly continuous altimeter data may be obtained by Geosat, ERSI and Topex/Poseidon. Our... more
The experience gained by analysing the Seasat altimetric observation of the ocean dynamic topography is getting more valuable as almost 10 years of nearly continuous altimeter data may be obtained by Geosat, ERSI and Topex/Poseidon. Our group worked on all aspects of the question: the accuracy of the measurements and more particularly the improvements of the orbit determination and its
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... Fabrice PAPA (l), Nelly MOGNARD (2), ED JOSBERGER (3), Frederique REMY (1) (1) LEGOS/CNES, 18 Av. ... ke = extinction the ERS2 data over continents and Legresy Benoit for his (1) constructive comments and suggestions. ...
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The relatively stable conditions of the sea ice cover in the Antarctic, observed for almost 40 years, seem to be changing recently. Therefore, it is essential to provide sea ice thickness (SIT) and volume (SIV) estimates in order to... more
The relatively stable conditions of the sea ice cover in the Antarctic, observed for almost 40 years, seem to be changing recently. Therefore, it is essential to provide sea ice thickness (SIT) and volume (SIV) estimates in order to anticipate potential multi-scale changes in the Antarctic sea ice. For that purpose, the main objectives of this work are: (1) to assess a new sea ice freeboard, thickness and volume altimetry dataset over 2003–2020 and (2) to identify first order impacts of the sea ice recent conditions. To produce these series, we use a neuronal network to calibrate Envisat radar freeboards onto CryoSat-2 (CS2). This method addresses the impacts of surface roughness on Low Resolution Mode (LRM) measurements. During the 2011 common flight period, we found a mean deviation between Envisat and CryoSat-2 radar freeboards by about 0.5 cm. Using the Advanced Microwave Scanning Radiometer (AMSR) and the dual-frequency Altimetric Snow Depth (ASD) data, our solutions are compar...
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We present studies of ice cover of continental water bodies and rivers using the synergy of more than 15 years-long simultaneous active (radar altimeter) and passive (radiometer) observations from radar altimetric satellites... more
We present studies of ice cover of continental water bodies and rivers using the synergy of more than 15 years-long simultaneous active (radar altimeter) and passive (radiometer) observations from radar altimetric satellites (TOPEX/Poseidon, Jason-1, ENVISAT and Geosat Follow-On) complemented by SSM/I passive microwave data. Five largest Eurasian continental water bodies Caspian and Aral seas, Baikal, Ladoga and Onega lakes, and the Ob' river in the Western Siberia are selected as examples. We use an ice discrimination approach based on a combined use of the data, that has been validated using in situ and independent satellite data in the visible range. We then analyse evolution of ice conditions for the lakes and inland seas using historical data, recent satellite observations and our field studies on the lakes Ladoga and Baikal.
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Constraining past ice mass changes, identifying their cause(s) and determining rigorous error estimates, is important for closing the sea level budget and as an input for and test of numerical models. For the Antarctic ice sheet,... more
Constraining past ice mass changes, identifying their cause(s) and determining rigorous error estimates, is important for closing the sea level budget and as an input for and test of numerical models. For the Antarctic ice sheet, considerable uncertainty remains between different methods and groups. Estimates obtained from altimetry, gravimetry, and mass-budget methods can yield conflicting results with error estimates that do not always overlap, while the, commonly adopted, use of different forward models to isolate and remove the effects of glacio-isostatic adjustment (GIA) and surface mass balance (SMB) processes introduces another source of uncertainty which is hard to quantify. To address both these issues, we present a statistical modelling approach to the problem. We combine the observational data, including satellite altimetry, GRACE, GPS and InSAR, and use the different degrees of spatial and temporal smoothness to constrain the underlying geophysical processes. This is ach...
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We analyse, above continental ice, the various factors which affect the power return of the Seasat radar altimeter as measured by its Automatic Gain Control (AGC). Corrections of effects due to the AGC loop control are first applied. AGC... more
We analyse, above continental ice, the various factors which affect the power return of the Seasat radar altimeter as measured by its Automatic Gain Control (AGC). Corrections of effects due to the AGC loop control are first applied. AGC is then normalized by positioning the half-power point at the middle of the instrument receiving window. This operation is valid for both surface and volume scattering. Over a part of Antarctica between long. 90° and 150°E., the remaining variations of AGC are of the order of 15 dB. Most of these variations occur on a large scale (>100km) and are correlated with the katabatic wind intensity. This indicates that AGC measures either surface roughness of the ice, which is related to wind intensity, or grain-size which could also be dependent on the wind. In-situ measurements support the evidence that the radar altimeter is more sensitive to surface scattering. These data could therefore provide a measurement of the intensity of katabatic winds over ...
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The Seasat A satellite scatterometer radar, initially designed to measure ocean-wind intensity and direction, also provided observations on the Antarctic ice sheet. The signal of the back-scatter coefficient decreases strongly from 10 to... more
The Seasat A satellite scatterometer radar, initially designed to measure ocean-wind intensity and direction, also provided observations on the Antarctic ice sheet. The signal of the back-scatter coefficient decreases strongly from 10 to −20 dB when the incidence angle of the observations increases from 0° to 65°. An additional 5 dB signal is found, which is correlated with the direction and intensity of katabatic winds, independent of the incidence angle and polarization of the signal. By using simplified models of the volume-scattering within the snowpack (which is mostly sensitive to snow grain-size) and surface-scattering from the air-snow interface (which depends on roughness), it is evident that the signal of the scatterometer could result from the effects of snow dunes at low incidence angle, and of micro-roughness and volume back-scatter at incidence angles greater than 25°. The instrument therefore provides a means of measuring the direction and intensity of katabatic winds.
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For the first time high-quality coverage of the ERS-1 radar altimeter provides a very accurate surface topographic map covering 80% of the Antarctic ice sheet that can contribute significantly to glaciological studies such as ice-sheet... more
For the first time high-quality coverage of the ERS-1 radar altimeter provides a very accurate surface topographic map covering 80% of the Antarctic ice sheet that can contribute significantly to glaciological studies such as ice-sheet flow modelling. The topography allows estimation of the ice-flow direction, the balance velocity and the basal shear stress. A relationship between shear stress, basal temperature and a parameter related to strain rate helps in mapping the behaviour anomalies of these parameters. Longitudinal stress, sliding, bedrock topography and variation in the pre-exponential factor of the flow law are found to play a major role in the ice-flow pattern. This relation can also be used to estimate rheological parameters: the Glen exponentnis found to be 1 forT< −10°C and 3–4 for higher temperatures, whereQis found to be 70 kJ mol−1.
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Large scale ice sheet models were mainly developped to reconstruct past ice sheets on long time scales. These models are now also used for projections to evaluate their behaviour in the close future and estimate their contribution to sea... more
Large scale ice sheet models were mainly developped to reconstruct past ice sheets on long time scales. These models are now also used for projections to evaluate their behaviour in the close future and estimate their contribution to sea level change. Short simulations in the future may require a different strategy for calibration and spinup than long paleo experiments. The
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The stochastic processes involved in the evolution of a hypothetical cloud of comets are investigated. The cloud is assumed to be randomly perturbed by passing stars approaching the Sun at a distance less than 1 pc. Within the frame of... more
The stochastic processes involved in the evolution of a hypothetical cloud of comets are investigated. The cloud is assumed to be randomly perturbed by passing stars approaching the Sun at a distance less than 1 pc. Within the frame of the impulse approximation the authors derive analytical expressions for the probability distribution of the impulse imparted to comets for both close and distant approaches. An application to the rate of ejection of comets is presented.
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Large uncertainties exist on the volume of ice transported by the Southern Ocean large icebergs, a key parameter for climate studies, because of the paucity of information, especially on iceberg thickness. Using icebergs tracks from the... more
Large uncertainties exist on the volume of ice transported by the Southern Ocean large icebergs, a key parameter for climate studies, because of the paucity of information, especially on iceberg thickness. Using icebergs tracks from the National Ice Center (NIC) and Brigham Young University (BYU) databases to select altimeter data over icebergs and a method of analysis of altimeter waveforms, a database of 5366 icebergs freeboard elevation, length, and backscatter covering the 2002–2012 period has been created. The database is analyzed in terms of distributions of freeboard, length, and backscatter showing differences as a function of the iceberg's quadrant of origin. The database allows to analyze the temporal evolution of icebergs and to estimate a melt rate of 35–39 m·yr−1 (neglecting the firn compaction). The total daily volume of ice, estimated by combining the NIC and altimeter sizes and the altimeter freeboards, regularly decreases from 2.2 104km3 in 2002 to 0.9 104km3 in...
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Satellite-altimeter data over ice sheets provide the best tool for mapping their topography and its possible climatic variations. However, these data are affected by measurement errors, orbit errors, and slope errors. We develop here a... more
Satellite-altimeter data over ice sheets provide the best tool for mapping their topography and its possible climatic variations. However, these data are affected by measurement errors, orbit errors, and slope errors. We develop here a three-step inversion technique which accommodates the a priori information on the expected topography and correctly handles and propagates the data errors: it estimates first a large-scale reference surface, then maps the residuals related to undulations, and finally iteratively corrects the slope error. The method is tested on overlapping small fragments of the Antarctic ice sheet, using a sub-set of Seasat data. Finally, a topographic map of Terre Adélie is produced. Over areas of small slopes, the a posteriori error should be of the order of 0.4 m. Using ERS-I data, it is therefore expected that climatic variations in the ice-sheet topography since the introduction of Seasat will be observable.
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Improved knowledge of geometrical boundary conditions, such as bedrock geometry and surface topography, can contribute significantly to glaciological studies including ice-sheet-flow modelling. Precise thickness and altimetric data allow... more
Improved knowledge of geometrical boundary conditions, such as bedrock geometry and surface topography, can contribute significantly to glaciological studies including ice-sheet-flow modelling. Precise thickness and altimetric data allow an estimation of ice-flow direction, the balance velocity and the basal shear stress. These parameters are calculated along a 1160 km profile in East Antarctica using a relationship between shear stress, basal temperature, the Glen flow exponent and a parameter related to strain rate. Strong variations of the flow-law parameters and basal conditions are found to play a major role in the ice-flow pattern. Sliding, anisotropy and longitudinal stress strongly perturb the validity of the law, but their signature can be identified.
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We show how the studies of ice and snow cover of continental water bodies can benefit from the synergy of more than 15 years-long simultaneous active (radar altimeter) and passive (radiometer) observations from radar altimetric satellites... more
We show how the studies of ice and snow cover of continental water bodies can benefit from the synergy of more than 15 years-long simultaneous active (radar altimeter) and passive (radiometer) observations from radar altimetric satellites (TOPEX/Poseidon, Jason-1, ENVISAT and Geosat Follow-On) and how this approach can be complemented by SSM/I passive microwave data to improve spatial and temporal coverage.
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Research Interests: Geology, Remote Sensing, Climate Change, Sea Level, Stereoscopy, and 15 moreAntarctica, global Climate change, Sea level rise, Greenland, Scientific Communication, Field Survey, Geomatic Engineering, Polar, Glacier, First Year, Accuracy Assessment, International Polar Year, Ice Sheets, Mass Balance, and digital terrain model
Research Interests: Geology, Remote Sensing, Climate Change, Sea Level, Stereoscopy, and 15 moreAntarctica, global Climate change, Sea level rise, Greenland, Scientific Communication, Field Survey, Geomatic Engineering, Polar, Glacier, First Year, Accuracy Assessment, International Polar Year, Ice Sheets, Mass Balance, and digital terrain model
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We attempt to quantify the Antarctica annual mass balance by estimating the seasonal water mass exchange among the main surface reservoirs (oceans, atmosphere, continents and ice sheets) using various data sources. These data include... more
We attempt to quantify the Antarctica annual mass balance by estimating the seasonal water mass exchange among the main surface reservoirs (oceans, atmosphere, continents and ice sheets) using various data sources. These data include Topex-Poseidon altimetry and in situ ocean temperature data, soil moisture and snow depth data, and atmospheric surface pressure data. We calculate the seasonal (annual) variations of