70 research outputs found

    GNSS transpolar earth reflectometry exploriNg system (G-TERN): mission concept

    Get PDF
    The global navigation satellite system (GNSS) Transpolar Earth Reflectometry exploriNg system (G-TERN) was proposed in response to ESA's Earth Explorer 9 revised call by a team of 33 multi-disciplinary scientists. The primary objective of the mission is to quantify at high spatio-temporal resolution crucial characteristics, processes and interactions between sea ice, and other Earth system components in order to advance the understanding and prediction of climate change and its impacts on the environment and society. The objective is articulated through three key questions. 1) In a rapidly changing Arctic regime and under the resilient Antarctic sea ice trend, how will highly dynamic forcings and couplings between the various components of the ocean, atmosphere, and cryosphere modify or influence the processes governing the characteristics of the sea ice cover (ice production, growth, deformation, and melt)? 2) What are the impacts of extreme events and feedback mechanisms on sea ice evolution? 3) What are the effects of the cryosphere behaviors, either rapidly changing or resiliently stable, on the global oceanic and atmospheric circulation and mid-latitude extreme events? To contribute answering these questions, G-TERN will measure key parameters of the sea ice, the oceans, and the atmosphere with frequent and dense coverage over polar areas, becoming a “dynamic mapper”of the ice conditions, the ice production, and the loss in multiple time and space scales, and surrounding environment. Over polar areas, the G-TERN will measure sea ice surface elevation (<;10 cm precision), roughness, and polarimetry aspects at 30-km resolution and 3-days full coverage. G-TERN will implement the interferometric GNSS reflectometry concept, from a single satellite in near-polar orbit with capability for 12 simultaneous observations. Unlike currently orbiting GNSS reflectometry missions, the G-TERN uses the full GNSS available bandwidth to improve its ranging measurements. The lifetime would be 2025-2030 or optimally 2025-2035, covering key stages of the transition toward a nearly ice-free Arctic Ocean in summer. This paper describes the mission objectives, it reviews its measurement techniques, summarizes the suggested implementation, and finally, it estimates the expected performance.Peer ReviewedPostprint (published version

    Azimuthal Dependence of GNSS‐R Scattering Cross‐Section in Hurricanes

    Full text link
    Global Navigation Satellite System‐Reflectometry (GNSS‐R) measurements of the ocean surface are sensitive to roughness scales ranging from a few cms to several kms. Inside a hurricane the surface roughness changes drastically due to varying sea age and fetch length conditions and complex wave‐wave interactions caused by its cyclonic rotation and translational motion. As a result, the relationship between the surface roughness at different scale sizes becomes azimuthally dependent, as does the relationship between scattering cross‐section and wind speed as represented by a Geophysical Model Function (GMF). In this work, the impact of this azimuthal variation on the scattering cross‐section is assessed. An empirical GMF is constructed using measurements by the NASA CYclone GNSS (CYGNSS) matched to HWRF reanalysis surface winds for 19 hurricanes in 2017 and 2018. The analysis reveals a 2–8% variation in scattering cross‐section due to azimuthal location, and the magnitude of the azimuthal dependence is found to grow with wind speed.Plain Language SummaryGlobal Navigation Satellite System‐Reflectometry (GNSS‐R) is a technique of studying reflected GPS signals to extract useful information about the surface. CYGNSS is the first of its kind GNSS‐R constellation mission selected by NASAs earth venture program. The goal of the mission is to understand inner core processes in hurricanes by making accurate surface wind speed measurements there. Wind speed at the surface is determined using a GMF that maps the reflection measurement to a wind speed. Due to the complex nature of sea state and wave interactions inside a hurricane, measured scattering cross‐section depends on the azimuthal location of the measurement inside the hurricane system. A modified GMF is proposed here that accounts for the azimuthal dependence. The model is developed by matching up CYGNSS measurements to hurricane winds estimated by the NOAA HWRF model for 19 hurricanes during 2017 and 2018. The new GMF accounts for a 2–8% variation in the measurements due to azimuthal location which increases with wind speed.Key PointsAzimuthal variations of GNSS‐R scattering cross‐section in hurricanes are modeled with sinusoidal harmonicsThe azimuthal harmonics explain 2–8% of the overall variation in scattering cross‐sectionThe magnitude of the azimuthal harmonics increases with increasing wind speedPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/156153/2/jgrc24060.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/156153/1/jgrc24060_am.pd

    Evaluating Impact of Rain Attenuation on Space-borne GNSS Reflectometry Wind Speeds

    Get PDF
    The novel space-borne Global Navigation Satellite System Reflectometry (GNSS-R) technique has recently shown promise in monitoring the ocean state and surface wind speed with high spatial coverage and unprecedented sampling rate. The L-band signals of GNSS are structurally able to provide a higher quality of observations from areas covered by dense clouds and under intense precipitation, compared to those signals at higher frequencies from conventional ocean scatterometers. As a result, studying the inner core of cyclones and improvement of severe weather forecasting and cyclone tracking have turned into the main objectives of GNSS-R satellite missions such as Cyclone Global Navigation Satellite System (CYGNSS). Nevertheless, the rain attenuation impact on GNSS-R wind speed products is not yet well documented. Evaluating the rain attenuation effects on this technique is significant since a small change in the GNSS-R can potentially cause a considerable bias in the resultant wind products at intense wind speeds. Based on both empirical evidence and theory, wind speed is inversely proportional to derived bistatic radar cross section with a natural logarithmic relation, which introduces high condition numbers (similar to ill-posed conditions) at the inversions to high wind speeds. This paper presents an evaluation of the rain signal attenuation impact on the bistatic radar cross section and the derived wind speed. This study is conducted simulating GNSS-R delay-Doppler maps at different rain rates and reflection geometries, considering that an empirical data analysis at extreme wind intensities and rain rates is impossible due to the insufficient number of observations from these severe conditions. Finally, the study demonstrates that at a wind speed of 30 m/s and incidence angle of 30°, rain at rates of 10, 15, and 20 mm/h might cause overestimation as large as ≈0.65 m/s (2%), 1.00 m/s (3%), and 1.3 m/s (4%), respectively, which are still smaller than the CYGNSS required uncertainty threshold. The simulations are conducted in a pessimistic condition (severe continuous rainfall below the freezing height and over the entire glistening zone) and the bias is expected to be smaller in size in real environments

    Engineering Calibration and Physical Principles of GNSS-Reflectometry for Earth Remote Sensing

    Full text link
    The Cyclone Global Navigation Satellite System (CYGNSS) is a NASA mission that uses 32 Global Positioning System (GPS) satellites as active sources and 8 CYGNSS satellites as passive receivers to measure ocean surface roughness and wind speed, as well as soil moisture and flood inundation over land. This dissertation addresses two major aspects of engineering calibration: (1) characterization of the GPS effective isotropic radiated power (EIRP) for calibration of normalized bistatic radar cross section (NBRCS) observables; and (2) development of an end-to-end calibration approach using modeling and measurements of ocean surface mean square slope (MSS). To estimate the GPS transmit power, a ground-based GPS constellation power monitor (GCPM) system has been built to accurately and precisely measure the direct GPS signals. The transmit power of the L1 coarse/acquisition (C/A) code of the full GPS constellation is estimated using an optimal search algorithm. Updated values for transmit power have been successfully applied to CYGNSS L1B calibration and found to signiïŹcantly reduce the PRN dependence of CYGNSS L1 and L2 data products. The gain pattern of each GPS satellite’s transmit antenna for the L1 C/A signal is determined from measurements of signal strength received by the 8-satellite CYGNSS constellation. Determination of GPS patterns requires knowledge of CYGNSS patterns and vice versa, so a procedure is developed to solve for both of them iteratively. The new GPS and CYGNSS patterns have been incorporated into the science data processing algorithm used by the CYGNSS mission and result in improved calibration performance. Variable transmit power by numerous Block IIF and IIR-M GPS space vehicles has been observed due to their flex power mode. Non-uniformity in the GPS antenna gain patterns further complicates EIRP estimation. A dynamic calibration approach is developed to further address GPS EIRP variability. It uses measurements by the direct received GPS signal to estimate GPS EIRP in the specular reflected direction and then incorporates them into the calibration of NBRCS. Dynamic EIRP calibration instantaneously detects and corrects for power fluctuations in the GPS transmitters and significantly reduces errors due to GPS antenna gain azimuthal asymmetry. It allows observations with the most variable Block IIF transmitters (approximately 37% of the GPS constellation) to be included in the standard data products and further improves the calibration quality of the NBRCS. A physics-based approach is then proposed to examine potential calibration errors and to further improve the Level 1 calibration. The mean square slope (mss) is a key physical parameter that relates the ocean surface properties (wave spectra) to the CYGNSS measurement of NBRCS. An approach to model the mss for validation with CYGNSS mss data is developed by adding the contribution of a high frequency tail to the WAVEWATCH III (WW3) mss. It is demonstrated that the ratio of CYGNSS mss to modified WW3 mss can be used to diagnose potential calibration errors that exist in the Level 1 calibration algorithm. This approach can help to improve CYGNSS data quality, including the Level 1 NBRCS and Level 2 ocean surface wind speed and roughness. The engineering calibration methods presented in this dissertation make significant contributions to the spatial coverage, calibration quality of the measured NBRCS and the geophysical data products produced by the NASA CYGNSS mission. The research is also useful to the system design, science investigation and engineering calibration of future GNSS-reflectometry missions.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/168052/1/wangtl_1.pd

    New Passive Instruments Developed for Ocean Monitoring at the Remote Sensing Lab—Universitat Politùcnica de Catalunya

    Get PDF
    Lack of frequent and global observations from space is currently a limiting factor in many Earth Observation (EO) missions. Two potential techniques that have been proposed nowadays are: (1) the use of satellite constellations, and (2) the use of Global Navigation Satellite Signals (GNSS) as signals of opportunity (no transmitter required). Reflectometry using GNSS opportunity signals (GNSS-R) was originally proposed in 1993 by Martin-Neira (ESA-ESTEC) for altimetry applications, but later its use for wind speed determination has been proposed, and more recently to perform the sea state correction required in sea surface salinity retrievals by means of L-band microwave radiometry (TB). At present, two EO space-borne missions are currently planned to be launched in the near future: (1) ESA's SMOS mission, using a Y-shaped synthetic aperture radiometer, launch date November 2nd, 2009, and (2) NASA-CONAE AQUARIUS/SAC-D mission, using a three beam push-broom radiometer. In the SMOS mission, the multi-angle observation capabilities allow to simultaneously retrieve not only the surface salinity, but also the surface temperature and an “effective” wind speed that minimizes the differences between observations and models. In AQUARIUS, an L-band scatterometer measuring the radar backscatter (σ0) will be used to perform the necessary sea state corrections. However, none of these approaches are fully satisfactory, since the effective wind speed captures some sea surface roughness effects, at the expense of introducing another variable to be retrieved, and on the other hand the plots (TB-σ0) present a large scattering. In 2003, the Passive Advance Unit for ocean monitoring (PAU) project was proposed to the European Science Foundation in the frame of the EUropean Young Investigator Awards (EURYI) to test the feasibility of GNSS-R over the sea surface to make sea state measurements and perform the correction of the L-band brightness temperature. This paper: (1) provides an overview of the Physics of the L-band radiometric and GNSS reflectometric observations over the ocean, (2) describes the instrumentation that has been (is being) developed in the frame of the EURYI-funded PAU project, (3) the ground-based measurements carried out so far, and their interpretation in view of placing a GNSS-reflectometer as secondary payload in future SMOS follow-on missions

    Investigating the Sensitivity of Spaceborne GNSS-R Measurements to Ocean Surface Winds and Rain

    Full text link
    Earth remote sensing using reflected Global Navigation Satellite System (GNSS) signals is an emerging trend, especially for ocean surface wind measurements. GNSS-Reflectometry (GNSS-R) measurements of ocean surface scattering cross section are directly related to the surface roughness at scale sizes ranging from small capillary waves to long gravity waves. These roughness scales are predominantly due to swell, surface winds and other meteorological phenomena such as rain. In this study we are interested in understanding and characterizing the impact of these phenomena on GNSS-R signals in order to develop a better understanding of the geophysical parameters retrieved from these measurements. In the first part of this work, we look at GNSS-R measurements made by the NASA Cyclone Global Navigation Satellite System (CYGNSS) for developing an effective wind retrieval model function for GNSS-R measurements. In a fully developed sea state, the wind field has a constant speed and direction. In this case, a single Fully Developed Seas (FDS) Geophysical Model Function (GMF) is constructed which relates the scattering cross-section to the near surface wind speed. However, the sea age and fetch length conditions inside a hurricane are in general not consistent with a fully developed sea state. Therefore, a separate empirical Young Sea Limited Fetch (YSLF) GMF is developed to represent the conditions inside a hurricane. Also, the degree of under development of the seas is not constant inside hurricanes and conditions vary significantly with azimuthal location within the hurricane due to changes in the relative alignment of the storms forward motion and its cyclonic rotation. The azimuthal dependence of the scattering cross-section is modelled and a modified azimuthal YSLF GMF is constructed using measurements by CYGNSS over 19 hurricanes in 2017 and 2018. Next, we study the impact of rain on CYGNSS measurements. At L-band rain has a negligible impact on the transmitted signal in terms of path attenuation. However, there are other effects due to rain, such as changes in surface roughness and rain induced local winds, which can significantly alter the measurements. In this part of the study we propose a 3-fold rain model for GNSS-R signals which accounts for: 1) attenuation; 2) surface effects of rain; and 3) rain induced local winds. The attenuation model suggests a total of 96% or greater transmissivity at L-Band up to 30mm/hr of rain. A perturbation model is used to characterize the other two rain effects. It suggests that rain is accompanied by an overall reduction in the scattering cross-section of the ocean surface and, most importantly, this effect is observed only up to 15 m/s of surface winds, beyond which the gravity capillary waves dominate the scattering in the quasi-specular direction. This work binds together several rain-related phenomena and enhances our overall understanding of rain effects on GNSS-R measurements. Finally, one of the important objectives for the CYGNSS mission is to provide high quality global scale GNSS-R measurements that can reliably be used for ocean science applications. In this part of the work we develop a Neural Network based quality control filter for automated outlier detection for CYGNSS retrieved winds. The primary merit of the proposed Machine Learning (ML) filter is its ability to better account for interactions between the individual engineering, instrument and measurement conditions than can separate threshold quality flags for each one.PHDClimate and Space Sciences and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/166140/1/rajibala_1.pd

    Application de la réflectométrie GNSS à l'étude des redistributions des masses d'eau à la surface de la Terre

    Get PDF
    GNSS reflectometry (or GNSS-R) is an original and opportunistic remote sensing technique based on the analysis of the electromagnetic waves continuously emitted by GNSS positioning systems satellites (GPS, GLONASS, etc.) that are captured by an antenna after reflection on the Earth’s surface. These signals interact with the reflective surface and hence contain information about its properties. When they reach the antenna, the reflected waves interfere with those coming directly from the satellites. This interference is particularly visible in the signal-to-noise ratio (SNR) parameter recorded by conventional GNSS stations. It is thus possible to reverse the SNR time series to estimate the reflective surface characteristics. If the feasibility and usefulness of thismethod are well established, the implementation of this technique poses a number of issues. Namely the spatio-temporal accuracies and resolutions that can be achieved and thus what geophysical observables are accessible.The aim of my PhD research work is to provide some answers on this point, focusing on the methodological development and geophysical exploitation of the SNR measurements performed by conventional GNSS stations. I focused on the estimation of variations in the antenna height relative to the reflecting surface (altimetry) and on the soil moisture in continental areas. The SNR data inversion method that I propose has been successfully applied to determine local variations of: (1) the sea level near the Cordouan lighthouse (not far from Bordeaux, France) from March 3 to May 31, 2013, where the main tidal periods and waves have been clearly identified ; and (2) the soil moisture in an agricultural plot near Toulouse, France, from February 5 to March 15, 2014. My method eliminates some restrictions imposed in earlier work, where the velocity of the vertical variation of the reflective surface was assumed to be negligible. Furthermore, I developed a simulator that allowed me to assess the influence of several parameters (troposphere, satellite elevation angle, antenna height, local relief, etc.) on the path of the reflected waves and hence on the position of the reflection points. My work shows that GNSS-R is a powerful alternative and a significant complement to the current measurement techniques, establishing a link between the different temporal and spatial resolutions currently achieved by conventional tools (sensors, radar, scatterometer, etc.). This technique offers the major advantage of being based on already-developed and sustainable satellites networks, and can be applied to any GNSS geodetic station, including permanent networks (e.g., the French RGP). Therefore, by installing a processing chain of these SNR acquisitions, data from hundreds of pre-existing stations could be used to make local altimetry measurements in coastal areas or to estimate soil moisture for inland antennas.La rĂ©flectomĂ©trie GNSS (ou GNSS-R) est une technique de tĂ©lĂ©dĂ©tection originale et pportuniste qui consiste Ă  analyser les ondes Ă©lectromagnĂ©tiques Ă©mises en continu par la soixantaine de satellites des systĂšmes de positionnement GNSS (GPS, GLONASS, etc.), qui sont captĂ©es par une antenne aprĂšs rĂ©flexion sur la surface terrestre. Ces signaux interagissent avec la surface rĂ©flĂ©chissante et contiennent donc des informations sur ses propriĂ©tĂ©s. Au niveau de l’antenne, les ondes rĂ©flĂ©chies interfĂšrent avec celles arrivant directement des satellites. Ces interfĂ©rences sont particuliĂšrement visibles dans le rapport signal-sur-bruit (SNR, i.e., Signal-to-Noise Ratio), paramĂštre enregistrĂ© par une station GNSS classique. Il est ainsi possible d’inverser les sĂ©ries temporelles du SNR pour estimer des caractĂ©ristiques du milieu rĂ©flĂ©chissant. Si la faisabilitĂ© et l’intĂ©rĂȘt de cette mĂ©thode ne sont plus Ă  dĂ©montrer, la mise en oeuvre de cette technique pose un certain nombre de problĂšmes, Ă  savoir quelles prĂ©cisions et rĂ©solutions spatio-temporelles peuvent ĂȘtre atteintes, et par consĂ©quent, quels sont les observables gĂ©ophysiques accessibles.Mon travail de thĂšse a pour objectif d’apporter des Ă©lĂ©ments de rĂ©ponse sur ce point, et est axĂ© sur le dĂ©veloppement mĂ©thodologique et l’exploitation gĂ©ophysique des mesures de SNR rĂ©alisĂ©es par des stations GNSS classiques.Je me suis focalisĂ© sur l’estimation des variations de hauteur de l’antenne par rapport Ă  la surfacerĂ©flĂ©chissante (altimĂ©trie) et de l’humiditĂ© du sol en domaine continental. La mĂ©thode d’inversion des mesures SNR que je propose a Ă©tĂ© appliquĂ©e avec succĂšs pour dĂ©terminer les variations locales de : (1) la hauteur de la mer au voisinage du phare de Cordouan du 3 mars au 31 mai 2013 oĂč les ondes de marĂ©es et la houle ont pu ĂȘtre parfaitement identifiĂ©es ; et (2) l’humiditĂ© du sol dans un champ agricole Ă  proximitĂ© de Toulouse, du 5 fĂ©vrier au 15 mars 2014. Ma mĂ©thode permet de s’affranchir de certaines restrictions imposĂ©es jusqu’à prĂ©sent dans les travaux antĂ©rieurs, oĂč la vitesse de variation verticale de la surface de rĂ©flexion Ă©tait supposĂ©e nĂ©gligeable. De plus, j’ai dĂ©veloppĂ© un simulateur qui m’a permis de tester l’influence de nombreux paramĂštres (troposphĂšre, angle d’élĂ©vation du satellite, hauteur d’antenne, relief local, etc.) sur la trajectoire des ondes rĂ©flĂ©chies et donc sur la position des points de rĂ©flexion. Mon travail de thĂšse montre que le GNSS-R est une alternative performante et un complĂ©ment non nĂ©gligeable aux techniques de mesure actuelles, en faisant le lien entre les diffĂ©rentes rĂ©solutions temporelles et spatiales actuellement atteintes par les outils classiques (sondes, radar, diffusiomĂštres, etc.). Cette technique offre l’avantage majeur d’ĂȘtre basĂ© sur un rĂ©seau de satellites dĂ©jĂ  en place et pĂ©renne, et est applicable Ă  n’importe quelle station GNSS gĂ©odĂ©sique, notamment celles des rĂ©seaux permanents (e.g., le RGP français). Ainsi, en installant une chaĂźne de traitement de ces acquisitions de SNR en domaine cĂŽtier, il serait possible d’utiliser les mesures continues des centaines de stations prĂ©-existantes, et d’envisager de rĂ©aliser des mesures altimĂ©triques Ă  l’échelle locale, ou de mesurer l’humiditĂ© du sol pour les antennes situĂ©es Ă  l’intĂ©rieur des terres

    Etude des processus de fracturation et vĂȘlage d'iceberg en Antarctique : une histoire du glacier Mertz

    Get PDF
    Ces travaux de thĂšse prĂ©sentent les processus de fracturation des glaciers Ă©missaires menant au vĂȘlage. Dans le cadre du programme CRAC-ICE, nous nous sommes intĂ©ressĂ©s Ă  l'Ă©volution du glacier Ă©missaire Mertz, situĂ© sur la cĂŽte George V en Antarctique de l'est. Avant son vĂȘlage qui a eu lieu en FĂ©vrier 2010, libĂ©rant un iceberg de 80 km de long par 35 km de large, ce glacier Ă©tait caractĂ©risĂ© par une langue de glace se dĂ©veloppant sur l'eau. Cette langue de glace, sĂ©parĂ©e par une faille depuis le dĂ©but des annĂ©es 1990, Ă©tait longue de 150 km par 35 km de large. GrĂące Ă  un ensemble de donnĂ©es in-situ, d'images satellite basse et haute rĂ©solution et le dĂ©veloppement d'un modĂšle ocĂ©anique (TUGO-Mertz), nous avons suivi l'Ă©volution de ce glacier ainsi que le devenir de son iceberg. La premiĂšre partie de ce travail a consistĂ© Ă  dĂ©velopper une stratĂ©gie de traitement de donnĂ©es GPS nous permettant ainsi d'obtenir la meilleure prĂ©cision possible sur nos donnĂ©es in-situ. Ces donnĂ©es ont Ă©tĂ© traitĂ©es via le logiciel GINS et une technique de traitement appelĂ©e IPPP basĂ©e sur le positionnement absolu et la rĂ©solution des ambiguĂŻtĂ©s en valeurs entiĂšre. La prĂ©cision des rĂ©sultats de positionnement nous a permis d'observer des oscillations d'am- plitude centimĂ©trique et de pĂ©riode de quelques minutes, qui, comparĂ©es Ă  un modĂšle de poutre d'Euler-Bernoulli, correspondent Ă  des modes de vibration de la langue de glace dans trois configurations diffĂ©rentes. Les pĂ©riodes de ces oscillations s'Ă©chelonnent de 5 minutes Ă  quelques heures. Dans ces gammes de valeurs, les principaux forçages ocĂ©a- niques sont la houle et les ondes d'Infra-GravitĂ©. De plus, nous avons pu dĂ©montrer que les mouvements associĂ©s Ă  la vibration du glacier entraĂźnent une torsion favorisant sa fracturation. A plus grande Ă©chelle, les courants de marĂ©e ainsi que la hauteur de surface impactent sur l'Ă©volution de la langue de glace. Les effets des courants se concentrent principalement sur l'ouverture des crevasses principales tandis que la hauteur de surface tend Ă  moduler la vitesse d'Ă©coulement du glacier. Cette modulation reprĂ©sente environ 5 cm/jour soit 14 % de la vitesse moyenne. Par ailleurs, nous avons suivi l'Ă©volution de la crevasse principale du glacier ; une originellement ouverte sur la partie est de la langue de glace et une ouverte sur la partie ouest. Elles ont ensuite continuĂ© son dĂ©veloppement jusqu'au vĂȘlage du glacier.Pour finir, nous nous sommes intĂ©ressĂ©s aux diffĂ©rents acteurs en jeu lors de la rupture de la langue de glace menant au vĂȘlage. L'action de l'iceberg B09B et la modification des courants a jouĂ© un rĂŽle lors de cet Ă©vĂšnement mĂȘme si la crevasse Ă©tait en cours de dĂ©veloppement. L'ensemble de ces rĂ©sultats nous a permis d'identifier un large spectre de processus opĂ©rant avant et pendant un Ă©pisode de vĂȘlage. Ces processus sont majoritaires dans l'Ă©volution du glacier Mertz, mais ne sont pas encore pris en compte dans les modĂšles universels de vĂȘlage.This thesis presents a study of the rifting processes of an outflow glacier leading to cal- ving. In the context of the CRAC-ICE program we worked on understanding the evolution of the Mertz Glacier, located in the King George V Land, East Antarctica. Before its cal- ving which occurred in February 2010, releasing an iceberg of about 80 km long and 35 km width, the Mertz Glacier was characterized by an ice tongue extending into the open ocean. This ice tongue, fractured by a large rift since the beginning of 1990, was 150 km long by 35 km in width. Using a range of in-situ GPS data, satellite images and an ocea- nic tide model (TUGO-Mertz), we followed the evolution of the glacier and the calving of its iceberg. The first part of this study consisted in developing a high precision GPS processing strategy allowing us to get the best accuracy possible for our measurements. These data were processed using the GINS software and a processing strategy named IPPP, based on absolute positioning. The accuracy of our results allowed us the ability to observe centimeter scale oscillations of the ice tongue. Then, we compared these observed signals with an Euler-Bernoulli beam based model, and found out that they matched with vibration modes of the ice tongue in three different configurations. The periods recorded varied from 5 minutes to a few hours. In this range of temporal values, the main oceanic forcing mechanisms are ocean swell and infra-gravity waves. We also demonstrated that the vibrations of the glacier tongue, lead to torsion movements and hence rifting. At larger spatial scale, tidal currents and ocean sea surface height impacts on the evolution of the ice tongue. Tidal currents mainly affect the rift opening, whereas the sea surface height tends to modulate the along flow current velocity. In addition to these mechanisms, we followed the evolution of the main rift, first opening on the eastern part of the ice tongue and then on the western part until the glacier calved. Finally, we focused on the different processes leading to calving. The action of the B09B iceberg through the modification of tidal currents, played an important role in this event even if the main rift was almost completely opened

    Interferometric GNSS-R processing : modeling and analysis of advanced processing concepts for altimetry

    Get PDF
    This PhD dissertation is focused on the use of the opportunity signals from the Global Navigation Satellite Systems (GNSS), that scatter-off the Earth's surface for perform ocean mesoscale altimetry (the so called GNSS-R technique). Specially, this work analyses the capabilities of the interferometric approach (iGNSS-R) originally proposed for PARIS IoD (which will be implemented on GEROS-ISS), comparing its performance with the one obtained by the conventional approach (cGNSS-R). The main content of this PhD dissertation includes: A comprehensive analysis of the GNSS-R cross-correlation waveform properties, analyzing the impact that the observation geometry and system parameters have on the GNSS-R observables, where parameters such as the receiver bandwidth, observation geometry, sea state, and thermal and speckle noises are analyzed. A detailed derivation of the statistics for both the voltage and power cross-correlations (for both conventional and interferometric processing cases) validated all of them with both simulated and real data from ground-based airborne, and spaceborne experiments. Study of the performance model of the altimetry precision based on the Cramer-Rao Bound statistical estimator theory. This study has been carried out for a wide variety of parameters concerning the overall observation system, including instrument, on-board and on-ground processing aspects, for both the conventional and interferometric GNSS-R techniques. Analysis of experimental data from the Typhoon Investigation using GNSS-R Interferometric Signals (TIGRIS) experiment. This analysis has been used to determine and establish the boundaries and capabilities of GNSS-R towards remote sensing of typhoons. In this part, aspects such as the mitigation of the direct cross-talk contamination, GNSS multipath contamination, and preliminary results (including a novel observable) are presented.Este PhD se centra en el uso de la señales GNSS (Global Navigation Satellite Systems) como señales de oportunidad, para realizar altimetrĂ­a. Es lo que se conoce como GNSS-R (Global Navigation Satellite Systems-Reflectometry). Especialmente este trabajo analiza las principales propiedades del mĂ©todo interferomĂ©trico (iGNSS-R) inicialmente propuesto para PARIS-IoD, y que serĂĄ implementado en GEROS-ISS. comparĂĄndolo con el mĂ©todo convencional (cGNSS-R). A continuaciĂłn se enumeran los principales puntos abordados en esta tesis doctoral: Se ha realizado un exhaustivo y detallado anĂĄlisis de las propiedades de las waveforms, evaluando la influencia de la geometrĂ­a (altitud, ĂĄngulo de incidencia, estado del mar, etc), y de los principales parĂĄmetros a nivel de instrumento sobre las waveforms. AsĂ­ mismo se ha analizado el impacto del ancho de banda del receptor, y del ruido tĂ©rmico y speckle. Por otra parte, se han derivado las estadĂ­sticas (matrices de covarianzas) tanto para las waveforms complejas, como para las waveforms de potencia, considerando el mĂ©todo convencional, y el interferomĂ©trico. Las diferentes estadĂ­sticas o matrices de covarianzas han sido validadas usando datos reales procedentes de diferentes experimentos y datos simulados, para diferentes escenarios (.espacio, aerotransportado, ground-base). Dichas matrices de covarianza, posteriormente han sido usadas para calcular la precisiĂłn altimĂ©trica, basada en el uso de estimadores estadĂ­sticos, como es el Cramer-Rad Bound. Dicho estudio ha sido realizado considerando un amplio nĂșmero de parĂĄmetros, tanto para el mĂ©todo convencional como para el interferomĂ©trico, comparando el performance de ambas tĂ©cnicas, y realizando una primera estimaciĂłn de la precisiĂłn altimĂ©trica que se obtendrĂ­a para una misiĂłn espacial como PARIS-IoD. Por Ășltimo se han analizado datos de la campaña experimental TIGRIS (Typhoon Investigation using GNSS-R Interferometric Signal). con dicho analisis se han estudio la capacidad de la tĂ©cnica GNSS-R para monitorizar y detectar huracanes. Este anĂĄlisis incluye aspectos como la mitigaciĂłn de la contaminaciĂłn procendente de la señal directa, y del multipath ocasionado por las diferentes señales GNSS. Al mismo tiempo durante este anĂĄlisis un nuevo observable se ha propuesto
    • 

    corecore