Formación - Tesis de Máster

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GNSS-­R as a Source of Opportunity for Remote Sensing of the Cryosphere

Estado: leído (2013-05-13)

Estudiante(s): Fran Fabra
Director(s): Estel Cardellach, Camps, A.
Universidad: Universitat Politècnica de Catalunya
During the last years, an awareness of global warming has appeared in the international community, giving room to discussion that sometimes goes beyond science topics and it gets more controversial. Several studies have arisen in order to find causes, consequences or simply to establish ways to predict the rate of change of the Earth's climate. These studies are usually based on experiments that perform measurements of variables at different environments. The conclusion and results that they may obtain will be then determined by the goodness of these data retrievals. Therefore, techniques to obtain data sets of good quality are valuable. It seems obvious that there exists a close relationship between the evolution of the Earth's climatology and the state of the polar regions. It is known that the large ice sheets particularly influence the global water cycle by locking up or releasing large amounts of fresh water; the radiation budget through the high albedo of ice­ and snow­covered surfaces; and the thermohaline circulation through the amount of fresh water released to the ocean by melting or iceberg calving; among other components of the climate system. It is also reported that due to the extreme polar environmental conditions, the surface mass balance and its most important parameter, the snow accumulation, are poorly retrieved. Remote sensing systems over satellite are specially helpful for monitoring these areas. The use of space­based systems for tracking the polar regions started approximately in 1978. Concretely, dealing with sea ice, a wide base of knowledge about microwave and optical signatures has been acquired since then, allowing the determination of the global extent of the ice cover over the oceans and to classify it into broad categories of age/thickness. The different techniques employed range from scatterometers and radiometers to multisensor classification. By these means, the gradual loss of the sea ice both in the Arctic and the Antarctic area during the last years have been shown. The characterization of the snow cover over large areas has been another potential use of these techniques. In 1993, a new approach for remote sensing appeared: PARIS (Passive Reflectometry and Interferometry System). It is based on the analysis of reflected L­band GPS signals and it was motivated by the availability of the GPS, GLONASS and (still) future GALILEO constellation of navigation satellites. For this reason, the concept is usually referred as GNSS­-R (Global Navigation Satellite Systems ­ Reflections). Despite being initially conceived for sea surface altimetry, this technique has been successfully proved for many other purposes, such as ocean wind speed, soil moisture changes, sea surface state determination, and sea ice detection and classification. GNSS-­R signals supposes an already existing and well characterized source of opportunity, since there is a lot of experience with the GPS system and precise orbits and other detailed information are available. On the other hand, the inversion of parameters like ice extent, type and snow depth from other satellite data is based on a collection of rather ad hoc techniques, which in most cases require a priori statistical knowledge of the ice conditions and is encumbered by internal ambiguities (e.g. salinity vs. roughness in backscatter data or surface effects using thermal emissions). Therefore, the utilization of scattered GPS signals affords an excellent opportunity to promote the theoretical study of microwave signatures of sea ice and has the potential to provide a validating reference to current active and passive observation techniques. In addition, GNSS­R systems only need to deploy the receivers, and it leads to consider an inexpensive constellation of LEOs (Low Earth Orbiters) with global coverage and high spatial/­temporal resolution (GNSS­-R providing other suitable applications at mid-­ and Equatorial ­latitudes). The aim of this thesis is to evaluate the potential of GNSS­-R to provide sea­ice and dry snow measurements. The work will be based on the analysis of the data gathered from fixed platforms at the GPS­-SIDS (Global Positioning System - Sea Ice Dry Snow) project. In addition, the same equipment will be used during a set of aircraft campaigns for the L­band radiometer CAROLS. At the end, the results obtained will be extrapolated to a spaceborne platform. GPS-­SIDS is an ESA's (European Space Agency) project carried on jointly by IEEC (Institut d'Estudis Espacials de Catalunya), GFZ (GeoForschungsZentrum), IFAC (Institute of Applied Physics), DMI (Danish Meteorological) and AD Telecom. With it, the gathering of relatively long term high quality data sets of reflected GPS signals from polar sea­ice and dry snow becomes possible.