1,721,007 research outputs found
Microphysical characterization of free space optical link due to hydrometeor and fog effects
No full textFree space optics (FSO) channel availability is affected by atmospheric water particles, which may introduce severe path attenuation. A unified microphysically oriented atmospheric particle scattering (MAPS) model is proposed and described to simulate particle scattering effects on FSO links. Atmospheric particles, such as raindrops, graupel particles, and snowflakes, together with fog droplets, are considered. Input data to characterize liquid and frozen water particle size distribution, density, and refractivity are derived from available literature data and measurements. Scattering, absorption, and extinction coefficients as well as the asymmetry factor are numerically simulated for each particle class and then parametrized with respect to particle water content, fall rate, and visibility, spanning from visible to infrared wavelengths. Both single- and multiple-scattering effects are discussed and quantified by using a radiative transfer model for small-angle approximation. MAPS simulations confirm that fog layers are those causing the largest power extinction on FSO links, but also several decibels of attenuation can be attributed to snow and rain conditions. Multiple-scattering effects, especially due to fog droplets, heavy rain, and dry snowflakes, typically tend to reduce the total attenuation by increasing the received power. An estimate of these effects, parameterized to single-scattering extinction, is proposed for near-infrared FSO link design
Atmospheric distortions of spaceborne SAR polarimetric signatures at X and Ka-band
No full textIn recent years operational missions and many investigations have assessed the potentialities of spaceborne Synthetic Aperture Radars (SARs) operating at X band and above to monitoring the Earth surface. The most appealing aspect of these instruments is the capability of observing the Earth at very high spatial resolution (of the order of meters) and with a well-known sensitivity to water content, ground roughness, ground displacement (i.e., interferometry applications) and so on. These characteristics make this kind of instrument very suitable for monitoring floods, earthquakes, volcanoes, urban areas, infrastructures, land use and marine surfaces and to produce Digital Elevation Models (DEMs) with a resolution of few meters (e.g. COSMO-SkyMed, TerraSAR-X/TanDEM-X systems). Moreover, fully-polarimetric SAR systems allow the complete characterization of target scattering properties to be measured, improving target classification capabilities. The experience gained throughout the mentioned missions has stimulated the development of SAR systems operating at higher frequencies, such as Ka-Band (around 35 GHz) and above, subject of many investigations. Nevertheless, sensitivity to atmospheric effects is a major concern. Numerous works in the last years have assessed how the atmosphere can influence spaceborne SARs operating at frequencies above C band. In particular, atmospheric water-based particles have demonstrated to condition SAR response echoes, both in amplitude and in phase, and in some extent the ground resolution, due to their turbulent motion in presence of precipitations. Such phenomenon are complex, and even if studied in the last decades, several issues are still open, especially respect polarimetric effects. Actually, the SAR slant-view echoes, are given by the ground surface backscattering, weighted by the two-way path attenuation, and significantly influenced by a volume contribution determined by the match water-based particles, characterized by their spatial distribution and electromagnetic properties (e.g. raindrops, snowflakes or ice droplets); moreover SAR band frequency and polarization differentiate the importance of such effects. To allow the comprehension of atmosphere on SAR received signal, we have developed a forward response model, where a known scenario is used to simulate the SAR slant echoes. The developed model is fully polarimetric and allows characterizing the SAR signature (in both amplitude and phase) in presence of a water-based particles distribution (e.g. precipitating cloud, such as cumulonimbus, or also non-precipitating one, such as cirrus), given a reference ground surface. In particular, we have model the Normalized Radar Cross Section (NRCS) and the complex correlation coefficient among copolarized returns, useful to characterize the signal differential phase shift (i.e., between horizontal and vertical polarizations: these represent the most significant elements of the measured covariance matrix, given by the one of the surface target with the superimposed scattering atmosphere. The polarimetric NRCS is model as combination of the clear-sky ground surface NRCS, weighted by the two-way atmospheric path attenuation, and by a volume contribution determined by the water-based particles reflectivity and extinction, still weighted by the two-way path attenuation. On the other hand, the observed complex correlation coefficient is modelled through the correlation coefficient of the ground target and hydrometeors reflectivity and specific differential phase, both weighted by the two-way path attenuation. The atmospheric scenario can be modeled through both synthetic scenarios, realized by stratified distributions of homogeneous particles (e.g. raindrops and ice) within a basic shape (e.g. rectangle or Gaussian) and realistic ones, produced by high resolution Cloud Resolving Models. The first scenarios are unrealistic but allow a better understating of the phenomenon and analysis of the SAR sensitivity range. On the other side, the second kind of target allows to establish a statistics of the applicability limits of the SAR atmospheric effects. In this work, realistic scenarios have been simulated using the System for Atmospheric Modeling (SAM) mesoscale model, that we have used to simulate high-resolution (250 m ground, in altitude ranging from 250 m to 800 m at 30 km height) tridimensional volumes (64 x 64 km2 and 30 km height) of water-based particles. In particular SAM simulates water content (g/cm3) distributions of precipitating (rain, snow, graupel) and non-precipitating (cloud ice and cloud liquid water) particles; on the other hand, their scattering properties have been characterized using T-Matrix simulation through the Hydrometeor-Ash Particle Ensemble Scattering Simulator (HAPESS). More exactly the SAR simulation datasets is composed by a significant number of vertical sections (each of them representing a simulation case study) has been selected among the available SAM 3-D structures, as representative of the variety of clouds systems (e.g. convective or stratiform). The ground surface scattering model completes the scenario. The framework can be easily configured to simulate bare soils, using for instance Semi-Empirical Models, as well as surface canonical targets, such as dihedrals or volume scattering, for an ease analysis of the effect of the atmosphere on the polarimetric signature and a comparison with literature works. In this work, we will use this model framework to analyze the distortions introduced by atmospheric effects, both in amplitude and phase, so focusing our attention not only on the most evident one, but also on other, less evident but still present, as phase distortions. This analysis will be performed on both X-band and Ka frequency bands. A verification of these results analysis will arise by the study of real COSMO-SkyMed and TerraSAR-X acquisitions
Effects of Multiple Scattering Due to Atmospheric Water Particles on Outdoor Free Space Optical Links
No full textFree Space Optics (FSO) systems, operating at near infrared or optical frequencies, represent an attractive possibility for wireless communications. Carrier wavelength and narrow beams ensure high data rates with reduced error rates and safety of communications. Nevertheless, their sensitivity to atmospheric conditions limits outdoor FSO applications. Fog droplets, raindrops and snowflakes, may introduce severe path attenuation, which drastically reduces the channel availability up to the channel interruption. In this respect, much work is still required, both in model and measurements perspectives. This work is devoted to a model analysis of multiple scattering effects on the FSO communication channel due to atmospheric hydrometeors. Within this framework, a parametric model is introduced to simulate and characterize scattering behavior of droplets. Several hydrometeor classes are analyzed and discussed, including fog droplets, raindrops, snowflakes and graupel particles
Quantitative estimation of precipitation on X-Band Synthetic Aperture Radar Imagery
No full textSpaceborne synthetic aperture radars (SARs) have become an important, in same extent fundamental, instrument for Earth observation and analysis. In particular, platforms operating at L-band and above have found a wide diffusion e.g. for flood areas detection and monitoring, earthquakes analysis, digital elevation model production, land use
monitoring and classification, while other application are under research, such as analysis of volcanic ashes. One of most interesting characteristic of these instruments is the ground spatial resolution (that can reach meters dimension). On the contrary, one of their traditional limitations is given by the reduced duty cycle and coverage: in this respect, recent space SAR missions, operational or near to completing deployment, have greatly reduced this limit. Other characteristic, traditionally allocated to SAR system, is the “all-weather” nature, which is insensitivity to meteorological phenomenon. Experience with simulated and observed data has indicated that this affirmation requires some refinements. Precipitations can significantly affect the signal backscattered from the ground surface (e.g. Ferrazzoli and Schiavon, 1997). Moreover meteorological phenomenon can directly alter the SAR received signal, both in amplitude and phase, as assessed by several authors in the last years (e.g. Marzano et al., 2010, Baldini et al., 2014) analyzing X-Band SAR data by COSMO-SkyMed (CSK) and TerraSAR-X (TSX) missions. Indeed the probability of matching a significant event is low (Danklmayer et al., 2009). If this sensitivity usually represents a problem to be addressed by SAR users, it could represent an interesting opportunity to detect and measure precipitations in wild areas. Moreover they offer the unique opportunity to ingest within flood forecasting model precipitation data at the catchment scale. In this work, we propose a processing framework aiming at producing precipitation maps and cloud masks by X-SARs data. Cloud masks are useful to SAR ground applications user to detect areas compromised by precipitations; in this
work, they are used also to improve the SAR precipitation product, using ancillary data. Precipitation maps, obtained at a very high ground resolution, as allowed in microwaves only by SAR systems, offer interesting opportunity not only in itself but also developing synergic uses with ground weather radar (WR), e.g. for WR calibration, development of improved WR precipitation retrieval algorithms or to improve cloud volume characterization, using the different operating frequency and observing geometry. In this respect, even if work has been done in the last years, several issues still need to be fully addressed. The developed procedure allows distinguishing flooded areas, precipitating clouds together with permanent water bodies, all appearing dark in the SAR image; this allows reducing the possibility of misinterpretations of the SAR data, which obviously have consequences on the precipitation map produced. Moreover, it allows estimating a cloud-free SAR image in order to retrieve the cloud attenuation. The following precipitation map procedure is based on the retrieval algorithm developed by Marzano et al. (2011), applied only to pixels where rain is known to be present. The developed procedure uses image segmentation techniques and fuzzy logic to perform the dark areas detection and recognition, while used ancillary data include local incident angle map and land cover (e.g. Pulvirenti et al. 2014 and Mori et al. 2012). The proposed methodology have been applied to 16 case study, acquired within TSX and CSK missions over Italy and United States, in order to analyzing both hurricane-like intense events and continental mid-latitude ones. Moreover this choice offer the possibility to establish the comparison with operational ground weather radar products, for both verify and validate the proposed methodology and to exploit the synergic use SAR-WR. We will discuss the results obtained until now in terms of improved rain cell localization and precipitation quantification. Produced precipitations map will be available online within the portal of the FP7 project EartH2Observe “Global Earth Observation for Integrated Water Resource Assessment” (http://www.earth2observe.eu
Analysis of Atmospheric Effects on X-BAND Synthetic Aperture Radar Observations and Precipitation Estimation
No full textThis paper proposes a new methodology for the detection and quantitative estimation of intense atmospheric precipitations on images acquired by Synthetic Aperture Radars (SARs) operating at X-Band wavelengths. The proposed methodology consists of two successive steps. The first one allows detecting and distinguishing areas subjected to intense precipitation events, permanent water surfaces, flood areas and snow coverage. The second step derives an estimation of the precipitation rate using the event attenuation estimated at the previous step. This methodology is applied on two COSMO-SkyMed (CSK) satellite case studies. The first one is relative to a severe precipitation weather event, occurred in northwestern Italy (close to Liguria region) on November 3-8, 2011. The second one is relative to Hurricane “Irene” event, occurred in Eastern United States (close to Delaware) on late August 2011. In both cases X-SAR echoes and estimated rain rate is compared with corresponding products derived by available ground Weather Radars (WRs). The correlation of the precipitating cloud fields between CSK X-SAR and WR images is significant in all case studies
Precipitation retrieval from satellite synthetic aperture radar measurements: numerical modeling and preliminary applications
No full textThis work explores the potential of space-borne X-SARs to estimate rainfall over land from both a model and retrieval point of view. The main objective is to provide a framework for a physically-based inversion of SARs measurements at X (9.6 GHz), Ku (14 GHz) and Ka (30 GHz) band over land. A forward model of SAR response will be illustrated for X, Ku and Ka bands. We will present inversion methodologies and a quantitative application to X-SAR data of the SIR-C mission in 1994
High-resolution rainfall retrieval over land from satellite synthetic aperture radar measurements at X, Ku and Ka band
No full textClimate modelers need global precipitation measurements because the released latent heat distribution has a profound effect on the performance of such models. Precipitation measurements are also required to facilitate water management strategies by hydrologists, and managers of transportation, agricultural and flood relief agencies. Although precipitation measurements are widely available in advanced countries, the measurement of precipitation over oceans, mountainous terrain and less developed regions leaves much to be desired. Since the 1980s much of our understanding of global precipitation has been provided by space-borne passive microwave radiometers and combination of microwave and infrared passive measurements. Unfortunately space-borne microwave radiometers, even in combination with infrared sensors, have had limited success in retrieving precipitation over land because they rely heavily on the scattering properties of ice in the upper regions of precipitating clouds. Those scattering properties may be poorly related to surface rainfall rates. This limitation can be overcome over land by space-based radars operating at X or Ku band.
The Ku band Precipitation Radar (PR) aboard the Tropical Rainfall Measurement Mission (TRMM) program has provided unique precipitation measurements over land. Mountainous terrain has presented challenges to both ground and space based radars. A new opportunity to measure precipitation from space may be afforded by the forthcoming availability of several X-band Synthetic Aperture Radars (X-SARs). The TerraSAR-X (TSX) was launched on June 15, 2007 by the Deutsches Zentrum f. Luft u. Raumfahrt (DLR). The Constellation of Small Satellites for Mediterranean basin Observations (COSMO-SkyMed, CSK) will be launched by the Agenzia Spaziale Italiana (ASI) within 2008. The first of four of these satellites was launched on June 7, 2007.
Space-borne X-SARs are generally not designed for atmospheric observation: in a way, they are always labeled as “all weather” sensors. However, there is relevant theoretical and experimental evidence that X-band radar may be significantly affected by precipitation occurrence within the synthetically scanned area. As a matter of fact, PR was designed at Ku band which is only 4 GHz far from X band. Several authors showed that X-SARs are more sensitive to rainfall effects than SARs operating at those longer wavelengths such as L and C bands, as demonstrated by the Shuttle Missions STS-59 and 68 of 1994 and the STS-99 Shuttle Radar Topography Mission (SRTM) of 2000 carrying the first X-SAR along with L and C band SARs. The high spatial resolution (less than 100 m) of X-SARs can provide new insights into the structure of precipitating clouds with respect to PR and its future upgrades. X-SAR platforms could also significantly enhance the planned constellation of satellites carrying microwave radiometers and radars that will be part of the foreseen Global Precipitation Measurements (GPM) mission.
This work is devoted to the exploration of the potential of space-borne microwave SAR to estimate rainfall over land from both a model and retrieval point of view. The main objective is to provide a framework for a physically-based inversion of SARs measurements at X, Ku and Ka band over land. Previous works have already shown X-SARs potentials for rainfall retrievals, but only recently there have been systematic approaches to design quantitative inversion algorithms. We will concentrate on SAR inversion over land in order to avoid the ambiguities of X-SAR response over ocean in the presence of rainfall and because the hydrological application seems to be very promising, as mentioned. A forward model of SAR response will be illustrated not only the X band, but also at Ku and Ka band where some SAR technology is already available. The inversion methodologies will be extensively illustrated and a quantitative application to X-SAR data of the SIR-C mission in 1994 will be discussed
Evidence of Rainfall Signatures on X-Band Synthetic Aperture Radar Imagery Over Land
No full textFive spaceborne X-band synthetic aperture radars (X-SARs) are nowadays operating, and several more will be launched in the coming years. These X-SAR sensors, able to image the Earth's surface at metric resolution, may provide a unique opportunity to measure rainfall over land with spatial resolution of about a few hundred meters due to the moving-target degradation effects. This work is devoted to experimentally demonstrate this X-SAR capability, which can also be exploited to correct synthetic aperture radar (SAR) imagery for rainfall attenuation effects. Several case studies, selected from TerraSAR-X (TSX) overpasses over Europe and the southern U. S. in 2008, are qualitatively analyzed in terms of rainfall signatures. Visual validation of these rainfall SAR signatures is carried out by using available data from ground-based weather radars. A detailed data analysis for the case study of Hurricane "Gustav" on September 2, 2008, is carried out to assess a quantitative correlation among X-SAR response and near-surface precipitation rain rate. Two simplified empirical inversion algorithms, based on statistical regression and probability matching, are developed to retrieve rain rate from TSX cross-track ground-range measurements. The TSX-retrieved rain fields are compared to those estimated from the Next Generation Weather Radar (NEXRAD) in Mobile (Alabama, U. S.), showing a root-mean-square error less than 15 mm/h and a correlation of about 0.7
Modeling Scintillation Effects on Free Space Optical Links using Radiosounding Profile Data
No full textWireless communications using free space optics (FSO) are sensitive to atmospheric conditions. Hydrometeors, but also clear-air turbulence, may introduce severe impairments reducing FSO channel availability. Yearly radiosounding profiles, available near Rome (Italy), are used to estimate the power scintillation index through a new physical turbulence structure constant model and to estimate scintillation fade statistics for near-infrared FSO
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