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Observations of Titan liquid bodies by means of the Cassini RADAR altimeter
No full textTwenty years after its great departure from Cape Canaveral the Cassini mission is going to have the “Grand Finale” it deserves. This spacecraft definitely marked a milestone in astronautical history as one of the best examples of collaboration among different space agencies (NASA-ESA-ASI) and a great model for any other future mission for planetary exploration. A discovery machine that unveiled the Titan’s lively world and that doesn’t stop giving surprises at every new observation. I’m Valerio Poggiali, phD candidate, and with my colleagues and professors of La Sapienza University of Rome I’m involved in the Cassini RADAR altimeter data processing, a task bequeathed to us by our Prof. Emeritus Giovanni Picardi who started believing in this project already at the beginning of nineties when he published a paper entitled “The Radar System for the Exploration of Titan”. It is 1992, and he states: “the knowledge of the ocean depth is particularly important and its determination is one of the main requirements for the radar instrument”. Actually he announced twenty-two years in advance our work on “The Bathymetry of a Titan Sea” in which we reported about an incredible observation made by the Cassini radar altimeter on May 23st, 2013. We were waiting for that fly-by by years as it was representing the only opportunity to test the instrument capability to plumb the bottom of a Titan sea and infer about its dielectric properties (besides proving once for all the liquid nature of those dark features). After Kraken and before Punga, the Ligeia Mare is the second greatest sea of Titan (roughly 260 x 217 miles). We have been able to track a deepest point of 180 yards and estimate its liquid to be composed by an extremely pure mixture of liquid hydrocarbons (laboratory experiments at JPL eventually confirmed components to be methane, ethane and nitrogen). The following period has been very busy for us, but also extremely rewarding. After the publication of Ligeia Mare bathymetry, the Cassini radar altimeter observed the Kraken Mare on August 21st, 2014 and the Punga Mare on January 11st, 2015. The measurements we had to perform were very challenging. Main difficulties came from the necessity to track the subsurface also in the shallowest parts of seas where we have to push the radar capabilities to the limits of its vertical resolution (nominally 33 yards but with some super-resolution algorithms improvable of a factor of about two). Another great test that we faced was in the production of the bathymetry of the largest lake (43 x 124 miles) of the southern polar area of Titan, the Ontario Lacus. The severe saturation of the receiver united to the shallowness of this liquid body (preliminary results suggested a 50 yards maximum depth along the altimeter ground track, that of course could be not the deepest point) imposed the necessity of developing a different method for determine its bathymetry and dielectric properties. Also if the Cassini radar will not observe Titan seas anymore (we will have just a last look to northern polar small lakes during Cassini’s last flyby of Titan on April 2017) we have still many analyses to perform on the already collected data, these will engage us for years to come. The more Cassini mission will come closer to its end on September 15, 2017, the more we will have to stay tuned to not miss anyone of the discoveries that will certainly follow as the spacecraft will finally approach Saturn, receiving, as if it was an engagement gift, the unique opportunity of a closer observation of its precious rings.
In this PhD dissertation I would like to resume the main steps we followed for analyzing the Cassini radar altimeter data from which we obtained the first bathymetries of extraterrestrial liquid bodies, starting from a brief introduction to the RADAR instrument characteristics and its capabilities. Later I will focus on the backscattering models we adopted in order to effectively interpret the radar altimeter nadiral power returns from the surface of Titan.
A detailed description will follow of the simulator we developed in order to reconstruct as accurately as possible how the Cassini radar receiving chain processes on board the incoming signal. Then, I will describe the results we obtained on Ligeia Mare and Ontario Lacus, giving some anticipations on the work ongoing for Punga and Kraken Mare.
The last part will be finally dedicated to our discovery on Titan of liquid-filled canyons directly connected to seas. I will describe how the radar altimeter was able to measure their depth and infer about their nature of sea flooded valleys by means of a comparison of the levels of their liquids.
Far from being just a library-based theoretical dissertation, this document will intentionally focus on the work we carried out and on the consequent results we obtained in the framework of the Cassini mission. The techniques described herein and, in particular, the simulator and the Bayesian method for the estimation of unknown parameters we adopted for this work are the result of the fruitful collaboration between our Dipartimento di Ingegneria Elettronica e Telecomunicazioni (DIET) of “La Sapienza” University of Rome and the Astronomy department of the Cornell University in Ithaca (NY)
Dune Height Estimation on Titan Exploiting Pairs of Synthetic Aperture Radar Images With Different Observation Angles
No full textWidespread longitudinal dunes have been identified on Titan thanks to the 2.2-cm wavelength Cassini Synthetic Aperture Radar (SAR) instrument. Understanding the properties of these surface features, such as material composition and dune height, is very important for giving new clues about the Titan geology and climate. One of the major difficulties in the estimation of dune heights using SAR occurs when the material composition of the dunes is heterogeneous. In this paper, we propose a novel method for dune height estimation, which takes into account material heterogeneity, and in particular, the case in which the interdune exhibits different dielectric properties with respect to the remaining part of the dune. Paired data acquisitions with orthogonal observations are considered for separating the dielectric from the geometric effect on the backscattering coefficients in order to retrieve the slope and thus the height of the dunes. The results for a test area located in the Fensal region indicate that the slopes of the dune faces are generally lower than 5° and the heights range between 40 and 110 m
Titan dune heights retrieval by using Cassini Radar Altimeter
No full textThe Cassini Radar is a Ku band multimode instrument capable of providing topographic and mapping information. During several of the 93 Titan fly-bys performed by Cassini, the radar collected a large amount of data observing many dune fields in multiple modes such as SAR, Altimeter, Scatterometer and Radiometer. Understanding dune characteristics, such as shape and height, will reveal important clues on Titan's climatic and geological history providing a better understanding of aeolian processes on Earth. Dunes are believed to be sculpted by the action of the wind, weak at the surface but still able to activate the process of sand-sized particle transport. This work aims to estimate dunes height by modeling the shape of the real Cassini Radar Altimeter echoes. Joint processing of SAR/Altimeter data has been adopted to localize the altimeter footprints overlapping dune fields excluding non-dune features. The height of the dunes was estimated by applying Maximum Likelihood Estimation along with a non-coherent electromagnetic (EM) echo model, thus comparing the real averaged waveform with the theoretical curves. Such analysis has been performed over the Fensal dune field observed during the T30 flyby (May 2007). As a result we found that the estimated dunes' peak to trough heights difference was in the order of 60-120 m. Estimation accuracy and robustness of the MLE for different complex scenarios was assessed via radar simulations and Monte-Carlo approach. We simulated dunes-interdunes different composition and roughness for a large set of values verifying that, in the range of possible Titan environment conditions, these two surface parameters have weak effects on our estimates of standard dune heights deviation. Results presented here are the first part of a study that will cover all Titan's sand seas. (C) 2014 Published by Elsevier Inc
Radar sounding using the Cassini altimeter waveform modeling and Monte Carlo approach for data inversion observations of Titan's seas
Get PDFRecently, the Cassini RADAR has been used as a sounder to probe the depth and constrain the composition of hydrocarbon seas on Saturn's largest moon, Titan. Altimetry waveforms from observations over the seas are generally composed of two main reflections: the first from the surface of the liquid and the second from the seafloor. The time interval between these two peaks is a measure of sea depth, and the attenuation from the propagation through the liquid is a measure of the dielectric properties, which is a sensitive property of liquid composition. Radar measurements are affected by uncertainties that can include saturation effects, possible receiver distortion, and processing artifacts, in addition to thermal noise and speckle. To rigorously treat these problems, we simulate the Ku-band altimetry echo received from Titan's seas using a two-layer model, where the surface is represented by a specular reflection and the seafloor is modeled using a facet-based synthetic surface. The simulation accounts for the thermal noise, speckle, analog-to-digital conversion, and block adaptive quantization and allows for possible receiver saturation. We use a Monte Carlo method to compare simulated and observed waveforms and retrieve the probability distributions of depth, surface/subsurface intensity ratio, and subsurface roughness for the individual double-peaked waveform of Ligeia Mare acquired by the Cassini spacecraft in May 2013. This new analysis provides an update to the Ku-band attenuation and results in a new estimate for its loss tangent and composition. We also demonstrate the ability to retrieve bathymetric information from saturated altimetry echoes acquired over Ontario Lacus in December 2008
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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