Browsing by Author "Thakur, Sanchari"

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3D radar sounder simulations of geological targets on Ganymede Jovian Moon
(SPIE, 2019-10) Thakur, Sanchari
Subsurface investigation of the Jovian icy moons is expected to disclose interesting information on the Jovian system. The Radar for Icy Moon Exploration (RIME) is the instrument in charge of characterizing the subsurface of the three icy moons Ganymede, Europa and Callisto. To provide a key for interpretation for the real acquired data, simulations of different possible scenarios on Ganymede are presented in this work. In this paper, we present an approach to performance analysis of RIME based on the 3D modelling and electromagnetic simulations of selected icy moon targets. These simulations are carried out using the Finite-Difference Time-Domain (FDTD) technique, which has been used in recent years to support radar sounder applications. In this work, we analyze in detail some interesting targets: 1) the dark terrain regolith, 2) the bright terrain dielectric profile, and 3) the grooved bright terrain. Our analysis is performed in two levels. First, the contribution of individual features is analyzed, varying their geometry and composition to understand how the measured fields vary accordingly. Second, a more realistic geological arrangement of a combination of subsurface features is considered. The results are very promising and indicate that the subsurface response is detectable in most of the cases.
Analysis of earth’s ionosphere effects on englacial layering detectability in spaceborne radar sounders data
(IEEE, 2022-06) Thakur, Sanchari
Several studies are in progress for proposing an Earth-orbiting radar sounder (EORS) mission. Some of them consider as baseline system architecture a recently proposed distributed radar sounding array in formation flight with enhanced capabilities of clutter suppression. Besides clutter, the detectability of subsurface targets may also be affected by the propagation of the radar signal through Earth’s ionosphere. These effects include frequency-dependent phase dispersion and scintillations. In this letter, we present a subsurface detection performance assessment of an EORS with distributed architecture focusing on the ionospheric effects. The novel contributions of this work are: 1) simulation of the coherent radar response of a representative polar ice target (englacial layering) in the distributed radar sounding case; 2) inclusion of spatially dependent ionospheric scintillation effects on the distributed beam pattern; 3) inclusion of phase dispersion effects for different values of total electron content (TEC); and 4) analysis of the subsurface detection performance. Detectability analysis is performed after applying a state-of-the-art technique for compensating ionospheric phase-dispersion effects. The results show that the englacial layering is detectable by compensating for the dispersion effects in the range between 1 and 21 TECU in the ionosphere. The layering is also detectable at higher values of TEC by improving the accuracy of TEC estimation. Moreover, even without compensation, the worst case ionospheric phase scintillations of 25° produce a negligible effect on the detectability.
Analysis of subsurface hypotheses through simulation of rime radargrams based on available analogous data
(IEEE, 2019) Thakur, Sanchari
Radar for Icy Moon Exploration (RIME) is designed to characterize the subsurface geology of the Jovian icy moons. The RIME radargrams will show the combined response of a number of geophysical and geological characteristics of the ice-shells of these moons. Thus, radar sounder simulations are needed to understand the relationship between these target variables and the RIME response. In this paper, we use a computationally simple simulation approach that is based on reprocessing the radargrams available from the geological analogs of RIME targets. Moreover, we present a case study for a particular RIME target using this simulation technique for the generation of a database of RIME radar-grams, and a technique for analyzing this database. From the preliminary analysis of the simulated radargrams, we could derive important information regarding the underlying target variables. This confirms the usefulness of the presented approach to support the geological interpretation of the RIME radargrams.
Analysis of surface clutter for subsurface radar sounding on Venus
(IEEE, 2022) Thakur, Sanchari
ESA's EnVision mission has been selected for Venus exploration, with launch scheduled in 2031. It will carryon-board the Subsurface Radar Sounder (SRS) to profile the shallow crust at low frequency and support the understanding of Venus' geological history. In the design and performance assessment of SRS, an important step is the analysis of off-nadir clutter due to rough surface scattering and its potential to mask the scientifically relevant subsurface echoes. While there have been studies to understand the roughness of Venusian terrains from data available from the previous missions, there are few studies that analyze the roughness and clutter at the wavelength-scale of a low-frequency radar sounder instrument. In this paper, we present a first step towards filling this gap by analyzing the clutter performance of selected sites on Venus where subsurface interfaces are expected, namely the plains, the impact craters, and the lava flows. Using surface roughness parameters and fractal modelling, we generate multiple surface realizations, simulate the SRS clutter response and analyze the probability distribution of clutter depth and power. The results show that for most of the sites, clutter is concentrated very close to the surface, and therefore does not significantly affect the subsurface detection.
An approach to the assessment of detectability of subsurface targets in polar ICE from satellite radar sounders
(IEEE, 2021-10) Thakur, Sanchari
A satellite mission onboard a radar sounder for the observation of the earth’s polar regions can greatly support the monitoring of the cryosphere and climate change analyses. Several studies are in progress proposing the design and demonstrating the performance of such an earth-orbiting radar sounder (EORS). However, one critical aspect of the cryospheric targets that are often ignored and simplified in these studies is the complex geoelectrical nature of the polar ice. In this article, we present a performance assessment of the polar ice target detectability by focusing on their realistic representation. This is obtained by simulating the orbital radargrams corresponding to different regions of the polar cryosphere by leveraging the data available from airborne campaigns in Antarctica and Greenland. We propose novel performance metrics to analyze the detectability of the internal reflecting horizons (IRHs), the basal interface, and to analyze the nature of the basal interface. This performance assessment strategy can be applied to guide the design of the signal-to-noise ratio (SNR) budget at the surface, which can further support the selection of the main orbital instrument parameters, such as the transmitted power, the two-way antenna gain, and the processing gains.
An approach to the generation and analysis of databases of simulated radar sounder data for performance prediction and target interpretation
(IEEE, 2021-02) Thakur, Sanchari
Radar sounders (RSs) are low-frequency instruments that profile the shallow subsurface of planetary targets to obtain valuable scientific information. The prediction of the RS performance and the interpretation of the target properties from the RS data are challenging due to the complex electromagnetic interaction among many acquisition variables. Simulation of RS data can address this issue by modeling the complex interaction and producing simulated radargrams representing the acquisition scenario. In this article, we present an approach to generate databases of geoelectrical models and simulated radargrams corresponding to combinations of: 1) target geoelectrical hypotheses; 2) RS parameters; and 3) acquisition geometry configurations. The proposed approach exploits this database for: 1) predicting the detection performance and sensitivity of the RS and 2) understanding the interpretability of the underlying hypotheses. In order to identify hypothesis combinations that can be unambiguously inverted from the radargrams, we analyze the similarity between pairs of geoelectrical models and between the simulated radargrams, and the statistical distance between radargram features. The approach is demonstrated for the case of Radar for Icy Moons Exploration (RIME), using three selected targets on the Jovian moon Ganymede, with three different simulation techniques. The results are very promising and reveal the effectiveness of the proposed approach in extracting valuable information regarding: 1) the target detection performance of RIME; 2) the sensitivity to the dielectric contrast; 3) the separability of radargram features; and 4) the identification of hypothesis combinations producing significantly different radar response, and thus invertible.
An approach to the simulation of radar sounder radargrams based on geological analogs
(IEEE, 2019-03) Thakur, Sanchari
Simulation of radar sounder (RS) data is important for understanding the radar response of subsurface features to facilitate the interpretation of the real data. Conventional electromagnetic simulators require the definition of complex geoelectrical models of the investigated targets. They also involve time-complex solutions of Maxwell's equations for computing the received electric field, which leads to very high computation time. Furthermore, the simulated radargrams are often not realistic as it is very difficult to model all the variables involved in the data acquisition. In this paper, we propose a novel simulation approach that exploits the data available from existing RSs in geologically analogous terrains, to produce realistic simulations of the investigated RS target. This simulation strategy is based on minimizing the difference between the analog and the investigated acquisition scenarios. This is done by applying a series of corrections, which depend on the relation between the radargram characteristics and the physical variables describing the acquisition process. The aim is to produce radargrams that resemble the investigated scenario in terms of the echo magnitude, bandwidth, range resolution, and along-track resolution. Experimental results present three case studies for different possibilities of the analog and the investigated scenarios. The validation of the simulated radargrams with actual data demonstrates the effectiveness of the proposed approach. Finally, we also present a real application of this approach for the simulation of Radar for Icy Moon Exploration (RIME) radargrams for a combination of instrument and target parameters, using the SHAllow RADar (SHARAD) radargram acquired over the geological analog of a selected RIME target.
The ASI integrated sounder-sar system operating in the uhf-vhf bands: first results of the 2018 helicopter-borne Morocco desert campaign
(MDPI, 2019-08) Thakur, Sanchari
This work is aimed at showing the present capabilities and future potentialities of an imaging radar system that can be mounted onboard flexible aerial platforms, such as helicopters or small airplanes, and may operate in the UHF and VHF frequency bands as Sounder and as Synthetic Aperture Radar (SAR). More specifically, the Sounder operates at 165 MHz, whereas the SAR may operate either at 450 MHz or at 860 MHz. In the work, we present the first results relevant to a set of Sounder and SAR data collected by the radar during a helicopter-borne campaign conducted in 2018 over a desert area in Erfoud, Morocco, just after the conclusion of a system upgrading procedure. In particular, a first analysis of the focusing capabilities of the Sounder mode and of the polarimetric and interferometric capabilities of the SAR mode is conducted. The overall system, originally developed by CO.RI.S.T.A. according to a ASI funding set up in 2010, has been upgraded in the frame of a contract signed in 2015 between ASI and different private and public Italian Research Institutes and Universities, namely CO.RI.S.T.A., IREA-CNR, Politecnico di Milano and University of Trento.
The ASI p-band helicopter-borne integrated sounder-sar system: preliminary results of the 2018 Morocco desert campaign
(IEEE, 2019-08) Thakur, Sanchari
The Italian Space Agency (ASI) has recently entrusted CO.RI.S.T.A. with the development of a radar system that can be mounted onboard small airplanes or helicopters and may operate, at different frequencies belonging to the P-Band, either as Synthetic Aperture Radar (SAR) or as Sounder. In this work, we present preliminary results of the helicopter-borne desert campaign carried out with this system in 2018 over the Erfoud area, Morocco, in the frame of a project that has involved different public Italian Research Institutes and Universities.
Assessing the detection performance on icy targets acquired by an orbiting radar sounder
(IEEE, 2019) Thakur, Sanchari
Radar sounders (RS) can be used to acquire data on ice sheets and provide direct evidence of the structures in the subsurface. Many acquisitions are available from airborne RS in the Antarctica and Greenland. However, airborne data are costly, have limited spatial coverage, and nonhomogeneous characteristics. To overcome these limitations, a potential satellite-mounted RS could provide uniform coverage and consistent data quality at the cost of lower resolution and higher path loss. In this paper, we assess the performance of a possible Earth-orbiting RS by simulating and analyzing its radargrams. The simulation approach reprocesses existing airborne RS to match the orbital RS characteristics. The simulated radargrams are analyzed to estimate the losses and understand the detection performance of icy targets using state-of-the-art data analysis techniques. The preliminary analysis of the simulated radargrams indicates that, under the simplified assumptions, an orbiting RS will be capable of imaging the investigated subsurface targets.
An automatic approach to map refreezing ice in radar sounder data
(SPIE, 2019-10) Thakur, Sanchari
Radar sounders mounted on airborne platforms have acquired data of the subsurface of the Earth's icy areas over the last decades. These data, called radargrams, contain information on the dielectric discontinuities in the ice-sheets, and thus on the buried geological structures and the related processes. Conventionally, these structures have been characterized and mapped by visually inspecting the radargrams. However, visual inspection is subjective and time-consuming and can lead to misinterpretations. Recently, state-of-the-art automatic techniques are proposed to map the position of the bedrock, the ice layering, and the noise in the radargram. However, there are no automatic techniques for mapping the basal refreezing, which is an important ice target that controls the rate of sea-ward ow of the ice-sheets. This paper proposes an automatic method to map the refreezing ice in radargrams. We model the refreezing ice considering its geophysical and radiometric properties. Then, we design a set of features considering this model to perform a classification of the radargrams into four classes, i.e., ice layering, echo-free zone (EFZ) and thermal noise, bedrock, and the refreezing ice. We applied the proposed method to radargrams acquired in the north Greenland by Multichannel Coherent Radar Depth Sounder (MCoRDS3), a radar sounder designed by the Center for Remote Sensing of Ice Sheets (CReSIS). The results indicate a good overall accuracy. The accuracy of refreezing ice is high, while that of the other classes is comparable with the state-of-the-art techniques. The results indicate the effectiveness of the proposed features in mapping the refreezing ice.
Calcrete-hosted surficial uranium systems in Western Australia: Prospectivity modeling and quantitative estimates of resources. Part 1 – Origin of calcrete uranium deposits in surficial environments: A review
(Elsevier, 2018-11) Thakur, Sanchari
This three-part paper reports the results of geochemical modeling, prospectivity modeling and quantitative resources assessments of calcrete-hosted surficial uranium deposits in the palaeochannels of geologically, physiographically, and climatologically permissive part of Western Australia. In this Part 1, geochemical dynamics of uranium mobilization and precipitation in near-surface oxidized groundwater systems are reviewed in order to understand the processes responsible for precipitation of uranyl vanadate minerals in valley/lacustrine calcrete within palaeochannels in arid and semi-arid desertic regions. The review indicates that uranium precipitation is essentially a function of concurrent changes in three mutually interdependent parameters of groundwaters, namely, (i.e. activities of all the carbonate bearing species), Eh-pH, and the activities of other ionic species. Geochemical modeling of groundwater data from northern part of the Yilgarn craton in Western Australia indicates that opening of the groundwater aquifer system to the atmosphere and consequent evaporation is likely the key process leading to precipitation of uranyl vanadate minerals in valley calcrete and playa lake sediments. Fluid mixing could also induce the precipitation below the water table, provided that the mixing groundwaters have contrasting geochemistry. These processes may operate in diverse geomorphic traps in palaeochannels. A generalized conceptual model of calcrete-hosted uranium systems is presented and regional-scale targeting criteria and their spatial proxies are identified, which, in turn, are used in Part 2 to develop a targeting model for calcrete-hosted uranium deposits in Western Australia.
Clutter reduction by estimation of echoes direction of arrival in distributed radar sounders in formation flying
(IEEE, 2022-08) Thakur, Sanchari
Spaceborne radar sounders are high frequency (HF)/very high frequency (VHF) nadir-looking sensors devoted to subsurface investigations. Their data interpretation can be severely hindered by off-nadir surface clutter. Recent literature showed that the clutter suppression capabilities of this class of systems can be greatly enhanced by deploying an array of orbiting sensors in formation flight synthesizing a narrow radar antenna beam. In this article, we assess the capability of distributed radar sounding to discriminate clutter from subsurface returns by exploiting direction of arrival (DOA) estimation techniques. This is achieved by first outlining an approach for designing and evaluating the distributed radar sounder DOA estimation performance as function of the radar system parameters (e.g., intersensor distance) and external noise factors such as ionospheric scintillations. Then, the theory is complemented by radar simulations of several acquisitions over Greenland assuming a variety of subsurface geometries. The simulations confirm that clutter discrimination through DOA estimation is a viable approach to further improve the array capability in disambiguation of subsurface echoes from surface ones.
Detecting near-surface melt-water and basal ice-water interfaces by VHF radar sounder data
(IEEE, 2024-07) Thakur, Sanchari
Very high-frequency (VHF) radar sounding from an orbital platform is a promising mission concept for subsurface observations of the Earth’s polar caps for mapping the ice sheets and ice shelves from the surface to the base. Previous feasibility studies have analyzed the detectability of the basal interface, the internal layers and the subglacial lakes in simulated VHF radargrams. However, there are few studies on the detection of near-surface melt-water and the ice-ocean interface, which are important for predicting the stability of the polar ice. This paper both presents a novel semi-supervised basal ice-water detection algorithm and exploits a state-of-the-art dielectric inversion technique to detect surface melt for the analysis of simulated VHF radargrams. Results show that about 90% of the ice-ocean interfaces are correctly detected with the proposed detection algorithm and that the dielectric inversion revealed a pool of melt-water in the Antarctic peninsula.
A dictionary-based integrated simulation approach to model large- and small-scale coherent surface scattering phenomena in radar sounder data
(IEEE, 2023-09) Thakur, Sanchari
With the increasing number of radar sounder (RS) instruments being used in planetary exploration, there is an increasing need for advanced and efficient RS data simulators. In this context, it is important to combine the advantages of the different simulators to produce end-to-end simulations at multiple scales in a reasonable time. This article addresses this problem by presenting a novel dictionary-based integrated simulation approach to model both large- and small-scale surface scattering phenomena at relatively low computational costs. The method combines the advantages of a ray-tracing approach for simulating large surface areas at low resolution and a numerical technique for simulating the small-scale wave-target interaction at higher resolution. The proposed approach generates an instrument dictionary (i.e., a finite set of high-resolution rough responses at the small-scale) that can be used for surface target simulations. The method is validated by comparing its response with that obtained by a numerical simulator (known to have high accuracy). Then, it is demonstrated for the forward modeling of small-scale roughness on a synthetic target and for the inversion of small-scale roughness from existing RS data. The results demonstrate the capability of the method to achieve high accuracy and computational efficiency in addressing the problem of small-scale surface scattering on a large-scale scenario.
Envision mission to Venus: subsurface radar sounding
(IEEE, 2020) Thakur, Sanchari
This paper presents the Subsurface Radar Sounder (SRS) instrument onboard European Space Agency's (ESA) EnVision mission. EnVision is one of the three candidates selected for the Cosmic Vision 2015-2025 M5 medium-class missions. It is aimed at exploring the activity, the geologic history and the atmosphere of Venus. SRS is an orbital ground-penetrating radar with the unique science objectives of understanding the evolution of Venus' surface by searching for subsurface dielectric interfaces in the top hundreds of metres of the crust. In the paper, we describe the main science objectives of SRS, the performance evaluation under expected target conditions, the instrument design and the acquisition strategy that maximize the scientific returns.
Exploring Venus subsurface: analysis of geological targets and their properties
(Elsevier, 2023-01) Thakur, Sanchari
The exploration of Venus is increasingly gaining importance in the planetary science community. Recently, EnVision has been selected as the European Space Agency's fifth Medium-class mission with a launch targeted in 2030. The subsurface radar sounder (SRS) instrument on board EnVision aims at profiling the shallow crust of Venus. The current phase of development of SRS focuses on a detailed performance analysis based on 3D simulations of the expected subsurface scenarios. This requires inputs on the composition, roughness, location and geometry of the geological features. However, the high uncertainty in the current knowledge of Venus due to the lack of high-quality data implies that several hypotheses exist for these inputs. The goal of this paper is to identify these hypotheses from the perspective of the scientific objectives of subsurface exploration of Venus' shallow crust. In this context, we identify geological features (a.k.a. targets) that are likely to be associated with subsurface dielectric interfaces. For each target, we review the literature for information on the simulation inputs, analogs of the targets (with possible detection by past radar sounders on other planetary bodies), expected subsurface models representing different hypotheses and the potential role of SRS data in resolving the ambiguities. The material presented here will be critical to support future activities on detailed performance assessment and radar sounder simulations. We also present a map of the potential targets for subsurface sounding, which can support operations planning of SRS.
Fuzzy inference systems for mineral exploration
(Springer, 2023-01) Thakur, Sanchari
A Fuzzy Inference System (FIS) is a system containing a set of if-then rules expressed in natural language to simulate inductive reasoning of an expert (Porwal et al. 2015). It is a knowledge-driven inference engine built upon the theory of fuzzy sets. A fuzzy set is the extension of a classical set and does not have clearly defined limits. The degree of membership to fuzzy sets grades from 0 to 1 (unlike classical sets, where it is either 0 or 1) (Zadeh 1973). A fuzzy set, hence, allows for a simplified representation of real-world phenomena including geological processes (see “Fuzzy Set Theory in Geosciences”). When more than one fuzzy set are identified in a dataset and combined using logical operators, it forms a fuzzy logic overlay. Numerous fuzzy logic overlays expressed as if-thenrules and integrated in an inference engine encompass a FIS. A FIS has the capabilities to capture the imprecision and vagueness of natural phenomena within a single system
Geophysical characterization of the interiors of Ganymede, Callisto, and Europa by ESA’s Jupiter Icy Moons Explorer (JUICE)
(Springer, 2024-07) Thakur, Sanchari
The JUpiter ICy moons Explorer (JUICE) of ESA was launched on 14 April 2023 and will arrive at Jupiter and its moons in July 2031. In this review article, we describe how JUICE will investigate the interior of the three icy Galilean moons, Ganymede, Callisto and Europa, during its Jupiter orbital tour and the final orbital phase around Ganymede. Detailed geophysical observations about the interior of the moons can only be performed from close distances to the moons, and best estimates of signatures of the interior, such as an induced magnetic field, tides and rotation variations, and radar reflections, will be obtained during flybys of the moons with altitudes of about 1000 km or less and during the Ganymede orbital phase at an average altitude of 490 km. The 9-month long orbital phase around Ganymede, the first of its kind around another moon than our Moon, will allow an unprecedented and detailed insight into the moon’s interior, from the central regions where a magnetic field is generated to the internal ocean and outer ice shell. Multiple flybys of Callisto will clarify the differences in evolution compared to Ganymede and will provide key constraints on the origin and evolution of the Jupiter system. JUICE will visit Europa only during two close flybys and the geophysical investigations will focus on selected areas of the ice shell. A prime goal of JUICE is the characterisation of the ice shell and ocean of the Galilean moons, and we here specifically emphasise the synergistic aspects of the different geophysical investigations, showing how different instruments will work together to probe the hydrosphere. We also describe how synergies between JUICE instruments will contribute to the assessment of the deep interior of the moons, their internal differentiation, dynamics and evolution. In situ measurements and remote sensing observations will support the geophysical instruments to achieve these goals, but will also, together with subsurface radar sounding, provide information about tectonics, potential plumes, and the composition of the surface, which will help understanding the composition of the interior, the structure of the ice shell, and exchange processes between ocean, ice and surface. Accurate tracking of the JUICE spacecraft all along the mission will strongly improve our knowledge of the changing orbital motions of the moons and will provide additional insight into the dissipative processes in the Jupiter system. Finally, we present an overview of how the geophysical investigations will be performed and describe the operational synergies and challenges.
Global grade-and-tonnage modeling of uranium deposits
(International Atomic Energy Agency, 2018) Thakur, Sanchari
This contribution presents models of grade and tonnage distribution of various uranium deposit types and sub-types. A p-value derived from the Kolmogorov-Smirnov test is used for evaluating the goodness-of-fit of the frequency-grade and frequency-tonnage distributions to the log-normal distribution. The results indicate that both frequency-grade and frequency tonnage distributions of most deposit types and sub-types conform to the log-normal distribution at the 95% confidence level. Models created by dividing deposits into sub-types show significant improvement in the goodness-of-fit. The t-tests reveal that the grade and tonnage distributions of sub-types of the sandstone-hosted type (namely, roll-front, tabular, basal channel and tectonic-lithologic sub-types) are significantly different from each other. Furthermore, country-wise models of the roll-front deposits in different countries do not necessarily correlate, which explains the lower goodness-of-fit for the global frequency grade and frequency-tonnage models for sandstone-hosted deposits. This implies that sandstone-hosted deposits may differ in their geological settings from country to country, thus requiring some reclassification. The same applies to basement-hosted and unconformity contact deposits in Canada and Australia.