BITS Faculty Publications
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Item Assessing the detection performance on icy targets acquired by an orbiting radar sounder(IEEE, 2019) Thakur, SanchariRadar 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.Item Analysis of subsurface hypotheses through simulation of rime radargrams based on available analogous data(IEEE, 2019) Thakur, SanchariRadar 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.Item Envision mission to Venus: subsurface radar sounding(IEEE, 2020) Thakur, SanchariThis 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.Item An approach to the generation and analysis of databases of simulated radar sounder data for performance prediction and target interpretation(IEEE, 2021-02) Thakur, SanchariRadar 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.Item Stratus: a new mission concept for monitoring the subsurface of polar and arid regions(IEEE, 2021) Thakur, SanchariThis paper presents the SaTellite RAdar sounder for earTh sUbsurface Sensing (STRATUS), which is a satellite mission for Earth Observation (EO) with an onboard instrument capable of probing the Earth's subsurface in polar and arid regions. STRATUS is based on an innovative distributed radar sounder (RS) with the unique capability to obtain continuous and large-scale subsurface measurements, with homogeneous and consistent quality in two of the least characterized and crucial frontiers of Earth: globally on the polar ice sheets, i.e., Greenland and Antarctica (primary objective), and regionally on the arid areas and deserts. STRATUS is a ground-breaking exploratory mission addressing crucial scientific questions. It provides new fundamental data that have not been acquired by any other past or present remote sensing mission on the Earth, with an expected high and genuine scientific return enabling the assessment of the climate change signature in the Earth subsurface.Item An approach to the assessment of detectability of subsurface targets in polar ICE from satellite radar sounders(IEEE, 2021-10) Thakur, SanchariA 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.Item Analysis of surface clutter for subsurface radar sounding on Venus(IEEE, 2022) Thakur, SanchariESA'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.Item Clutter reduction by estimation of echoes direction of arrival in distributed radar sounders in formation flying(IEEE, 2022-08) Thakur, SanchariSpaceborne 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.Item Exploring Venus subsurface: analysis of geological targets and their properties(Elsevier, 2023-01) Thakur, SanchariThe 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.Item Subsurface signatures of bright terrain formation models on Ganymede by 3D radar sounder simulations(Elsevier, 2023-04) Thakur, SanchariThe Radar for Icy Moon Exploration (RIME) on-board the JUICE mission will look for dielectric and mechanical interfaces below the icy crust of the Galilean moon Ganymede. Previous missions suggest that the surface of Ganymede is covered by two types of terrains, namely the dark terrain and bright terrain. The bright terrain covers two-thirds of the surface, and presents two main morphologies, i.e. the grooved and smooth bright terrains. For understanding the geological history of Ganymede, a still-open question regards the sequence and styles of processes that formed these morphologies. While in the literature different formation hypotheses are highly debated, direct evidence of these can be proven by subsurface probing, for which RIME will offer a unique opportunity. In this work, we aim to understand whether RIME can actually detect and differentiate between the different hypotheses. To this purpose, we perform 3D simulations of RIME radargrams, following a progressive insertion approach which allows to simplify the data interpretability. We focus on the bright terrain morphologies and analyze in detail the RIME detectability of geological components that can support the formation hypotheses of the grooved and smooth bright terrains. The results suggest that RIME can possibly detect the subsurface interfaces associated with the geological components, and differentiate between their radar responses. We also provide a key to interpretation of the RIME data over these targets, and link the radar responses to the formation hypotheses debated in the literature.