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.

Shallow aquifers are the primary water source to mitigate rising hydroclimatic fluctuations in arid areas, notably in North Africa and the Arabian Peninsula. The occurrence and dynamics of these expansive water bodies remain poorly characterized due to the reliance on sporadic monitoring wells. To address this deficiency, several studies are exploring the potential of low Earth orbit sounding radars as a large-scale mapping tool that can provide unique insights into the delineation and dynamics of these aquifers. Herein, we analyze the detectability of shallow aquifers (<10 m deep) using the radiometric analysis of surface reflections from a 45-MHz orbital sounder with an 8-MHz bandwidth. We use the ray tracing method to simulate the radar return from two realistic geoelectrical and topographic models of shallow aquifers in North African Sahara desert for omnidirectional and distributed array configurations. Our results suggest that the dielectric change induced by shallow aquifers that are up to 10-m deep can increase the 45-MHz radar surface return of the desiccated desert surface by 5 dB in areas with very low surface roughness of rms height <0.35 m. These preliminary results suggest a constrained potential for a monostatic VHF reflectometry to probe large sedimentary basins, which a distributed architecture can improve.

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 paper 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 as well as computational efficiency in addressing the problem of small-scale surface scattering on a large-scale scenario.

Spaceborne radar sounders are nadir-looking sensors devoted to subsurface investigations. The data interpretation of these sensors can be severely hindered by clutter originating from surface off-nadir reflections. Recently, the concept of distributed radar sounding has been proposed for synthesizing a narrow radar antenna beam with clutter suppression capability. The antenna beam is effectively synthesized by deploying an array of orbiting sensors in formation flight. In this paper, we assess the capability of distributed radar sounding to further discriminate clutter from subsurface returns by exploiting Direction of Arrival (DOA) estimation techniques. Accordingly, we propose an approach to design and evaluate the distributed radar sounder DOA estimation performance. The theory is complemented by radar simulations of several acquisitions over Greenland. The simulations confirm that clutter discrimination through DOA estimation is an effective approach for further improving the array capability in disam-biguating subsurface echoes from surface ones.

Radar sounders (RS) have the unique capability of providing direct measurements of subsurface structures. Due to the complex nature of subsurface acquisition scenarios, the RS design and data interpretation require the support of SAR focused electromagnetic simulations of the subsurface targets. While SAR focusing techniques applied to real RS data are well-established, the same techniques cannot be directly applied to the simulated radargrams, due to the inherent differences between the real data and the data simulation processing chains. In this paper, we approach this challenge by adopting a novel approach to the focusing of simulated raw RS data. In the proposed approach the main focusing parameters are extracted from the simulator impulse response to properly consider any artefact and the inherent assumptions exploited in the simulations. The method has been applied to previously published raw simulated radargrams of two RS instruments. The results show that the method provides good results for different target scenarios and RS instruments.

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.

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: (i) simulation of the coherent radar response of a representative polar ice target (englacial layering) in the distributed radar sounding case; (ii) inclusion of spatially-dependent ionospheric scintillation effects on the distributed beam pattern; (iii) inclusion of phase dispersion effects for different values of total electron content (TEC), and (iv) 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 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° produces a negligible effect on the detectability.

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 paper, 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, inter-sensor 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.

Radar sounders (RS) are gaining importance in planetary missions thanks to their unique capability of providing direct measurements of subsurface structures. To support their design and data interpretation, several electromagnetic (e.m.) simulation techniques have been developed with enhanced capabilities of emulating the RS acquisition process. However, the raw simulated radargrams obtained from e.m. simulators are difficult to interpret and analyze without a focusing operation, which results in an underestimation of the RS detection performance. While frequency methods for range and azimuth compression of real RS data are well-established, their use on simulated data is not addressed in the literature and requires major modifications. This paper presents a novel method that implements azimuth compression using unfocused and focused processing on simulated raw data. The proposed method is based on an adaptation of the Range-Doppler algorithm to the case of raw data generated by a coherent RS simulator. The method is demonstrated in three case-studies to show the similarity between simulated and real data processing: 1) simple geometries; 2) a simulated SHARAD radargram compared with the real data product; and 3) a real application scenario for supporting the design of a new RS instrument. The results indicate higher fidelity of the focused simulated data with the real data product and the target structure, confirming the usefulness of the proposed approach in obtaining a realistic processing of simulated radargrams.

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.

This 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.

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.

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.

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.

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.

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.

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.