Radar Sounding Research Articles

A Coherent Multilayer Simulator of Radargrams Acquired by Radar Sounder Instruments

Reliable electromagnetic simulators are of prime importance for the design of radar sounder instruments and for supporting the subsequent analysis of their data. In this paper, we present a coherent, facet method-based simulator that can compute radar echoes from the subsurface of a target area with an arbitrary number of geological layers, thus going beyond the surface-only or the two-layer descriptions so far implemented in coherent ray-tracing radar sounder simulators. Propagation of fields throughout the subsurface is computed according to Snell’s law following a ray-tracing approach. For each ray interacting with the surface, be it a direct reflection or a refracted ray coming from the subsurface, the phase contribution of each facet is calculated through the linear phase approximation, while the total field received at the antenna is computed using Huygen’s principle. Validation simulations have been performed against the radar data of lunar and martian areas characterized by a multilayer nature and collected by the Lunar Radar Sounder instrument of JAXA’s Kaguya lunar probe and the Shallow Radar instrument onboard NASA’s Mars Reconnaissance Orbiter, respectively. Results confirm the effectiveness of the proposed simulator.

Retrieval of Englacial Firn Aquifer Thickness From Ice‐Penetrating Radar Sounding in Southeastern Greenland

AbstractAn extensive perennial firn aquifer was previously mapped in Helheim Glacier, Southeast Greenland. However, critical constraints on aquifer thickness have been so far unobtainable without expensive field observations. Here we present a novel method that combines very high frequency airborne deep ice radar and ice sheet modeling to retrieve aquifer thickness and its evolution at Helheim Glacier. Using 2012–2014 radar measurements, we identify three aquifers of 4–25 m thick, with a total area of 1,934 km2 and water storage of 2.2 ± 1.5 Gt, about half the volume of previous estimates. The aquifer system is dynamic, and its thickness varies interannually at a rate similar to changes in surface mass balance. The rapid upstream migration of the saturated aquifer implies that this feature has the potential to increase its storage potential in upper Greenland. Together, a combination of very high‐ and ultra high frequency radar sounding provides a powerful tool to characterize englacial firn aquifers.

High Ice Purity of Martian Lobate Debris Aprons at the Regional Scale: Evidence From an Orbital Radar Sounding Survey in Deuteronilus and Protonilus Mensae

AbstractLobate Debris Aprons (LDA) are martian landforms with a strong morphologic resemblance to rock glaciers and debris‐covered glaciers. While the Shallow Radar (SHARAD) sounder has confirmed that a handful of LDA are composed of >80% water ice, viscous flow morphology can also be produced by as little as 30% ice. To distinguish between these endmembers, we conducted a comprehensive regional SHARAD survey of LDA, in Deuteronilus Mensae. We found that the majority of LDA are penetrated by SHARAD and determined that they are composed of a material with and tanδ≈0.002 < 0.005. These LDA are thus consistently composed of >80% water ice, which constrains the regional sequestered ice budget to a minimum of 0.9 − 1.0 × 105 km3 or a 62–69‐cm Global Equivalent Layer.

Compensating Earth Ionosphere Phase Distortion in Spaceborne VHF Radar Sounders for Subsurface Investigations

Spaceborne low-frequency and wide bandwidth radar sounders are a promising technology to regularly investigate at global-scale Earth’s icy and arid regions. However, Earth ionosphere distorts the radar signal impacting performance parameters, such as subsurface resolution, of the radar system. One of the most relevant distortions that a sounder signal in the lower part of the very high-frequency (VHF) band (e.g., 40–50 MHz) encounters is the distortion of the phase component that could become mission critical if not properly compensated. Low-frequency and high fractional bandwidth radar systems are particularly affected by this issue. Previous works on radar sounder ionosphere phase distortion compensation addressed the Martian ionosphere and used techniques based on the Taylor series expansion. In this letter, we focus on the Earth ionosphere and we exploit a recently proposed ionosphere compensation technique based on the Legendre orthogonal polynomials expansion, which proved to be more accurate than the compensation based on Taylor expansion. Simulations show that the method allows a nominal compensation of the phase distortions under realistic ionosphere scenarios expected during the acquisitions. Furthermore, it proved to be accurate and robust for total electron content conditions expected during nighttime for all the geomagnetic latitudes. The results confirm that the method can accurately compensate the distorting effects on the phase component of a spaceborne VHF radar sounder.

Geometric Power Fall-Off in Radar Sounding

This paper reports the analysis of the geometric power fall-off of Fresnel zone scattering in radar sounding. Radar sounders can take advantage of strong coherent scattering from Fresnel zones at nadir which grow with sensor altitude. The strength of the signal and the actual rate of power fall-off, however, depend heavily on the surface properties. We first use the radar equation to separate geometric versus scattering mechanisms driving $R^{2}$ , $R^{3}$ , or $R^{4}$ fall-off. The Fresnel zone for planetary surfaces, where body curvature has an effect, is derived and its implications discussed. We show the impact of Gaussian and fractal surface roughness on the exponent of power fall-off and the transition from coherent to incoherent scattering. This is done in simulation and analytically to derive coherence loss functions. Finally, we study the effect of incoherent area fraction within the Fresnel zone. These results are intended to be used in radar link budgets, performance metrics, and scientific interpretation of sounding data.

Method of estimating linear dimensions of object by subsurface radar sounding

The methods for recognizing subsurface objects by the length of radar range portraits in a georadar with wideband probing signals, optimized for the maximum likelihood criterion for complex hypotheses, for the case of a known internal noise power, have been developed. It is shown that, using the optimal method, an energy gain is achieved when the threshold value of the probability of correct recognition is reached.

Probing supraglacial debris on Mars 1: Sources, thickness, and stratigraphy

Geomorphic and geophysical evidence supports a debris-covered glacier origin for a suite of landforms at the mid-latitudes of Mars, including lobate debris aprons (LDA), lineated valley fill (LVF), and concentric crater fill (CCF). These large reservoirs of ice and their near-surface structure provide a rich record for understanding the planet's climate and history of global volatile exchange over the past billion years. LDA, LVF, and CCF are also potential sites for future robotic and human missions but the accessibility of glacial ice for direct sampling and in situ resource utilization depends largely on the geotechnical properties of the surface debris (“supraglacial debris”), including its thickness, grain sizes, and density structure. The physical properties of this supraglacial debris layer have been poorly constrained. We use images of morphology, digital elevation models, thermal inertia data, and radar sounding data to probe the near surface of LDA, LVF, and CCF in Deuteronilus Mensae in order to place constraints on the sources, grain sizes, thickness, and stratigraphy of supraglacial debris. We find evidence for at least a two-layer stratigraphy. Layered mantle consisting of atmospherically emplaced dust and ice superposes boulder-rich sediment sourced by rockfalls glacially transported downslope. High thermal inertia, boulder-rich termini and debris bands reminiscent of medial moraines are found throughout the study region, supporting a rockfall origin for at least a fraction of the debris exposed at the surface. This supraglacial debris layer would have thickened with time from sublimation of glacial ice and liberation of englacial sediment and dust. At present, the entire supraglacial debris package is a minimum of a few meters in thickness and is likely tens of meters in thickness in many locations, possibly thinning regionally at lower latitudes and locally thinning toward the headwalls. The lack of terracing or interior structures in craters formed within LDA, LVF, and CCF and the absence of near-surface reflectors in SHARAD radar data further suggest that no strong contrasts in permittivity or strength occur at the interface of the layers.

ENHANCEMENT OF TRANSPORTATION SAFETY BY REMOTE SENSING DEVICES

ABSTRACT Improving safety of rail transportation in the face of increasing speeds and traffic intensity and the presence of a terrorist threat requires introduction of new technical means of surveillance and control. A particular task is development of measures to prevent accidents and catastrophes, as well as to eliminate their consequences. The use of Earth remote sensing means, which allow to receive information from large areas and with a sufficiently high resolution, can significantly help in implementation of these tasks. The article deals with variants of satellites and remotely piloted aircrafts systems receiving information in various ranges of the electromagnetic spectrum. Their advantages and disadvantages, opportunities of joint use are analyzed. Keywords: rail transportation safety, remote sensing of the Earth, satellites, remotely-piloted aircraft, aerial photography, radar sounding, image processing. The study was carried out with the financial support of the Russian Foundation for Basic Research in the framework of the scientific project No. 17-20-02086. The authors are grateful to the staff of the Experimental Railway Ring in Shcherbinka for their assistance in conducting RPA flights to obtain the necessary information.

The formation and stability of buried polar CO2 deposits on Mars

Shallow Radar soundings of the south polar layered deposits (SPLD) have revealed buried CO2 ice with as many as three distinct layers separated by thinner layers of water ice (Bierson et al., 2016). This layering is suggestive of formation by cyclical processes such as obliquity, eccentricity and advance of perihelion. If obliquity is the main driver, then there were many opportunities for CO2 deposits to have formed within the last 3.5-4 Myr. To persist, however, CO2 deposits must be followed by the deposition of an insulating layer of porous water ice that can seal in the CO2 to survive the obliquity maxima that follow. We suggest that the existing deposits were formed within the last 350 kyr. Each obliquity minimum was quickly followed by a period in which perihelion occurred in the northern summer (southern winter) solstice. Under these conditions, an enhanced deposition of low porosity, fine-grained water ice could occur on the winter pole. A similar series of depositions and burials by water ice could have occurred between 2.75 and 2.2 million years ago, but these deposits were unlikely to have survived the many high obliquity swings that occurred between 2.2 Ma and 400 ka. The formation and burial of CO2 ice could also have occurred in similar fashion earlier during the last ∼ 100 Myr in which the southern polar layered deposits were formed. Although at current times the geothermal flux may be too low to cause basal melting of CO2 deposits, in earlier times, deposits formed in deeper scarps and valleys could have led to basal CO2 melting and sequestration into the regolith. By extension, basal melting and sequestration in the regolith may provide an explanation for the fate of the thick CO2 atmosphere (pCO2 ≳ 0.5 bar) implied by climate models of early Mars.

HF (5 MHz) Imaging of the Moon by Kaguya Lunar Radar Sounder Off Nadir Echo Data

HF (5MHz) imaging of lunar surface was attempted using off nadir echo data of Kaguya Lunar Radar Sounder (LRS). LRS observation data of multiple orbits were processed and mapped onto a surface which was defined based on the Kaguya Digital Elevation Model. The transmitting/receiving antenna of LRS is a dipole antenna which illuminates lunar surface on both sides of orbit, consequently, in a single-orbit observation, a detected target location has ambiguity in terms of the side of the orbit. However, use of multiple orbit data enables to resolve this ambiguity problem, i.e., radar illumination is controlled. We demonstrated this HF imaging technique by reconstructing surface images of Rupes Recta region using LRS observation data of 61 orbits. Control of radar illumination was confirmed by the presence/absence of Rupes Recta image in the reconstructed surface images. As it was anticipated, the reconstructed surface images presented false images which were identified as mirror images of major surface features. We also carried out simulation of these LRS observations of 61 orbits over the Rupes Recta site using Kirchhoff-approximation Surface Scattering (KiSS) simulation code. Comparison of the images of LRS observation and those of KiSS simulation exposed some discrepancies. Our interpretation is that the discrepancies are attributed to shallow subsurface scatterings which the KiSS simulation does not take into account. This implies the possibility of imaging shallow lava tubes by LRS, although we did not find one in this particular site of Rupes Recta region.

Radar Sounding of Open Basin Lakes on Mars

AbstractOrbital observations of the surface of Mars indicate that ancient basins were once host to lakes that may have been depocenters of sedimentary materials, including hydrated minerals like phyllosilicates. Later volcanic and sedimentary resurfacing may have developed a stratigraphy within the basins detectable through radar sounding data from the Shallow Radar instrument. Our radar survey of 61 open basin lakes (OBLs) revealed only one plausible reflector in a single basin east of Hellas Planitia. We investigated possible factors contributing to lack of radar detections in the other basins through detailed analysis of radargrams and subsurface characteristics of OBLs. As possible hosts to previous habitable environments, OBLs on Mars are important sites for future robotic and human missions. A full understanding of the factors influencing the radar signal, as addressed here, is important for more fully characterizing the subsurface structure and geology at these locations.

HF/VHF Radar Sounding of Ice from Manned and Unmanned Airborne Platforms

Ice thickness and bed topography of fast-flowing outlet glaciers are large sources of uncertainty for the current ice sheet models used to predict future contributions to sea-level rise. Due to a lack of coverage and difficulty in sounding and imaging with ice-penetrating radars, these regions remain poorly constrained in models. Increases in off-nadir scattering due to the highly crevassed surfaces, volumetric scattering (due to debris and/or pockets of liquid water), and signal attenuation (due to warmer ice near the bottom) are all impediments in detecting bed-echoes. A set of high-frequency (HF)/very high-frequency (VHF) radars operating at 14 MHz and 30–35 MHz were developed at the University of Kansas to sound temperate ice and outlet glaciers. We have deployed these radars on a small unmanned aircraft system (UAS) and a DHC-6 Twin Otter. For both installations, the system utilized a dipole antenna oriented in the cross-track direction, providing some performance advantages over other temperate ice sounders operating at lower frequencies. In this paper, we describe the platform-sensor systems, field operations, data-processing techniques, and preliminary results. We also compare our results with data from other ice-sounding radars that operate at frequencies both above (Center for Remote Sensing of Ice Sheets (CReSIS) Multichannel Coherent Depth Sounder (MCoRDS)) and below (Jet Propulsion Laboratory (JPL) Warm Ice Sounding Explorer (WISE)) our HF/VHF system. During field campaigns, both unmanned and manned platforms flew closely spaced parallel and repeat flight lines. We examine these data sets to determine image coherency between flight lines and discuss the feasibility of forming 2D synthetic apertures by using such a mission approach.

Using the Subsurface Soil Sounding Radar for Investigating the Structure and Total Electron Content of the Martian Ionosphere

The possibilities of using the Martian soil subsurface sounding radar for investigating the structure of the plasma shell surrounding the planet have been considered. Based on the numerical modeling results and actual soil sounding data, it has been shown that the soil sounding mode of the radio-locating MARSIS radar can be used to assess the structure of the Martian ionosphere. As the emitted signals pass to the planet’s surface, it is possible to use the reflected signals to estimate the total electron content of the Martian ionosphere along the flight track of the spacecraft.

A multi-frequency radar sounder for lava tubes detection on the Moon: Design, performance assessment and simulations

Lunar lava tubes have attracted special interest as they would be suitable shelters for future human outposts on the Moon. Recent experimental results from optical images and gravitational anomalies have brought strong evidence of their existence, but such investigative means have very limited potential for global mapping of lava tubes. In this paper, we investigate the design requirement and feasibility of a radar sounder system specifically conceived for detecting subsurface Moon lava tubes from orbit. This is done by conducting a complete performance assessment and by simulating the electromagnetic signatures of lava tubes using a coherent 3D simulator.The results show that radar sounding of lava tubes is feasible with good performance margins in terms of signal-to-noise and signal-to-clutter ratio, and that a dual-frequency radar sounder would be able to detect the majority of lunar lava tubes based on their potential dimension with some limitations for very small lava tubes having width smaller than 250 m. The electromagnetic simulations show that lava tubes display an unique signature characterized by a signal phase inversion on the roof echo. The analysis is provided for different acquisition geometries with respect to the position of the sounded lava tube. This analysis confirms that orbiting multi-frequency radar sounder can detect and map in a reliable and unambiguous way the majority of Moon lava tubes.

A Method for Automatic Three-Dimensional Reconstruction of Ice Sheets by Using Radar Sounder and Altimeter Data

Understanding the processes occurring at the ice sheets requires reliable three-dimensional (3-D) models of the ice sheet geometry. To address this challenge, we propose a technique for the 3-D reconstruction of the ice sheet geometry that uses radar sounder (RS) and altimeter (ALT) data to automatically identify the scale (or grid size) for interpolation. Existing studies derive the interpolation scale empirically, by qualitatively analyzing the RS data sampling and often neglecting the surface topography effects. Our method initially performs the interpolation of RS data at several potential scales. At each scale, it uses the ordinary kriging interpolation method that enables the quantitative analysis of both the RS data sampling and the surface topography. The optimal scale for the estimation of the surface map is identified according to an objective criterion that minimizes the difference to a subset of reference ALT data. Thereafter, the identified optimum scale on the surface is used to estimate the bedrock and ice thickness maps. Thus, the technique is a best-effort approach to the reconstruction of the ice sheet geometry, given the reference surface data and in the absence of reference bedrock data. Results obtained by applying the method to RS and ALT data acquired over the Byrd Glacier in Antarctica, in four regions characterized by different RS sampling and surface topography, confirm its effectiveness. Moreover, they point out that the method could be used for guiding future RS surveys, since the identified optimal scales are typically larger than those needed for addressing specific science objectives.

Ionospheric Electron Densities at Mars: Comparison of Mars Express Ionospheric Sounding and MAVEN Local Measurements

AbstractWe present the first direct comparison of Martian ionospheric electron densities measured by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) topside radar sounder on board the Mars Express spacecraft and by the Langmuir Probe and Waves (LPW) instrument on board the Mars Atmosphere and Volatile Evolution Mission (MAVEN) spacecraft. As low electron densities are not measured by MARSIS due to the low power radiated at low sounding frequencies, MARSIS electron density profiles between the local electron density and the first data point from the ionospheric sounding (≈104 cm−3) rely on an empirical electron density profile shape. We use the LPW electron density measurements to improve this empirical description and thereby the MARSIS‐derived electron density profiles. We further analyze four coincident events, where the two instruments were measuring within a 5° solar zenith angle interval within 1 h. The differences between the electron densities measured by the MARSIS and LPW instruments are found to be within a factor of 2 in 90% of measurements. Taking into account the measurement precision and different locations and times of the measurements, these differences are within the estimated uncertainties.

Solving for ambiguities in radar geophysical exploration of planetary bodies by mimicking bats echolocation

Sounders are spaceborne radars which are widely employed for geophysical exploration of celestial bodies around the solar system. They provide unique information regarding the subsurface structure and composition of planets and their moons. The acquired data are often affected by unwanted artifacts, which hinder the data interpretation conducted by geophysicists. Bats possess a remarkable ability in discriminating between a prey, such as a quick-moving insect, and unwanted clutter (e.g., foliage) by effectively employing their bio-sonar perfected in million years of evolution. Striking analogies occur between the characteristics of bats sonar and the one of a radar sounder. Here we propose an adaptation of the unique bat clutter discrimination capability to radar sounding by devising a novel clutter detection model. The proposed bio-inspired strategy proves its effectiveness on Mars experimental data and paves the way for a new generation of sounders which eases the data interpretation by planetary scientists.

Assessing the potential for measuring Europa's tidal Love number h2 using radar sounder and topographic imager data

The tidal Love number h2 is a key geophysical measurement for the characterization of Europa's interior, especially of its outer ice shell if a subsurface ocean is present. We performed numerical simulations to assess the potential for estimating h2 using altimetric measurements with a combination of radar sounding and stereo imaging data. The measurement principle exploits both delay and Doppler information in the radar surface return in combination with topography from a digital terrain model (DTM). The resulting radar range measurements at cross-over locations can be used in combination with radio science Doppler data for an improved trajectory solution and for estimating the h2 Love number. Our simulation results suggest that the absolute accuracy of h2 from the joint analysis of REASON (Radar for Europa Assessment and Sounding: Ocean to Near-surface) surface return and EIS (Europa Imaging System) DTM data will be in the range of 0.04–0.17 assuming full radio link coverage. The error is controlled by the SNR budget and DTM quality, both dependent on the surface properties of Europa. We estimate that this would unambiguously confirm (or reject) the global ocean hypothesis and, in combination with a nominal radio-science based measurement of the tidal Love number k2, constrain the thickness of Europa's outer ice shell to up to ±15 km.

An Interferometric Approach to Cross-Track Clutter Detection in Two-Channel VHF Radar Sounders

Surface cross-track clutter can corrupt both earth and planetary radar sounder (RS) observations preventing definitive interpretation of subsurface features, which are often of primary interest to geologists and planetary scientists. This clutter is usually identified either by manual or automatic techniques that require ancillary information about the topography of the surface, or by using multichannel RS systems with arrays of antennas. However, topographic information is not always available and multichannel systems are generally too massive and costly to mount on satellites for the planetary exploration. In this paper, we propose a novel approach to clutter discrimination that is independent of ancillary information and limits the hardware complexity of the RS system. This approach uses a two-channel RS and exploits cross-channel interferometric phase differences to discriminate the clutter. Our approach includes three main steps: 1) manual feature extraction and theoretical phase-difference estimation; 2) RS interferogram formation; and 3) comparison of theoretical and real phase difference distributions. The proposed method was validated on RS data acquired in Greenland and provides a proof of concept for the surface clutter discrimination using RS data.

Detection of Intact Lava Tubes at Marius Hills on the Moon by SELENE (Kaguya) Lunar Radar Sounder

AbstractIntact lunar lava tubes offer a pristine environment to conduct scientific examination of the Moon's composition and potentially serve as secure shelters for humans and instruments. We investigated the SELENE Lunar Radar Sounder (LRS) data at locations close to the Marius Hills Hole (MHH), a skylight potentially leading to an intact lava tube, and found a distinctive echo pattern exhibiting a precipitous decrease in echo power, subsequently followed by a large second echo peak that may be evidence for the existence of a lava tube. The search area was further expanded to 13.00–15.00°N, 301.85–304.01°E around the MHH, and similar LRS echo patterns were observed at several locations. Most of the locations are in regions of underground mass deficit suggested by GRAIL gravity data analysis. Some of the observed echo patterns are along rille A, where the MHH was discovered, or on the southwest underground extension of the rille.