Ice-penetrating radars are a valuable tool for studying subsurface processes and conditions on Earth’s ice sheets. Vast amounts of ice-penetrating radar data have been collected from coherent chirped airborne ice-penetrating radar systems. Operating these airborne radar sounders is resource intensive, requiring large teams and significant financial resources. Here, we present a coherent chirped ground-based ice-penetrating radar system that can leverage the wealth of processing methods built for coherent chirped airborne radar data at a much lower cost. Our system is based on a commercially available software-defined radio (SDR) and open-source radar code platform, enabling widespread community adoption and use of the system. Furthermore, use of the SDR platform increases flexibility of the system and allows for easy reconfiguration of specific intrinsic radar and survey parameters. We present the architecture of our system alongside data it collected on the McMurdo Ice Shelf in Antarctica and at Summit Station in Greenland. We estimate the bed echo SNR it would achieve throughout Antarctica, which can be used to guide survey design. Finally, we discuss potential future system architectures which the system can be easily adapted to.

We explore and present a range of strategies for quantifying and mitigating the effects of receiver saturation in ice-penetrating radars. Ice-penetrating radars must be able to detect very weak reflected signals while maintaining dynamic ranges greater than 120 dB or more. Historically, ice-penetrating radars have done this by increasing pulse length, increasing transmit power, and interleaving short and long pulses to improve dynamic range. In most ice-penetrating radar architectures, steps are taken to prevent receiver saturation. Here we investigate the performance trade-offs associated with receiver saturation, and explore techniques to impact the negative effects of receiver saturation.

Satellite radar altimetry signals from glaciers and ice sheets represent a combination of scattering from ice-surface topography and the snow and firn volume. Changes in surface topography through time, even at sub-footprint scales, will change the interaction between the wavefront and surface, impacting the shape of the return waveform. Here, we describe a new facet-scattering model that uses arbitrary topographic inputs and can be used for testing hypotheses about ice-sheet surface change in the historical, three-decade-long pulse-limited radar altimetry record. We demonstrate the scattering model on both synthetic and real topographies, showing how off-nadir features and a deforming surface impact the returned waveform. We anticipate this simulator can be used to quantify sub-footprint surface change and the evolution of dynamic ice-sheet processes on rugged, high velocity ice streams and outlet glaciers in the decades before our modern, continuous laser altimetry and high-resolution stereophotogrammetric records capable of observing such changes began.

Orbital and autonomous radar sounding platforms promise to enable widespread mapping of subglacial topography and englacial layers with greater uniformity of data and observing conditions than the current patchwork of distinct radar systems and surveys. They also have the potential to collect time-series observations of evolving subsurface conditions including ice-shelf melting, ocean access across grounding zones, 3D ice flow, and dynamic ice-sheet hydrology. Here, we investigate the impact of platform altitude and velocity on the detectability of subsurface interfaces beneath ice, sand, and permafrost in the presence of both noise and range sidelobes from surface echoes. Specifically, we evaluate the potential performance of radar sounding from orbital satellites, stratospheric UAVs, and low-altitude UAVs, relative to one another and to existing piloted platforms.

Ice-penetrating radar (IPR) instruments are a widely used tool to understand the structure and dynamics of Earth’s ice sheets and glaciers. Originally primarily designed to image the bedrock beneath ice, IPR systems are now being used for a wider range of scientific investigations. At the same time, new hardware architectures and customized radar systems are emerging. This combination of factors makes it worth re-visiting common assumptions about the noise characteristics of IPR systems and the phase coherence of measured reflections. In this work, we explore what it means for an IPR instrument to be coherent, how architectural choices in the design of software-defined radio-based instruments may impact phase coherence, and what instrument specifications should be considered for IPR applications that rely on measurements of small phase changes.

Airborne 0.5–2 GHz brightness temperatures were collected along a transect of the Priestley Glacier, Northern Victoria Land, Antarctica that coincides with previously acquired 189–199 MHz depth sounding radar data from NASA’s Operation IceBridge (OIB). The measured brightness temperature spectra evolve from negative spectral gradients (brightness temperature decreases with frequency) over the inland ice sheet and upper reach of the outlet glacier toward positive spectral gradients in the central region of the glacier. The spectra then increase almost linearly with frequency over the floating portion of the glacier terminus. The positive spectral gradients are more similar to sea ice on the ocean as compared to the grounded interior ice sheet. Comparison with modeling studies at Ross Ice Shelf sites where physical temperature has been measured in boreholes shows that positive gradients are to be expected where glacier ice overlies a water base. Calculations of radar reflectivity support the assumption of patchy regions of basal water and help to resolve ambiguities associated with processes such as interface roughness and near-surface firn layering.

Recent work has highlighted the simulated performance of passive synthetic aperture radar (SAR) using Jupiter’s radio emissions to probe the icy moons of Jupiter. Terrestrially, passive radar sounding using the Sun as a source for echo detection, ranging, imaging, and measuring ice thickness has also been recently demonstrated for the first time. With increasing advancements in passive radar sounders that use extended, incoherent radio-astronomical sources for echo detection, we revisit a potential limitation of the technique in terms of the sources’ spatial coherence properties. While previous work has considered the spatial coherence effects of extended sources for passive sounding in terms of pulse broadening, there has been little work to date that has examined the spatial coherence constraints for passive sounding imposed by source size, wavelength, incidence angle, and altitude—all of which govern the potential performance of passive SAR focusing. Starting from antenna theory, the Van Cittert-Zernike (VCZ) theorem, and the coherence function for passive sounding, we derive additional bounds set by these parameters and the expected source extent to estimate the maximum orbital altitudes when using radio-astronomical sources; in particular, we analyze the scenarios for a spacecraft using the Sun and Jovian bursts as sources for passive sounding of the Earth, Mars, and Europa. While the results of our analysis and simulations show that the coherence requirements (in terms of both pulse broadening and spatial radius of coherence) are met for terrestrial ground-based experiments up to large incidence angles, the limited spatial coherence at these greater altitudes creates an upper bound for orbital passive radar sounding. Our results therefore provide a richer understanding of the passive sounding technique, its viability, and a critical design constraint when planning future planetary and terrestrial passive sounding experiments.

Here we show the initial design and results from MAPPERR: The Multi-frequency Active Passive Polar Exploration Radar-Radiometer. MAPPERR addresses fundamental limitations in traditional ice-penetrating radar sounders and microwave radiometers stemming from non-unique contributions of different glaciological conditions. By utilizing active radar channels at 2, 22, 330, and 1000 MHz MAPPERR is designed to disentangle basal roughness and basal material conditions, while also providing important constraints on englacial temperature via measurements of attenuation. Passive radiometer channels at 330 and 1000 MHz provide additional constraints on englacial temperature via measurements of thermal emission and attenuation. Here, the system design is described and initial field results from campaigns in Svalbard, Iceland, and Antarctica are presented.

The ice sheet dynamics in Antarctica that directly impact the polar ice mass balance and the glacier erosion caused to the bedform are predicted by models that rely on several hard-to-estimate variables, including the bed topography itself. Antarctica’s bed topography is hard to estimate because it is covered by several layers of ice that could be up to several kilometers thick. Sparse, higher-resolution along-track measurements of its bed topography collected using Ice-Penetrating Radar (IPR) data are interpolated to create coarser-resolution gridded bed topography models. However, the significant gaps between IPR profiles mean there is significant scope for improving the measurements and interpolation approaches to fill those gaps. Here, we propose a deep learning (DL) generative adversarial network (GAN) approach to generate a realistic model of the bed topography from single-channel IPR acquisitions. The model takes advantage of the clutter caused by the IPR antenna to predict a Digital Elevation Model (DEM) accordingly to the information in the IPR acquisitions. The method is tested with synthetic data from regions with a high-resolution DEM available.

We seek to understand Jupiter’s radio emissions as a source for passive sounding by characterizing its signal properties for echo detection and ranging in terms of both temporal and frequency availability. As Jupiter is one of the loudest natural sources of radio emissions in our solar system outside of the Sun’s radio signals, this work explores the extent to which we can utilize these emissions for echo detection in terrestrial passive radar experiments. Our results show that while Jupiter is temporally available year-round, the source experiences elevation angle limitations at extreme latitudes. Our results also show the source’s minimum usable frequency can vary both diurnally and seasonally. We conclude with a discussion of three cases (ideal, conservative, and pessimistic) for the maximum terrestrial bandwidth based on the described constraints.

During the 1970s, the first large-scale ice-penetrating radar surveys were conducted over Antarctica and Greenland through a collaboration between the Scott Polar Research Institute at the University of Cambridge, the National Science Foundation, and the Technical University of Denmark [1]. This pioneering work represents the earliest available large-scale measurements of the sub-surface conditions of Earth’s two major ice sheets. In many cases, data collected as part of this survey pre-dates other available data in the area by decades. As we experience increasingly rapid changes in Antarctica and Greenland, understanding the historical states of these ice sheets takes on increased importance. Although 45-55 years is a short time in the evolution of continent-scale ice sheets, this dataset is unique in that it represents direct measurements over massive spatial scales. Unfortunately, integration of this data into models has proven extremely difficult due to unique challenges in how the data was collected and stored. We introduce our efforts to build digital tools for this analog dataset to bring this data into the modern era and make it accessible to researchers.

A significant portion of the sea level contributions of Antarctica and Greenland comes from ice streams, but the physical processes controlling ice stream width are poorly understood, especially when topographic controls are absent. Recent modeling studies have indicated that ice stream width may be controlled by elevated temperatures inside ice stream shear margins. While radio echo sounders can provide measurements of englacial water storage and subglacial conditions, existing radar-sounding techniques cannot measure temperature profiles at the scale required to test this hypothesis. We propose using a wide-angle radar survey and tomographic inversion to resolve temperature profiles, gradients, and anomalies at the scale required to study the thermophysical controls on shear margins. Recent work produced a bistatic radar system capable of obtaining the long offsets required for well-constrained inversions; however, shear-margin-specific temperature inversion techniques have not been developed for this system. In this article, we develop Newton’s method and alternating direction method of multipliers’ inversions for estimating temperature distribution and basal material across ice stream shear margins. We evaluate the performance of these inversion techniques on simulated bistatic radar-sounding data. Our results suggest that bistatic radar tomography experiments should be able to produce temperature maps on 50 m$\times50$m grids with$0.83~^{\circ} \text{C}~\pm ~0.084~^{\circ} \text{C}$mean temperature error,$3.58~^{\circ} \text{C}~\pm ~0.20~^{\circ} \text{C}$maximum temperature error, and an error in relative basal permittivity of$0.63~\pm ~0.08$for a 4-km transect.

Measurement of ice-sheet thermal state via microwave remote sensing techniques has the potential to provide critically needed observations on englacial temperature at the local and continental scale. Better constraints of the vertical englacial temperature structure are needed to improve understanding of thermomechanical processes, ice rheology, and basal sliding, and to reduce uncertainty in interpretations of basal conditions such as material, roughness, and thermal state. We investigate the potential to combine active and passive microwave remote sensing techniques, namely ice-penetrating radar sounding and microwave radiometry, to enable more precise, accurate, and robust measurement of ice-sheet thermal state across widely varying thermal regimes. We simulate the effects of englacial temperature profiles on the attenuation and brightness temperature and explore the performance tradeoffs for joint radar-radiometer system architectures. Our analysis shows that active and passive microwave measurements have complementary sensitivities to englacial temperature as a function of depth, and that a ground-based joint radar-radiometer system can reduce the requirements and complexity demanded of each single instrument.

Orbital radar sounding systems could significantly improve our understanding of dynamic processes at ice sheet margins by enhancing the spatial and temporal coverage of subglacial observations. However, surface crevasses in these fast-flowing regions may contribute significant radar clutter that can impede detection of the ice sheet bed. It is currently unclear if particular radar center frequency choices might enhance or mitigate this clutter. To investigate this question, we analyze crevasse clutter returns in airborne radar sounding data collected at 195 MHz, 300 MHz, and 750 MHz. Additionally, we rescale the derived empirical angular scattering functions to simulate crevasse clutter in orbital geometries. In this data set, we find that, unlike rough surface clutter, crevasse clutter is reduced at higher frequencies. However, the small angles of incidence at orbital altitudes lead to sufficient clutter at all frequencies to dominate over the theoretical bed echo power. Our results suggest that surface crevassing may not necessarily limit the viability of UHF frequencies for orbital sounding. However, synthetic aperture focusing and narrow cross-track antenna beam patterns will be crucial regardless of system center frequency.

Studies of sea level rise, ice sheet mass loss, and other glacial processes are hindered by the sparsity of observational measurements. Orbital radar sounding of the Martian ice caps has been successfully conducted, showing the promise for orbital sounding of terrestrial glaciers for improved coverage. Concepts for terrestrial orbital radar sounders, such as the Distributed Element Beamformer Radar for Ice and Subsurface Sounding (DEBRIS) mission concept, use an array of CubeSats to obtain narrow beam patterns and reduce cross-track clutter. To loosen wireless synchronization requirements, we investigate the application of orthogonal wave beamforming to DEBRIS. We compare orthogonal wave beamforming to traditional phased arrays and investigate Sinusoidal Frequency Modulated Continuous Waves (SFMCW) as an orthogonality scheme for beamforming.

Accurately measuring the vertical temperature profiles of ice sheets and glaciers is important for providing observational constraints on how temperature impacts ice rheology and flow. Directly measuring englacial temperature is challenging, often involving resource-intensive boreholes. Ice-penetrating radars and radiometers have been utilized independently to probe englacial temperatures, highlighting the potential of remote sensing to collect glacier- and catchment-scale temperature data. Here, we investigate the sensitivity to temperature at depth of active and passive microwave systems and highlight the benefits of combining their measurements to constrain and improve top-to-bottom temperature retrievals. These simulations allow us to understand how each instrument performs when applied to the unique electromagnetic and geophysical properties found on an ice sheet, enabling us to tailor our system design and implementation in a target-informed acquisition and analysis approach.

Perennial firn aquifers may play an important role in modulating the surface mass balance and ice dynamics of the Greenland Ice Sheet by storing surface meltwater in a porous firn layer. Ice-penetrating radar has been shown to be an effective tool for mapping the spatial extent and thickness of these subsurface water bodies. However, quantitative estimates of aquifer properties from radar observations require extensive a priori knowledge of the local firn structure and electrical properties of the aquifer water. Here, we develop a Markov Chain Monte Carlo joint inversion of radar reflectivity and attenuation to simultaneously estimate firn structure and aquifer properties and their associated uncertainty. We show that using this inversion method, the only additional field measurement required to robustly and accurately constrain the total water storage is the conductivity of the aquifer water. However, the inversion is largely insensitive to firn structure parameters, and aquifer properties like thickness and bulk porosity are poorly constrained on an individual basis due to the large number of solutions that map to the same total water storage. Regardless, our results suggest that, when combined with field measurements of water conductivity, ice-penetrating radar surveys can be an effective method for remotely characterizing kilometer-scale spatial and temporal variations in firn aquifer storage capacity.

The Autonomous Phase-Sensitive Radio Echo Sounder (ApRES) is a relatively inexpensive ice-penetrating Frequency-Modulated Continuous-Wave (FMCW) radar that is widely utilized in the glaciological community to obtain estimates of ice-sheet basal melt, vertical strain, and compaction rates [1]. However, these instruments are designed for stationary deployments, which prevents glacier- and catchment-scale surveys [2]. To expand the range of available applications, we assess the feasibility of mobile ApRES surveys. Our in-vestigation reveals that utilizing the ApRES in this manner introduces artifacts into the raw data. This paper character-izes the two types of artifacts (Doppler Blurring and grating lobes), investigates the conditions for when they occur, and attempts to correct them by modifying synthetic aperture radar (SAR) focusing algorithms for FMCW radars. We ul-timately identify the main obstacle in focusing radargrams from mobile ApRES surveys to be grating lobes; future work that reduces the presence of this artifact could enable more widespread use of mobile ApRES surveys.

Surface crevasses impact ice sheet mass loss by initiating hydrofracturing and calving at the margins and transporting supraglacial meltwater to the subglacial drainage sys-tem. This process subsequently modulates basal sliding and glacier motion. However, the development of robust models for calving and hydrofracture has been limited by a lack of field observations of crevasse formation and geometry. In this paper, we analyze a two-year Multi-Input Multi-Output Autonomous Phase-Sensitive Radio-Echo Sounder (MIMO ApRES) dataset collected at Store Glacier in West Greenland, which documents the formation of a crevasse that opened under the instrument. We present methods for processing the data as well as identifying and removing artifacts, including clipping, radio frequency interference (RFI), receiver failure events such as elevated thermal noise, and signal leakage between channels. Specifically, we perform a mean squared error (MSE) analysis, clipping detection and quantification, and calculations of total power over time in the frequency domain and the time domain. After characterizing and min-imizing these artifacts, we find that the bottom of a crevasse can be detected in the processed images. Our results suggest that, with appropriate data processing, the MIMO ApRES is a promising geophysical system for investigating future crevasse evolution.

We revisit a potential limitation for passive radar sounders that propose to use extended, incoherent radio-astronomical sources for echo detection and ranging. Recent work has considered the spatial coherence effects of extended sources for passive sounding in terms of pulse broadening; we expand on this work by analyzing the spatial coherence limitations imposed by source size, wavelength, incidence angle, and altitude. Moreover, we derive additional bounds set by these parameters and the expected source extent to estimate the maximum orbital altitudes for a spacecraft using Jovian bursts as a source for passive sounding of the Moon and Europa. While the results of our analysis and simulations show that the coherence requirement is met for terrestrial ground-based experiments up to large incidence angles, the limited spatial coherence creates an upper bound for an orbital passive radar sounder at Europa. Our results therefore provide a richer understanding of the passive sounding technique, its feasibility, and its potential limitations when designing future missions.

Vegetation water stress is a key control on wildfire risk, tree mortality, and ecosystem water and carbon fluxes. Although active microwave remote sensing methods have been used to estimate vegetation water, they remain poorly validated because of the immense mismatch between the scale of radar pixel resolutions (100 m to 25 km) and field measurements (individual trees). In this study, we present a new plot-scale vegetation water measurement technique using a side-facing bistatic radar. Using field experiments and a matched filtering technique to isolate the radar signal from noise, we show that radar amplitude is sensitive to xylem water potential (a measure of tree water status). However, our results are affected by periodic noise (period of~12 hours), which may be due to radio frequency interference. We discuss potential pathways to isolate the signal and the implications of the new tree water status measurement system for global validation of microwave remote sensing.

Ice-penetrating radar is the primary geophysical tool for large-scale measurements of the geometry and internal prop-erties of the Antarctic and Greenland Ice Sheets. These low-frequency radar instruments are typically mounted on crewed aircraft or towed behind snowmobiles, both of which introduce significant logistical challenges and costs. The availability of inexpensive, portable, and fully-autonomous uncrewed aerial vehicles (UAVs) promises to reduce the cost, logistical complexity, and risk of collecting ice-penetrating radar data. We introduce a chirped radar system built around a software-defined radio (SDR) that can be carried by a low-cost and easily-transportable fixed-wing UAV. The antennas for the radar are fully integrated with the wings of the UAV and have a usable frequency range from 300–450 MHz. We detail the most critical design challenges and the solutions we have chosen.

Radar tomography of glaciers promises to improve imaging and estimates of subsurface ice-sheet structures and properties, including temperature distributions, basal materials, ice fabric, and englacial water content. However, bistatic radar data with long (i.e., larger than the ice thickness) walk-away surveys are required to constrain high-fidelity tomographic inversions. These long-offset data have proven difficult to collect due to the hardware complexity of existing synchronization techniques. Therefore, we remove the hardware complexity required for real-time synchronization by synchronizing in postprocessing. Our technique transforms an Autonomous phase-sensitive Radio Echo Sounder (ApRES) system and a software-defined radio receiver into a coherent bistatic radar capable of recovering basal echoes at long offsets. We validated our system at Whillans Ice Stream, West Antarctica, with a walk-away survey up to 1300 m (797 m thick) and at Store Glacier, Greenland, up to 1450 m (1028 m thick). At both field sites, we measured the basal echo at angles beyond the point of total internal reflection (TIR), whose previous literature had set as a hard physical limit. We support our experimental results with high-frequency structure simulation, which shows that ground-based radar systems capture evanescent waves and are not hindered by TIR. Our analysis and experiments demonstrate a system capable of executing wide-angle bistatic radar surveys for improved geometric and radiometric resolution of inversions for englacial and subglacial properties.

Ice-penetrating radar sounders are powerful geophysical tools for studying the englacial and subglacial conditions of Earth’s ice sheets. Data from these instruments is critical to enhancing understanding and modeling of ice-sheet dynamics and ice-sheet contributions to global sea level. Traditionally, ice-penetrating radar sounders have a single center frequency creating inherent ambiguity in their pulse-by-pulse received power due to the confounding effects of basal material and roughness. Interpreting ice-penetrating radar data in light of these ambiguities requires assumptions about the physical state of the ice sheet, large-scale empirical analyses, or ancillary data. Here, we demonstrate that a high radiometric fidelity, narrowband, multi-frequency radar sounder can overcome these issues by separating the frequency-dependent basal roughness signature from the frequency-independent basal material signature. We present a radar system architecture with three narrow frequency bands spaced orders of magnitude apart (center frequencies of 1, 10, and 100 MHz, each with 20% fractional bandwidth) and high radiometric resolution (±0.5 dB). This architecture will allow us to precisely constrain ice-sheet basal conditions on a pulse-by-pulse basis by disambiguating the effects of basal material and basal roughness in ice-penetrating radar data.

Passively sounding icy and rocky bodies in our solar system provides a way to observe the surface and subsurface of these objects without the need for costly transmitters. Jupiter’s decametric radiation provides a suitable source of radio frequency signals for sounding on geological scales of interest, but its spectral structure can introduce undesired artifacts. Recent studies have shown that Jovian bursts could be effectively applied as a source of passive sounding. However, it has been noted that the spectral properties of Jovian bursts, as measured with JUNO data, are not perfectly modeled as white noise, which can result in degraded sensitivity and vertical resolution for passive sounding. In response to this finding, we present conditioning processes that improve the echo detectability and sounding resolution for Jovian burst-like signals. More than 18 h of Jovian burst recordings are used to simulate conditioning of the natural spectral variation, demonstrating that high-quality corrections are possible with these processes for noise conditions in both the Jovian and Earth/Moon systems.

Low-frequency radar sounders have the potential to generate altimetric profiles, but the feasibility of utilizing planetary radar sounding data as an alternative to laser altimetry has not been assessed using existing data to date. Therefore, we have developed, implemented, and evaluated an algorithm to process SHAllow RADar sounder (SHARAD) data on Mars (Experiment Data Records as available on the planetary data system) first into altimetry profiles and ultimately into digital terrain models (DTMs). The minimally processed data are pulse compressed, corrected for ionospheric distortion, zero-Doppler filtered, and incoherently summed. We then apply pulse re-tracking techniques adapted from terrestrial ocean altimetry to identify the surface return. From the surface return we compute the time-of-flight and hence the range from the spacecraft to the surface of the planet. The altimetry groundtracks are then co-registrated with Mars Orbiter Laser Altimeter (MOLA) to remove any biases resulting from residual ionospheric effects or timing issues. The altimetric profiles are finally used to create DTMs based on SHARAD data. While the SHARAD altimetry data have coarser inherent resolution than laser altimeters or imaging radars, we demonstrate that radar sounding data is still a viable source for satellite-based altimetry measurements. This is particularly important for future planetary missions not carrying laser altimeters but radar sounders, such as the upcoming Europa Clipper mission.

Accurately measuring the vertical temperature profiles of ice sheets and glaciers can provide critical data for numerical ice sheet models used in global sea level projections. Ice-penetrating radars and radiometers have been utilized independently in remote sensing campaigns to measure englacial ice temperatures. Here we show that by combining these two instruments, which have complimentary sensitivities to lower and upper depths of the ice column, respectively, the overall uncertainty in temperature profile retrievals can be significantly reduced. We present unified simulations of radar attenuation and radiometer brightness temperature as functions of depth and frequency to demonstrate the value in this combined active/passive remote sensing approach.

Englacial layering is a ubiquitous feature of ice penetrating radar images of continental ice sheets. These reflections occur at subsurface interfaces of changing density, conductivity, or crystal orientation fabric and can record the past atmospheric conditions and flow history of a region. As a result, both the physical cause of this layering and its geometry are of scientific interest. We develop a method for simulating englacial radio stratigraphy using measurements of density and conductivity from deep ice cores and show that it can reproduce coincident radar measurements. We then extend this method to two dimensions to simulate physically realistic synthetic radargrams. Our model incorporates both geophysical parameters and radar system parameters including center frequency, bandwidth, impulse response, aircraft attitude, and post-processing. As a result, our model can be used to generate labeled training data for machine learning layer tracking algorithms, connect layers observed by different radar systems to ice core stratigraphy, or as a forward model for testing hypotheses for the formation and evolution of the geometric and chemical properties of englacial layers away from deep drilling sites.

Quantifying uncertainty in subglacial topography measured with airborne radio-echo sounding (RES) is important for studying ice sheet dynamics and constraining future ice sheet behavior. Measurement accuracy is influenced by bed geometry. However, measurement uncertainties from off-nadir reflections are insufficiently characterized for quantifying topographic measurement uncertainty from off-nadir reflections. In this paper, we use a clutter simulator to quantify this source of uncertainty. We find that measurement uncertainty from clutter is spatially correlated and is strongly dependent on topographic roughness. This method could be used to estimate measurement uncertainty in RES measurements and enable more robust uncertainty quantification in bed topography.

We present a passive radar sounding approach that performs direct signal suppression (DSS) with a single-channel receiver. The modified passive sounding signal processing algorithm leverages a blind deconvolution filter to estimate the direct path of the ambient noise source before using CLEAN processing for DSS. We then highlight the results of our single-channel DSS technique with both synthetic and experimental data, and we analyze its performance in terms of SNR improvement. We find that by combining an adaptive blind deconvolution approach with the CLEAN algorithm, we obtain an SNR improvement of over 10 dB for both synthetic and experimental data. The single-channel DSS technique is a further step towards low-resource, high SNR passive sounding with ambient radio noise, as it eliminates the requirement for long integration times and a two-channel DSS system.

Orbital radar sounding of terrestrial ice sheets is an area of increasing research interest with mission concepts at 45 MHz, P-Band, and L-Band under development. However, large uncertainties remain in impact of glacial conditions and platform altitude on their link budgets. Here, we present a collection of empirically and glaciologically informed constraints on orbital sounder link budgets using airborne radar sounding data. We also analyze the effects of geometric spreading and englacial water. Finally, we discuss link-budget considerations for investigations beyond bed mapping including observing basal reflectivity, englacial hydrology, ice-shelf thickness, englacial layers, and estimating vertical ice velocity.

The full velocity structure of glacial ice is critical to understanding glacier dynamics, but such measurements are rare. We propose a novel data processing method to resolve the horizontal and vertical velocity profile of an ice column using existing autonomous phase-sensitive radio echo sounder (ApRES) systems. The availability and relatively low cost of these systems can enable more widespread measurement of the 3D velocity structure of ice, with important implications for observational constraints on ice dynamics and basal drag.

The subglacial topography of the Earth's ice sheets is a critical input to models of the evolution of ice sheets and sea level rise. Direct measurements of ice thickness, however, are sparse, necessitating techniques for interpolating these measurements. One class of interpolation methods enforces physical constraints to transform the problem into an inversion. A challenge with these approaches is that multiple unknown parameters must be solved for simultaneously. We introduce a new numerical approach to solving for mass conservation-constrained ice thickness maps. This technique, based on a physics-informed neural network, allows for the flexible incorporation of a range of soft constraints. In the future, this could enable simultaneous estimation of ice velocity, bed topography, and sliding parameters.

Englacial layer velocity can provide insights on the vertical-velocity structure of the ice sheets. We present a repeat-pass interferometric approach that allows the estimation of the vertical englacial layer velocity using the radar sounder data. In contrast to the ground-based sensors, the airborne radar sounder data can potentially be used to estimate the layer velocity on a continental scale. When merged with the horizontal surface velocity and the numerical models, layer velocity can support the 3-D analysis of the ice flow and structure. Our aim is to provide the proof-of-concept demonstration that, similar to the side-looking synthetic aperture radar for imaging, the airborne radar sounder data can be used to estimate the subwavelength displacement of the englacial radio-stratigraphic layers. To achieve this, we use the phase and magnitude acquired repeatedly over the same region of interest. After the extraction of the crossing points, two acquisitions are finely registered. Then, we compute the interferometric phase for the englacial layers (which are shown to behave as PSs in the nadir sounding geometry), to estimate a vertical displacement and a velocity profile. We present the results over East Antarctica using data from the high-capability radar sounder (HiCARS) system. We show two scenarios that demonstrate the feasibility, limitations, and requirements of this approach.

Recent studies have proposed that Jovian decametric radiation (DAM) can be effectively exploited for probing the subsurface of Jupiter's icy moons by using passive radio sounding. However, these studies were based on the assumption that Jovian noise is white and stationary. Therefore, additional investigations into the temporal stability, predictability, and spectral properties of Jovian noise are required to fully assess the potential of passive radar sounding and improve the acquisitions planning. In this article, we investigate these properties of the Jovian DAM to understand their impact on radar sounding performance. This is done by analyzing the recently available radio spectra acquired by the JUNO Waves instrument. Results are also evaluated for the specific case of the Radar for Icy Moon Exploration (RIME) and the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) that have been selected for European Space Agency (ESA) and NASA missions to Ganymede and Europa. Our results show that the Jovian DAM is not perfectly white, but no severe distortions in the range response should be expected. The results on spatiotemporal occurrence show that Jupiter's DAM has a variable probability of occurrence, which is rather sporadic for some frequency ranges. The results on RIME and REASON flybys show that the Jovian DAM occurrence probability is relatively low for selected sub-Jovian flybys at Europa. For the RIME Ganymede orbital phase, a large number of high occurrence passive acquisition opportunities are expected. The experimental results also show that a large bandwidth receiver would enhance the chance of recording Jovian noise.

Recent work has demonstrated a passive radio sounding approach that uses the Sun as a source for echo detection and ranging. As the Sun is a moving source with a position that is knowna priori, we evaluate this technique’s capabilities to measure the echo’s phase history, map topography, and perform synthetic aperture radar (SAR) focusing. Here, we present our approach to implementing passive SAR using a compact, temporally incoherent radio-astronomical source as a signal of opportunity. We first evaluate the passive system’s capabilities to obtain an echo from a rough surface by determining the critical signal-to-noise ratio (SNR) for reliably observing the Sun’s echo reflection with our passive instrument. We then demonstrate that our technique can detect the necessary changes in range, phase, and reflectivity of an echo from the Sun. We next present the experimental results of our passive radar testing using the Sun at Dante’s View, Death Valley, to highlight this technique’s ability to perform 2-D imaging. Finally, with synthetic data, we demonstrate that we can use time-domain backprojection to focus a planar white noise signal, perform passive SAR imaging, and improve the measurement’s SNR and azimuth resolution. The results of passive SAR focusing on white noise highlight the potential for the Sun and Jupiter’s radio emissions to perform surface and subsurface imaging for planetary and terrestrial observations.

We develop and test a bistatic radar system and processing chain that recovers weak echoes at large antenna separations, which is a necessary step towards high precision temperature inversions. Traditional ice penetrating radars have limited capacity to infer temperature distributions because monostatic measurements do not provide sufficient information to resolve the depth-dependent ice temperature profile. Bistatic radar introduces quasi-independent measurements that can address this challenge, but existing systems are unable to attain the large antenna separations necessary to resolve the small temperature gradients important to glaciological processes. Existing bistatic systems are either limited in antenna separation by losses in synchronization cables or by poor signal-to-noise-ratio (SNR) for unsynchronized systems. We address this challenge through coherent summation of phase re-aligned signals to recover the basal and internal layer reflections at large antenna separations without requiring hardware synchronization. The system consists of an Autonomous Phase-sensitive Radio Echo Sounder (ApRES) as the transmitter and a Software Defined Radio (SDR) as the receiver. We assess the system's capacity to achieve high SNRs and large offsets at Whillans Ice Stream, West Antarctica, with up to a 1.3 km antenna separation. This experiment charts a course for even larger antenna separations to resolve small temperature signals with high fidelity.

Traditional radar sounders operating at single frequencies provide inherently ambiguous information due to the impact of subsurface basal material and roughness effects on relative along-track echo power. This ambiguity forces assumptions to be made about the nature of the ice sheet in order to definitively interpret subglacial conditions on a pulse by pulse basis. Here, we show that a multi-frequency radar sounder is best suited to discriminate basal material properties by correcting for basal roughness effects. By using frequencies spanning multiple orders of magnitude (3, 30, 300 MHz), each with high radiometric resolution (±0.5dB), we establish that such an architecture will enable determination of the basal material reflectivity to within 1.5 dB for roughness scales less than λ0/4, where λ0is the reference wavelength.

The formation of impermeable ice layers within Greenland's firn can significantly increase surface meltwater runoff by capping percolation, thus increasing the continent's immediate contributions to sea level rise. Detection of these layers by airborne radar sounding would permit large scale assessment of their spatial coverage and temporal evolution. We present an electromagnetic forward model for radar scattering in dry firn, as well as a statistical model of firn density profiles, which together allow us to robustly simulate airborne radar sounding measurements in the ice sheet near-surface. We use these models to simulate the response of the University of Kansas Accumulation Radar to refrozen ice layers thinner than the radar vertical resolution. Our results suggest that continuous ice layers are detectable as anomalously bright reflections within the firn, so long as the background density does not exceed 0.72 g/cm3. We find that approximately 81% of single ice layers thicker than 2 cm are detectable, as well as over 90% of all multi-layer configurations. This suggests that the Accumulation Radar is an effective tool for studying the spatial and temporal coverage of thin ice layers in Greenland's firn, but such a survey would likely still underestimate the total areal extent.

The topography beneath ice sheets is an important parameter in numerous ice sheet analyses including sea level rise prediction. Subglacial topography is measured with airborne ice-penetrating radar. Radar coverage of ice sheets is incomplete so gaps must be interpolated. However, traditional interpolation techniques such as kriging create unrealistically smooth surfaces. Alternatively, topography can be geostatistically simulated such that the interpolated topography has similar spatial statistics to the data. The direct sampling technique simulates topography by sampling from a training dataset, or exposed topography. However, bed measurements are often hundreds or thousands of kilometers away from exposed topography, so selecting a representative training dataset is difficult. Therefore, we propose a technique that uses radar bed measurements to generate synthetic training data. This approach uses a Fourier transform to simulate topography based on the frequency content of the available bed measurements. The simulated training data can then be used to perform a direct sampling simulation. We demonstrate this concept for exposed bed topography in Greenland.

Radar sounding from airborne and ground-based platforms is one of the most powerful geophysical techniques in terrestrial glaciology. It can be used to probe a wide range of conditions from the near surface to the base of ice sheets and ice shelves. However, although these conditions are known to evolve on timescales from hours to centuries, the vast majority of terrestrial radar sounding observations are the lone observation at their given location. This has prevented the study of temporally evolving subglacial conditions using radar sounding in all but the most heavily-surveyed regions. Fortunately, recent advances in multi-system data analysis, digitized archival radar film, high-altitude autonomous sounding, and sounder sensor networks are putting terrestrial multitemporal radar sounding within reach. This paper discusses those advances as well as pathways to their large-scale implementation.

Radar sounding is a powerful tool for constraining subglacial conditions, which influence the mass balance of polar ice sheets and their contributions to global sea-level rise. A satellite-based radar sounder, such as those successfully demonstrated at Mars, would offer unprecedented spatial and temporal coverage of the subsurface. However, airborne sounding studies suggest that poorly constrained radar scattering in polar firn may produce performance-limiting clutter for terrestrial orbital sounders. We develop glaciologically constrained electromagnetic models of radar interactions in firn, test them against in situ data and multifrequency airborne radar observations, and apply the only model we find to be consistent with observation to assess the implications of firn clutter for orbital sounder system design. Our results show that in the very high-frequency (VHF) and ultrahigh-frequency (UHF) bands, radar interactions in the firn are dominated by quasi-specular reflections at the interfaces between layers of different densities and that off-nadir backscatter is likely the result of small-scale roughness in the subsurface density profiles. As a result, high frequency (HF) or low VHF center frequencies offer a significant advantage in near-surface clutter suppression compared to the UHF band. However, the noise power is the dominant constraint in all bands, so the near-surface clutter primarily constrains the extent to which the transmit power, pulselength, or antenna gain can be engineered to improve the signal-to-noise ratio. Our analysis suggests that the deep interior of terrestrial ice sheets is a difficult target for orbital sounding, which may require optimizations in azimuth processing and cross-track clutter suppression which complement existing requirements for sounding at the margins.

Radar sounding of ice from orbit has been successful on Mars [1], is planned for the Galilean satellites [2], and is attractive for earth [3] as a complement to current airborne instruments [4], but of major concern is the poorly constrained but potentially seriously limiting contribution of firn clutter [5]. To inform this issue, we analytically model electromagnetic scattering in the upper 100 meters of the ice column for continental ice sheets and evaluate the effects of variable platform altitude, frequency, and range resolution on clutter power. Our results show that volume scattering from air inclusions is insignificant and unlikely to constrain deep ice sounding. Rather, firn scattering is dominated by quasispecular reflections from layers of varying density which, at orbital altitudes, may contribute significantly to clutter due to the small angles of illumination. This layer clutter can be mitigated by a careful choice of range resolution for center frequencies below 200 MHz, but is practically unavoidable above 250 MHz. Firn layer clutter is likely to significantly constrain UHF orbital ice sounding, making a VHF instrument the more practical choice.

Recent work has demonstrated a passive radio sounding approach using the Sun as a source for echo detection. We expand on our passive autocorrelation-based technique by demonstrating its potential to map topography as the Sun moves throughout the entire day. Here, we show with synthetic data and our experimental results of passive radar testing with the Sun at Dante's View, Death Valley, our approach to implementing two dimensional image formation with passive radar. We also determine the critical signal to noise ratio (SNR) required to reliably observe a Sun echo with our passive instrument. Finally, we show that our autocorrelation-based technique can obtain changes in range, reflectivity, and phase, which are measurements normally acquired with traditional active radar systems. Demonstrating that our technique can acquire these changes is the first step to developing passive SAR processing using the Sun.

Ice penetrating radar sounders are the primary geophysical instruments for profiling the subsurface of terrestrial [1] and planetary [2]-[6] cryospheres. The capability of radar sounders to detect subsurface interfaces beneath thick and/or lossy ice is a function of their link budget [6]-[7] which must be much greater than surface imaging or altimetric radars [8] to achieve deep penetration. We propose a coherent along-feature processing approach for radar sounding data that maximizes the achievable azimuth gain for applications, like the exploration of icy moons [6]-[7] or mapping of deep/warm outlet glacier beds [9], where detecting very weak reflections from continuous interfaces is critical.

Ice crystal orientation fabric (COF) records information about past ice-sheet deformation and influences the present-day flow of ice. Polarimetric radar sounding provides a means to infer anisotropic COF patterns due to the associated birefringence of polar ice. Here, we develop a polarimetric coherence (phase-based) method to determine horizontal properties of the COF. The method utilizes the azimuth and depth dependence of the vertical gradient of the hhvv coherence phase to infer the dielectric principal axes and birefringence, which are then related to the second-order fabric orientation tensor. Specifically, under the assumption that one of the orientational eigenvectors is vertical, we can determine the horizontal eigenvectors and the difference between the horizontal eigenvalues (a measure of horizontal fabric asymmetry). The method exploits single-polarized data acquired with varying antenna orientation. It applies to ground-based “multi-polarization” surveys and is demonstrated using data acquired by Center for Remote Sensing of Ice Sheets (CReSIS) using Multi-Channel Coherent Radar Depth Sounder (MCRDS) from the North Greenland Eemian Ice Drilling (NEEM) ice core region in Greenland. The analysis is validated using a combination of polarimetric matrix backscatter simulations and comparison with COF data from the NEEM ice core. The results are consistent with a conventional model of ice deformation at an ice divide where a lateral tension component is present, with minor horizontal COF asymmetry and the greatest horizontal concentration of crystallographic axes orientated near parallel to the ice divide.

Jupiter is one of the Solar System's most powerful source of radio waves in the MHz range. Very recent studies proposed that Jovian Decametric Radiation can be effectively exploited for probing the subsurface of Jupiter's icy moons Europa and Ganymede by using passive radio sounding. However, these studies results were based on the assumption that the Jovian noise is white and stationary. Therefore additional investigation into the temporal stability, predictability, and spectral properties of Jovian noise are required to fully assess the potential of passive radar sounding. In this paper, we investigate the actual spectral structure, temporal occurrence and general characteristics of the Jovian Decametric Radiation for understanding their impact on the radar sounding performance metrics. This is done by analyzing the recently available Jupiter's radio spectra acquired by JUNO Waves. The results show that the Jovian Decametric Radiation (i) is not perfectly white and some performance deterioration in the range response should be expected and (ii) has a varying temporal occurrence versus probing frequency.

Radar sounders represent a powerful remote sensing solution to probe icy surface and detect subsurface layers by tracking vertical dielectric discontinuities. These englacial layers are fundamental indicator of the dynamic, rheological, and subglacial configuration of an ice sheet. However, steep layers can disappear when along-track processing is applied to radar sounder data. In this work, we present a novel Synthetic Aperture Radar (SAR) processing technique that aims to enhance the post processing Signal to Noise Ratio (SNR) of subsurface layers and avoiding interference in data processing. The proposed technique uses the phase and magnitude of a single channel RS system in a four step approach, made up of: i) sidelobes weighting, ii) range compression, iii) phase-shifted along-track coherent summation, and iv) multilooking. Additionally, the proposed technique allows the automatic estimation of the slope of layers. The proposed processing method has been validated on airborne radar sounder data acquired in Antarctica, while the slope estimation result is theoretically demonstrated.

While radio echo sounders are powerful tools used to constrain subglacial conditions, current ice-penetrating radar systems are too resource intensive for multiyear deployment at a large scale. To address this, we present passive radio sounding as a low resource approach for observing the subsurface of ice sheets and glaciers. Although passive radar has been used for target tracking and military purposes, it has never been implemented for ice sounding. Nevertheless, recent work has proposed the passive radio sounding of Europa's icy shell using Jupiter's decametric radiation as a source for echo detection [1], [2]. Expanding on this idea, we evaluate and discuss the challenges of developing a passive radio sounder that uses the Sun for echo detection. Our prototype measures the Sun's direct and reflected path off the ocean to obtain the height of a cliff along the California coast. This serves as the first in-situ demonstration of an autocorrelation-based passive-sounder using a compact astronomical white noise source.

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 R2, R3, or R4fall-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.

Ice sheet contributions to sea level rise present one of the greatest challenges that our society will face in the next century. However, models predicting sea level rise due to ice melt lack critical information regarding processes at the base of ice sheets. Although radio echo sounders are powerful tools that are currently used to constrain subglacial conditions, existing ice-penetrating radar systems are too resource-intensive in terms of cost, power, and logistics for multiyear deployment at a large scale. To address this, we present passive radio sounding as a low-resource approach for observing ice sheets across a range of spatial and temporal scales. While passive radar has been used for target tracking and military purposes, it has never been used for the sounding of ice sheets. Some recent work has proposed using passive radio sounding of Europa's icy shell using Jupiter's decametric radiation. We expand on this idea by evaluating and discussing challenges and opportunities for developing a passive radio sounder using the Sun as an illuminator of opportunity for echo detection. Here, our prototype instrument sits on the side of a cliff and measures the Sun's direct and reflected path off the ocean surface. We then use an autocorrelation-based method to extract the amplitude and delay of the reflection. This serves as the first in situ demonstration of an autocorrelation-based passive sounder using a compact astronomical white noise signal.

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.

Surface clutter can corrupt both Earth and planetary Radar Sounder (RS) observations preventing definitive interpretation of subsurface features, which are often the primary interest of geologists and planetary scientists. Clutter is usually detected by manual or automatic techniques that require ancillary information about the topography of the surface. However, this topography information is not always available. In this paper, we propose a novel method for clutter detection that is independent from ancillary information. This method uses a two channel RS system to exploit the cross-channel interferometric phase difference and is made up three main steps: i) feature extraction and theoretical phase difference estimation, ii) RS interferogram formation and iii) comparison of theoretical and real phase difference distributions. The proposed method has been validated on RS data acquired in Greenland.

Airborne radar sounding is an established tool for observing the bed conditions and subglacial hydrology of ice sheets and glaciers. The specularity content of radar bed echoes has also been used to detect the hydrologic transition of a subglacial water system from a network of distributed canals to a network of concentrated channels beneath the Thwaites Glacier. However, the physical dimensions of the distributed water bodies in these networks have not been constrained by observations. In this letter, we use a variety of simple radar scattering, attenuation, and cross-sectional models to provide a first estimate of the subglacial water body geometries capable of producing the observed anisotropic specularity of the Thwaites Glacier catchment. This approach leads to estimates of ice/water interface root mean square roughnesses less than about 15 cm, thicknesses of more than about 5 cm, lengths of more than about 15 m, and widths between about 0.5 and 5 m.