Passive radar sounding is a promising remote sensing technique that uses ambient radio emissions as signals of opportunity for terrestrial and planetary radar observations. Recent work has focused on improving passive sounding’s signal-to-noise ratio (SNR) by optimizing a single-channel direct signal suppression (DSS) algorithm to remove direct path interference. However, this technique requires an accurate estimate of the echo delay time, which can be significantly impacted by the ionosphere’s total electron content (TEC). This research adapts a recently developed DSS technique to consider the effects of the ionosphere’s TEC with respect to the echo delay time to provide further improvements to passive radar SNR.

Firn aquifers play an important role in the mass balance of the Greenland Ice Sheet, but changes in their water storage are challenging to measure. Here, we explore the feasibility of passive radar sounding to monitor changes in the firn aquifer water table level throughout the year. Our simulations consider the time-varying environmental challenges for passively monitoring fluctuations in firn aquifer water table height. Our results suggest that passive sounding could be a novel observational tool for quasi-continuous monitoring of firn aquifer properties in geographic regions of interest.

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.

We present the results of a preliminary statistical analysis and classification of solar radio burst candidates detected by the Mars Reconnaissance Orbiter (MRO) Shallow Radar (SHARAD). We first analyze the histograms of the MRO SHARAD burst candidates as a function of MRO-STEREO true anomaly difference and received peak power. We then show the results of performing logistic regression to classify the MRO SHARAD burst candidates. Our results highlight the need for additional burst data to further refine the classifier, additional parameters to determine if bursts are present, and potentially explore a different classification technique to assign burst candidates with improved accuracy. Analyzing SRBs detected by MRO/SHARAD (as a potential additional solar radio-observatory) would enhance our understanding of solar radio burst propagation physics and behavior. We conclude by discussing the potential application, and challenges, of using these bursts as a source for subsurface radio sounding for future terrestrial and Mars missions.

This research aims to optimize a previously developed direct signal suppression (DSS) algorithm for single-channel passive radars that use radio-astronomical sources (e.g., the Sun and Jupiter’s radio emissions) as ambient noise. Such passive radars can be used in extreme environments such as polar regions to measure ice sheet thickness and space-based experiments as a low-resource solution. One possible path to optimize DSS algorithms is by accurately estimating the direct signal and performing the Wiener deconvolution. This requires constructing the impulse response function, where the two most critical parameters are the echo peak power (α) and delay time (τ). Although only a minor increase in signal to noise ratio (SNR) was achieved compared to the previous approach, our results highlight the importance of correctly estimating the phase of the impulse response function. We observe significant losses in SNR if the phase information is not accurately estimated; while, on the other hand, the amplitude of the impulse response has a negligible impact on SNR compared to the correct phase estimation.

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.

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.

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.

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.

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.

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.

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.

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.

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.