Irrigation informed by soil moisture conditions maximizes crop yield by mitigating stress from under-watering and nutrient deficiencies, stunted growth, and fruiting reduction from over-watering. Scalable methods for monitoring depth-dependent soil moisture at crop root zones are needed to fully quantify water available to crops. We aim to develop a drone-based multistatic radar system for 3D retrievals of water content and soil composition. Here, we present our current progress towards this goal which includes a trade study of penetration depth versus attenuation and our current implementation of a Finite-Difference Time-Domain Full Wave Inversion implemented in python. Once completed, we hope that this multistatic radar tomography method will enable root-zone soil moisture sensing to inform irrigation practices for maximized yield and improved food security.

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.

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.

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.

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.

The innovations in high-performance, low-power electronics and low-cost space access are unlocking affordable distributed radar systems and new remote sensing opportunities. The Distributed Element Beamformer Radar for Ice and Subsurface sounding (DEBRIS) is a concept to implement a 2D sparse radar aperture to improve the radar's spatial resolution and sounding investigation depth through the reduction of surface clutter. Here, we introduce this system and highlight its applications, orbital configurations, and the implementation considerations to achieve state-of-the-art performance for spaceborne radar sounders.

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.

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.