A novel high frequency (HF) to very high frequency (VHF) wire-mesh dipole antenna design for use in polar regions is discussed and evaluated. The antenna was designed to be lightweight, readily demountable, and acceptably robust. This is an initial step in the development of a ground-based, phasesensitive, synthetic aperture imaging system for use on an autonomous rover platform. The results of initial trials on the Rhône Glacier, Switzerland in August 2019 are discussed, with particular attention being paid to the effect on antenna performance of high surface water content. Including the effects of surface water resulted in good agreement between field results and modelled performance.
<p>Acquisition of quad-polarimetric radar data on ice sheets gives insights about the ice-fabric variability with depth and consequently can deliver essential constraints on the spatially variable ice rheology. Polarimetric measurements are collected manually in most ground-based surveys, discretely sampling a limited profile range. Measurements are time-intensive and often do not cover critical areas such as shear zones where field safety is a concern. Autonomous rovers can provide an alternative that optimizes for time, sampling resolution and safety. &#160;</p><p>Here, we present an autonomous acquisition technique of quad-polarimetric radar data using a rover. This technique is based on a previous layout that has proven its capacity to navigate in various snow conditions but did not yet actively trigger the geophysical instruments attached. We upgraded the rover with a novel Robotic Operating System (ROS2) that interfaces simultaneously with a real-time positioning GPS and an automatic phase-sensitive radio-echo sounder (ApRES) with multiple transmitters multiple receivers. Like this, the rover can autonomously steer to pre-destined waypoints and then take static measurements at those locations also in areas where field safety might be compromised. We demonstrate this proof-of-concept on the Ekstr&#246;m Ice Shelf Antarctica, where we acquired densely spaced polarimetric radar data measurements. The rover&#8217;s operating system offers many opportunities for other measurement principles, e.g., densely spaced co-polarized data suitable for synthetic aperture radar (SAR) processing.</p>
<p>The internal stratigraphy of alpine glaciers entails information about its past dynamics and accumulation rates. It further can be used for intercalibrating the age-depth scales of ice cores. The internal ice stratigraphy is often imaged using radar, but similar to polar ice sheets the deeper stratigraphy is often difficult to resolve with classical pulsed radar systems. For polar ice sheets, the introduction of phase coherent radars has illuminated this former echo-free zone (EFZ) and now patterns of folded, buckled and disrupted ice stratigraphy are clearly visible. Unfortunately, the new airborne and ground-based radar systems applied in polar regions are typically too heavy to be deployed in an alpine environment.</p><p>Here, we transfer the lightweight autonomous phase-sensitive radio-echo sounder (ApRES) to an alpine glacier targeting its echo-free zone (Colle Gnifetti, Italy/Switzerland). The ApRES is a coherent frequency modulated continuous wave radar with an integration time of 1 s per trace which we deployed in combination with a GNSS used in real time kinematic (RTK) mode. The latter allows repositioning of the antennas with sub-wavelength accuracy (approximately 5 cm) required to exploit the coherent signal. Like this, the radio-stratigraphy of the former EFZ at this site could be imaged using a matched filtering SAR method. The resulting radargrams cover former ice core sites (e.g., Ice Memory and KCC) and can be used to harmonize conflicting age-depth scales. This dataset will be analysed further in conjunction with ice-fabric measurements from ice cores to reveal how the anisotropic ice rheology imprints on the flow field of glaciers.</p>
The design of an HF-VHF frequency modulated continuous wave radar intended for use with an autonomous rover in the polar regions is presented. The RF front-end, antenna design and deramped filter for the radar are described and validated using laboratory measurements and an outdoor field measurement.
A ray-based 2D modelling approach is proposed to reduce computation times involved in the forward-modelling of deramped frequency-modulated continuous wave (FMCW) radar signals for subglacial ice features. The model generation procedure and link-budget are described, prior to validation and benchmarking of the ray-based approach using a commercial 3D FDTD simulator. The proposed model results in a reduced computation time of three orders of magnitude per range profile, for comparable power and phase outputs. This provides a highly efficient numerical method for investigating subglacial ice structures with more realistic sizes and features.
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