Radar sounding is a powerful geophysical approach for characterizing the subsurface conditions of terrestrial and planetary ice masses at local to global scales. As a result, a wide array of orbital, airborne, ground-based, and in situ instruments, platforms and data analysis approaches for radioglaciology have been developed, applied or proposed. Terrestrially, airborne radar sounding has been used in glaciology to observe ice thickness, basal topography and englacial layers for five decades. More recently, radar sounding data have also been exploited to estimate the extent and configuration of subglacial water, the geometry of subglacial bedforms and the subglacial and englacial thermal states of ice sheets. Planetary radar sounders have observed, or are planned to observe, the subsurfaces and near-surfaces of Mars, Earth's Moon, comets and the icy moons of Jupiter. In this review paper, and the thematic issue of the Annals of Glaciology on ‘Five decades of radioglaciology’ to which it belongs, we present recent advances in the fields of radar systems, missions, signal processing, data analysis, modeling and scientific interpretation. Our review presents progress in these fields since the last radio-glaciological Annals of Glaciology issue of 2014, the context of their history and future prospects.

Russian (former Soviet) systematic studies of Antarctica by radio-echo sounding (RES) and ground-penetrating radar technique (GPR) were commenced in 1964. Since that time airborne RES surveys have covered about 5.5 × 106 km2 of the icy continent discovering remarkable geographic objects such as Subglacial Gamburtsev Mountains, and allowed studies of Filchner-Ronne Ice Shelf, Amery Ice Shelf and Lambert Glacier. Ground-based investigations during the 1990s and 2000s revealed the structure of the Lake Vostok area and surveyed along the Mirny to Vostok and Progress to Vostok traverse routes. GPR studies during the 2010s were to select the site for a new snow-runway at Mirny Station, with the resumption of the aviation after a 25 year hiatus.

Since the mid-1970s radio-echo soundings have been conducted on Iceland's temperate glaciers. Since then, low-frequency radar technology has furthered the study of most of the island's ice caps. Their masses and volumes have been quantified and detailed subglacial topographic maps produced which demarcate glacial drainage basins and identify subglacial lakes and volcanoes. Even internal tephra layers have been charted. The resulting data have been used to force and validate models of past and future glacier evolution. Many practical applications in glacier hydrology have come into being, including hydropower management, road and bridge planning and the prediction of catastrophic flood paths from subglacial eruption sites. Finally, emerging landscapes can now be foreseen in places where glaciers may soon disappear. These achievements would not have been possible without the advances in RES technology.

The thickness of glaciers in High-Mountain Asia (HMA) is critical in determining when the ice reserve will be lost as these glaciers thin but is remarkably poorly known because very few measurements have been made. Through a series of ground-based and airborne field tests, we have adapted a low-frequency ice-penetrating radar developed originally for Antarctic over-snow surveys, for deployment as a helicopter-borne system to increase the number of measurements. The manoeuvrability provided by helicopters and the ability of our system to detect glacier beds through thick, dirty, temperate ice makes it well suited to increase greatly the sample of measurements available for calibrating ice thickness models on the regional and global scale. The Bedmap Himalayas radar-survey system can reduce the uncertainty in present-day ice volumes and therefore in projections of when HMA's river catchments will lose this hydrological buffer against drought.

Airborne radio-echo sounding (RES) surveys are widely used to measure ice-sheet bed topography. Measuring bed topography as accurately and widely as possible is of critical importance to modelling ice dynamics and hence to constraining better future ice response to climate change. Measurement accuracy of RES surveys is influenced both by the geometry of bed topography and the survey design. Here we develop a novel approach for simulating RES surveys over glaciated terrain, to quantify the sensitivity of derived bed elevation to topographic geometry. Furthermore, we investigate how measurement errors influence the quantification of glacial valley geometry. We find a negative bias across RES measurements, where off-nadir return measurement error is typically −1.8 ± 11.6 m. Topographic highlands are under-measured an order of magnitude more than lowlands. Consequently, valley depth and cross-sectional area are largely under-estimated. While overall estimates of ice thickness are likely too high, we find large glacier valley cross-sectional area to be under-estimated by −2.8 ± 18.1%. Therefore, estimates of ice flux through large outlet glaciers are likely too low when this effect is not taken into account. Additionally, bed mismeasurements potentially impact our appreciation of outlet-glacier stability.

This paper provides an update and overview of the Center for Remote Sensing of Ice Sheets (CReSIS) radars and platforms, including representative results from these systems. CReSIS radar systems operate over a frequency range of 14–38 GHz. Each radar system's specific frequency band is driven by the required depth of signal penetration, measurement resolution, allocated frequency spectra, and antenna operating frequencies (often influenced by aircraft integration). We also highlight recent system advancements and future work, including (1) increasing system bandwidth; (2) miniaturizing radar hardware; and (3) increasing sensitivity. For platform development, we are developing smaller, easier to operate and less expensive unmanned aerial systems. Next-generation platforms will further expand accessibility to scientists with vertical takeoff and landing capabilities.

The interpretations of relevant interfaces (i.e. the surface and bed) in radar sounding datasets over glaciers and ice sheets are primary boundary conditions in a variety of climate studies and particularly subglacial water routing models. It is therefore necessary to ensure these interpretations are consistent and not affected by cross-track clutter. For the surface interface, interferometry and a family of methods relying on digital elevation models have been used to successfully discriminate cross-track surface clutter. Here we present how interferometry can be applied to the problem of basal clutter from cross-track bed topography. Our approach is based on a comparison of the differential phases of ambiguous reflectors that may represent bed clutter and the differential phase of a reflector in an adjacent area that appears unaffected by basal clutter. The reflector yielding the smallest interferometric phase difference relative to the unambiguous bed reflector is considered to represent its consistent continuation. We successfully demonstrate our approach using 60 MHz center frequency MARFA data collected over Devon Ice Cap in the Canadian Arctic. Finally, we investigate the effects of clutter-affected and interferometry-corrected bed interpretations on ice layer thickness estimates, basal hydraulic head gradients and the potential extent of inferred subglacial water bodies.

Here we use polarimetric measurements from an Autonomous phase-sensitive Radio-Echo Sounder (ApRES) to investigate ice fabric within Whillans Ice Stream, West Antarctica. The survey traverse is bounded at one end by the suture zone with the Mercer Ice Stream and at the other end by a basal ‘sticky spot’. Our data analysis employs a phase-based polarimetric coherence method to estimate horizontal ice fabric properties: the fabric orientation and the magnitude of the horizontal fabric asymmetry. We infer an azimuthal rotation in the prevailing horizontal c-axis between the near-surface (z ≈ 10–50 m) and deeper ice (z ≈ 170–360 m), with the near-surface orientated closer to perpendicular to flow and deeper ice closer to parallel. In the near-surface, the fabric asymmetry increases toward the center of Whillans Ice Stream which is consistent with the surface compression direction. By contrast, the fabric orientation in deeper ice is not aligned with the surface compression direction but is consistent with englacial ice reacting to longitudinal compression associated with basal resistance from the nearby sticky spot.

The Askaryan Radio Array (ARA) experiment at the South Pole is designed to detect high-energy neutrinos which, via in-ice interactions, produce coherent radiation at frequencies up to 1000 MHz. Characterization of ice birefringence, and its effect upon wave polarization, is proposed to enable range estimation to a neutrino interaction and hence aid in neutrino energy reconstruction. Using radio transmitter calibration sources, the ARA collaboration recently measured polarization-dependent time delay variations and reported significant time delays for trajectories perpendicular to ice flow, but not parallel. To explain these observations, and assess the capability for range estimation, we use fabric data from the SPICE ice core to model ice birefringence and construct a bounding radio propagation model that predicts polarization time delays. We compare the model with new data from December 2018 and demonstrate that the measurements are consistent with the prevailing horizontal crystallographic axis aligned near-perpendicular to ice flow. The study supports the notion that range estimation can be performed for near flow-perpendicular trajectories, although tighter constraints on fabric orientation are desirable for improving the accuracy of estimates.

The Antarctic Impulsive Transient Antenna (ANITA) balloon experiment was designed to detect radio signals initiated by high-energy neutrinos and cosmic ray (CR) air showers. These signals are typically discriminated by the polarization and phase inversions of the radio signal. The reflected signal from CRs suffer phase inversion compared to a direct ‘tau neutrino’ event. In this paper, we study subsurface reflection, which can occur without phase inversion, in the context of the two anomalous up-going events reported by ANITA. It is found that subsurface layers and firn density inversions may plausibly account for the events, while ice fabric layers and wind ablation crusts could also play a role. This hypothesis can be tested with radar surveying of the Antarctic region in the vicinity of the anomalous ANITA events. Future experiments should not use phase inversion as a sole criterion to discriminate between down-going and up-going events, unless the subsurface reflection properties are well understood.

Stationary ice-penetrating radar (sIPR) systems can be used to monitor temporal changes in electromagnetically sensitive properties of glaciers and ice sheets. We describe a system intended for autonomous operation in remote glacial environments, and document its performance during deployments in cold and temperate settings. The design is patterned after an existing impulse radar system, with the addition of a fibre-optic link and timing module to control transmitter pulses, a micro-UPS (uninterruptable power supply) to prevent uncontrolled system shutdown and a customized satellite telemetry scheme. Various implementations of the sIPR were deployed on the Kaskawulsh Glacier near an ice-marginal lake in Yukon, Canada, for 44–77 days in summers 2014, 2015 and 2017. Pronounced perturbations to englacial radiostratigraphy were observed commensurate with lake filling and drainage, and are interpreted as changes in englacial water storage. Another sIPR was deployed in 2015–2016 on ice island PII-A-1-f, which originated from the Petermann Glacier in northwest Greenland. This system operated autonomously for almost a year during which changes in thickness of the ice column were clearly detected.

Several autonomous phase-sensitive radio-echo sounders (ApRES) were deployed at Greenland glaciers to investigate ice deformation. Different attenuation settings were tested and it was observed that, in the presence of clipping of the deramped ApRES signal, each setting produced a different result. Specifically, higher levels of clipping associated with lower attenuation produced an apparent linear increase of diurnal vertical cumulative displacement with depth, and obscured the visibility of the basal reflector in the return amplitude. An example with a synthetic deramped signal confirmed that these types of artifacts result from the introduction of harmonics from square-wave-like features introduced by clipping. Apparent linear increase of vertical displacement with depth occurs when the vertical position of a near-surface internal reflector changes in time. Artifacts in the return amplitude may obscure returns from internal reflectors and the basal reflector, making it difficult to detect thickness evolution of the ice and to correctly estimate vertical velocities. Variations in surface melt during ApRES deployments can substantially modulate the received signal strength on short timescales, and we therefore recommend using higher attenuator settings for deployments in such locations.

Despite widespread use of radio-echo sounding (RES) in glaciology and broad distribution of processed radar products, the glaciological community has no standard software for processing impulse RES data. Dependable, fast and collection-system/platform-independent processing flows could facilitate comparison between datasets and allow full utilization of large impulse RES data archives and new data. Here, we present ImpDAR, an open-source, cross-platform, impulse radar processor and interpreter, written primarily in Python. The utility of this software lies in its collection of established tools into a single, open-source framework. ImpDAR aims to provide a versatile standard that is accessible to radar-processing novices and useful to specialists. It can read data from common commercial ground-penetrating radars (GPRs) and some custom-built RES systems. It performs all the standard processing steps, including bandpass and horizontal filtering, time correction for antenna spacing, geolocation and migration. After processing data, ImpDAR's interpreter includes several plotting functions, digitization of reflecting horizons, calculation of reflector strength and export of interpreted layers. We demonstrate these capabilities on two datasets: deep (~3000 m depth) data collected with a custom (3 MHz) system in northeast Greenland and shallow (<100 m depth, 500 MHz) data collected with a commercial GPR on South Cascade Glacier in Washington.

Radio-echo sounding (RES) can be used to understand ice-sheet processes, englacial flow structures and bed properties, making it one of the most popular tools in glaciological exploration. However, RES data are often subject to ‘strip noise’, caused by internal instrument noise and interference, and/or external environmental interference, which can hamper measurement and interpretation. For example, strip noise can result in reduced power from the bed, affecting the quality of ice thickness measurements and the characterization of subglacial conditions. Here, we present a method for removing strip noise based on combined wavelet and two-dimensional (2-D) Fourier filtering. First, we implement discrete wavelet decomposition on RES data to obtain multi-scale wavelet components. Then, 2-D discrete Fourier transform (DFT) spectral analysis is performed on components containing the noise. In the Fourier domain, the 2-D DFT spectrum of strip noise keeps its linear features and can be removed with a ‘targeted masking’ operation. Finally, inverse wavelet transforms are performed on all wavelet components, including strip-removed components, to restore the data with enhanced fidelity. Model tests and field-data processing demonstrate the method removes strip noise well and, incidentally, can remove the strong first reflector from the ice surface, thus improving the general quality of radar data.

This work is a collection of radar equations for low-frequency radar sounding and radar in general that emphasize the form of the radar equation for different target and source geometries. This is meant as a handbook for scientists and engineers that work with or analyze radar sounder systems and interpret radar sounding echoes. Lookup tables summarize the results and derivations are provided for each equation.

The Northeast Greenland Ice Stream (NEGIS) is an important dynamic component for the total mass balance of the Greenland ice sheet, as it reaches up to the central divide and drains 12% of the ice sheet. The geometric boundary conditions and in particular the nature of the subglacial bed of the NEGIS are essential to understand its ice flow dynamics. We present a record of more than 8000 km of radar survey lines of multi-channel, ultra-wideband radio echo sounding data covering an area of 24 000 km2, centered on the drill site for the East Greenland Ice-core Project (EGRIP), in the upper part of the NEGIS catchment. Our data yield a new detailed model of ice-thickness distribution and basal topography in the region. The enhanced resolution of our bed topography model shows features which we interpret to be caused by erosional activity, potentially over several glacial–interglacial cycles. Off-nadir reflections from the ice–bed interface in the center of the ice stream indicate a streamlined bed with elongated subglacial landforms. Our new bed topography model will help to improve the basal boundary conditions of NEGIS prescribed for ice flow models and thus foster an improved understanding of the ice-dynamic setting.

Recent advances in the measurement of bedforms beneath active ice streams have been made using ground-based grid profiling using impulse radar systems operating with centre frequencies in the 3–5 MHz range. Surveys of Rutford Ice Stream and Pine Island Glacier have shown that features such as mega-scale glacial lineations with topographic relief of as little as 3 m can be traced for many kilometres downstream under more than 2 km of fast-moving ice. In the discussion of these data, it is often asked ‘How is it possible to map such fine-scale topography with such a low-frequency radar’. In answering that question, the key point is the distinction between the precision of a radar range measurement to a single, isolated reflective interface and the ability to resolve the presence of two closely-spaced interfaces of similar reflectivity (commonly referred to as the vertical resolution). This paper will discuss and illustrate this distinction and use the case study of data acquired over Pine Island Glacier to examine the limits of precision of the radar range measurement.

Basal motion of ice sheets depends in part on the roughness and material properties of the subglacial bed and the occurrence of water. To date, basal motion represents one of the largest uncertainties in ice-flow models. It is that component of the total flow velocity that can change most rapidly and can, therefore, facilitate rapid variations in dynamic behaviour. In this study, we investigate the subglacial properties of the East Antarctic Ice Sheet by statistically analysing the roughness of the bed topography, inferred from radio-echo sounding measurements. We analyse two sets of roughness parameters, one derived in the spatial and the other in the spectral domain, with two roughness parameters each. This enables us to compare the suitability of the four roughness parameters to classify the subglacial landscapes below the ice sheet. We further investigate the relationship of the roughness parameters with observed surface flow velocity and modelled basal temperatures of the ice sheet. We find that one of the roughness parameters, the Hurst exponent derived in the spatial domain, coincides with the thermal condition at the base of the ice sheet for slow flow velocities and varies with flow velocity.

All radar power interpretations require a correction for attenuative losses. Moreover, radar attenuation is a proxy for ice-column properties, such as temperature and chemistry. Prior studies use either paired thermodynamic and conductivity models or the radar data themselves to calculate attenuation, but there is no standard method to do so; and, before now, there has been no robust methodological comparison. Here, we develop a framework meant to guide the implementation of empirical attenuation methods based on survey design and regional glaciological conditions. We divide the methods into the three main groups: (1) those that infer attenuation from a single reflector across many traces; (2) those that infer attenuation from multiple reflectors within one trace; and (3) those that infer attenuation by contrasting the measured power from primary and secondary reflections. To assess our framework, we introduce a new ground-based radar survey from South Pole Lake, comparing selected empirical methods to the expected attenuation from a temperature- and chemistry-dependent Arrhenius model. Based on the small surveyed area, lack of a sufficient calibration surface and low reflector relief, the attenuation methods that use multiple reflectors are most suitable at South Pole Lake.

Numerical ice-sheet models are commonly matched to surface ice velocities from InSAR measurements by modifying basal drag, allowing the flow and form of the ice sheet to be simulated. Geophysical measurements of the bed are rarely used to examine if this modification is realistic, however. Here, we examine radio-echo sounding (RES) data from the Weddell Sea sector of West Antarctica to investigate how the output from a well-established ice-sheet model compares with measurements of the basal environment. We know the Weddell Sea sector contains the Institute, Möller and Foundation ice streams, each with distinct basal characteristics: Institute Ice Stream lies partly over wet unconsolidated sediments, where basal drag is very low; Möller Ice Stream lies on relatively rough bed, where basal drag is likely larger; and Foundation Ice Stream is controlled by a deep subglacial trough with flow-aligned bedrock landforms and smooth unconsolidated sediments. In general, the ice-sheet model represents each ice-stream system well. We also find that ice velocities do not match perfectly in some locations, and that adjustment of the boundaries of low basal drag, to reflect RES evidence, should improve model performance. Our work showcases the usefulness of RES in calibrating ice-sheet model results with observations of the bed.