Fig 13 - available via license: Creative Commons Attribution 4.0 International
Content may be subject to copyright.
Source publication
We have built and operated an ultra-wideband UHF pulsed-chirp radar for measuring firn stratigraphy from airborne platforms over the ice sheets of Greenland and West Antarctica. Our analysis found a wide range of capabilities, including imaging of post firn–ice transition horizons
and sounding of shallow glaciers and ice shelves. Imaging of horizon...
Similar publications
Old ice for paleo-environmental studies, traditionally accessed through deep core drilling on domes and ridges on the large ice sheets, can also be retrieved at the surface from ice sheet margins and blue ice areas. The practically unlimited amount of ice available at these sites satisfies a need in the community for studies of trace components req...
Definitive interpretation of ice-sheet basal conditions from radar-sounding data beneath outlet-glacier grounding zones and shear margins can be problematic due to poorly constrained and spatially variable englacial attenuation rates and losses from propagation through a rough ice surface. To correct for spatially variable attenuation rates, we dev...
Citations
... (1) the high transmit power and correspondingly high detection range enabled by the internal battery in the transmitters (∼60 − 80 m in air and ∼2.5 m in water) and (2) the relatively favourable attenuation properties of UHF radio signals in pure, cold glacier ice (Fujita and others, 2000;Barwick and others, 2005;Martinez and others, 2005;Lewis and others, 2015;Hashmi and Brennan, 2022). These off-the-shelf RFID systems have the potential to be rapidly deployed with a minimum of technical radio-systems knowledge. ...
We present a method for tracking radio-tagged pebbles and cobbles through subglacial meltwater channels under shallow temperate glaciers. Natural particles tagged with active radio transmitters were injected directly into a large subglacial channel 300 m up-glacier from the terminus of the Glacier d'Otemma, Switzerland. A roving antenna was developed to localise tagged particles planimetrically in subglacial and proglacial channel reaches (350 and 150 m long, respectively) using a probabilistic technique, delivering records of the change in particle location and transport distance over time with uncertainty. The roving antenna had a ±5−15 m planimetric precision, a 75% particle localisation rate and operated at a maximum ice depth of 47 m. Additionally, stationary supraglacial and proglacial antennas continuously monitored the passage of tagged particles through consecutive reaches of the channel, constraining the timing of particle transport events. The proglacial antenna system had a 98.1% detection rate and was operational to 0.89 m water depth during testing. Roving and stationary antenna records were combined to create a transport distance model for each particle, which may be used in conjunction with hydraulic data to investigate the kinematics of particle motion. When applied at scale in future studies, this method may be used to reveal the mechanisms and timescales of coarse sediment export from Alpine glaciers.
... To evaluate the role of these feature in modulating SMB, it is critical to determine how commonly ice blobs form and whether they develop before, after, or in conjunction with ice slabs. Therefore, we use the more extensive OIB Accumulation Radar data record (Lewis et al., 2015) to map ice blobs and isolated firn aquifers across Northwest Greenland by identifying similar low-reflectivity anomalies in data collected in 2011, 2013, 2014, and 2017 (Text S10 in Supporting Information S1). ...
Plain Language Summary
Much of the surface of the Greenland Ice Sheet is covered in a porous layer of old snow, known as firn. However, in some areas, surface melt has refrozen to form thick layers of solid ice in the upper few meters of this porous layer. It is generally assumed that once these ice slabs form, the firn can no longer absorb meltwater. Therefore, subsequent surface melt must either flow over the surface into the ocean or drain to the bottom of the ice sheet through cracks, ultimately leading to more mass loss. Here we show that in Northwest Greenland, this is not always the case. Instead, some of this water drains through shallow cracks in the ice slabs and is stored in the remaining porous firn underneath. This reduces immediate mass loss from this region and limits the amount of water that can contribute to speeding up the flow of ice. As a result, even after ice slabs form, the ice sheet may lose mass at a somewhat slower rate than previously assumed.
... To fill the gap of measurements between the ICESat-1 and ICESat-2 missions, NASA implemented the OIB Program to acquire airborne remote sensing observations in the areas undergoing rapid changes. The multifrequency radar instrumentation package, including Multichannel Coherent Radar Depth Sounder/Imager (MCoRDS/I), Accumulation Radar, Ku-Band Radar, and Snow Radar, designed and operated by the Center for Remote Sensing of Ice Sheets (CReSIS), has been aboard NASA airborne platforms since 2009 as a part of the OIB program and measures ice surface topography, near-surface internal layers, bedrock topography and snow thickness over sea ice [42,43]. The locations of FAs over the GrIS are derived from the Accumulation Radar data and MCoRDS/I data of the OIB program [29,31]. ...
... To fill the gap of measurements between the ICESat-1 and ICESat-2 missions, NASA implemented the OIB Program to acquire airborne remote sensing observations in the areas undergoing rapid changes. The multifrequency radar instrumentation package, including Multichannel Coherent Radar Depth Sounder/Imager (MCoRDS/I), Accumulation Radar, Ku-Band Radar, and Snow Radar, designed and operated by the Center for Remote Sensing of Ice Sheets (CReSIS), has been aboard NASA airborne platforms since 2009 as a part of the OIB program and measures ice surface topography, near-surface internal layers, bedrock topography and snow thickness over sea ice [42,43]. The locations of FAs over the GrIS are derived from the Accumulation Radar data and MCoRDS/I data of the OIB program [29,31] [33]. ...
Surface meltwater runoff is believed to be the main cause of the alarming mass loss in the Greenland Ice Sheet (GrIS); however, recent research has shown that a large amount of meltwater is not directly drained or refrozen but stored in the form of firn aquifers (FAs) in the interior of the GrIS. Monitoring the changes in FAs over the GrIS is of great importance to evaluate the stability and mass balance of the ice sheet. This is challenging because FAs are not visible on the surface and the direct measurements are lacking. A new method is proposed to map FAs during the 2010–2020 period by using the C-band Advanced Scatterometer (ASCAT) data based on the Random Forests classification algorithm with the aid of measurements from the NASA Operation IceBridge (OIB) program. Melt days (MD), melt intensity (MI), and winter mean backscatter (WM) parameters derived from the ASCAT data are used as the input vectors for the Random Forests classification algorithm. The accuracy of the classification model is assessed by ten-fold cross-validation, and the overall accuracy and Kappa coefficient are 97.49% and 0.72 respectively. The results show that FAs reached the maximum in 2015, and the accumulative area of FAs from 2010 to 2020 is 56,477 km2, which is 3.3% of the GrIS area. This study provides a way to investigate the long-term dynamics in FAs which have great significance for understanding the state of subsurface firn and subglacial hydrological systems.
... AR and MCoRDS were flown over the GrIS on a P-3 aircraft in April and May between 2010 and 2017. The AR instrument operates at a center frequency of 750 MHz with a bandwidth of 300 MHz, resulting in a range resolution in firn of 0.53 m(Lewis et al., 2015). The collected data have an along-track resolution of approximately 30 m with 15 m spacing between traces in the final processed radargrams. ...
Perennial firn aquifers are subsurface meltwater reservoirs consisting of a meters-thick water-saturated firn layer that can form on spatial scales as large as tens of kilometers. They have been observed within the percolation facies of glaciated regions experiencing intense seasonal surface melting and high snow accumulation. Widespread perennial firn aquifers have been identified within the Greenland Ice Sheet (GrIS) via field expeditions, airborne ice-penetrating radar surveys, and satellite microwave sensors. In contrast, ice slabs are nearly continuous ice layers that can also form on spatial scales as large as tens of kilometers as a result of surface and subsurface water-saturated snow and firn layers sequentially refreezing following multiple melting seasons. They have been observed within the percolation facies of glaciated regions experiencing intense seasonal surface melting but in areas where snow accumulation is at least 25 % lower as compared to perennial firn aquifer areas. Widespread ice slabs have recently been identified within the GrIS via field expeditions and airborne ice-penetrating radar surveys, specifically in areas where perennial firn aquifers typically do not form. However, ice slabs have yet to be identified from space. Together, these two ice sheet features represent distinct, but related, sub-facies within the broader percolation facies of the GrIS that can be defined primarily by differences in snow accumulation, which influences the englacial hydrology and thermal characteristics of firn layers at depth. Here, for the first time, we use enhanced-resolution vertically polarized L-band brightness temperature (TVB) imagery (2015–2019) generated using observations collected over the GrIS by NASA's Soil Moisture Active Passive (SMAP) satellite to map perennial firn aquifer and ice slab areas together as a continuous englacial hydrological system. We use an empirical algorithm previously developed to map the extent of Greenland's perennial firn aquifers via fitting exponentially decreasing temporal L-band signatures to a set of sigmoidal curves. This algorithm is recalibrated to also map the extent of ice slab areas using airborne ice-penetrating radar surveys collected by NASA's Operation IceBridge (OIB) campaigns (2010–2017). Our SMAP-derived maps show that between 2015 and 2019, perennial firn aquifer areas extended over 64 000 km2, and ice slab areas extended over 76 000 km2. Combined together, these sub-facies are the equivalent of 24 % of the percolation facies of the GrIS. As Greenland's climate continues to warm, seasonal surface melting will increase in extent, intensity, and duration. Quantifying the possible rapid expansion of these sub-facies using satellite L-band microwave radiometry has significant implications for understanding ice-sheet-wide variability in englacial hydrology that may drive meltwater-induced hydrofracturing and accelerated ice flow as well as high-elevation meltwater runoff that can impact the mass balance and stability of the GrIS.
... During the early years of OIB, the accumulation radar was operated as a step-frequency chirpedpulse radar with low-speed data converters (Table 7). It was later upgraded to operate as a chirped pulsed radar using high-speed data converters at a 50 kHz PRF (Lewis et al., 2015), followed by the current system, which directly generates and samples the signal's entire bandwidth. A pulsed chirp permits much higher peak Tx powers (400 W) and overall improved performance. ...
The National Aeronautics and Space Administration (NASA)’s Operation IceBridge (OIB) was a 13‐year (2009–2021) airborne mission to survey land and sea ice across the Arctic, Antarctic, and Alaska. Here, we review OIB’s goals, instruments, campaigns, key scientific results, and implications for future investigations of the cryosphere. OIB’s primary goal was to use airborne laser altimetry to bridge the gap in fine‐resolution elevation measurements of ice from space between the conclusion of NASA’s Ice, Cloud, and land Elevation Satellite (ICESat; 2003–2009) and its follow‐on, ICESat‐2 (launched 2018). Additional scientific requirements were intended to contextualize observed elevation changes using a multisensor suite of radar sounders, gravimeters, magnetometers, and cameras. Using 15 different aircraft, OIB conducted 968 science flights, of which 42% were repeat surveys of land ice, 42% were surveys of previously unmapped terrain across the Greenland and Antarctic ice sheets, Arctic ice caps, and Alaskan glaciers, and 16% were surveys of sea ice. The combination of an expansive instrument suite and breadth of surveys enabled numerous fundamental advances in our understanding of the Earth’s cryosphere. For land ice, OIB dramatically improved knowledge of interannual outlet‐glacier variability, ice‐sheet, and outlet‐glacier thicknesses, snowfall rates on ice sheets, fjord and sub‐ice‐shelf bathymetry, and ice‐sheet hydrology. Unanticipated discoveries included a reliable method for constraining the thickness within difficult‐to‐sound incised troughs beneath ice sheets, the extent of the firn aquifer within the Greenland Ice Sheet, the vulnerability of many Greenland and Antarctic outlet glaciers to ocean‐driven melting at their grounding zones, and the dominance of surface‐melt‐driven mass loss of Alaskan glaciers. For sea ice, OIB significantly advanced our understanding of spatiotemporal variability in sea ice freeboard and its snow cover, especially through combined analysis of fine‐resolution altimetry, visible imagery, and snow radar measurements of the overlying snow thickness. Such analyses led to the unanticipated discovery of an interdecadal decrease in snow thickness on Arctic sea ice and numerous opportunities to validate sea ice freeboards from satellite radar altimetry. While many of its data sets have yet to be fully explored, OIB’s scientific legacy has already demonstrated the value of sustained investment in reliable airborne platforms, airborne instrument development, interagency and international collaboration, and open and rapid data access to advance our understanding of Earth’s remote polar regions and their role in the Earth system.
... However, VHF/UHF bands for CubeSat based radar imaging have not yet been pursued due to the requirement of large antenna apertures. Concurrently, VHF/UHF frequencies are attractive for sensing ice thickness [7], soil moisture [8], vegetation biomass, and for exploring the nature of asteroids in our solar system [9]. Additionally, these frequencies can achieve greater propagation depths and penetrate through dense foliage [10]. ...
To date, CubeSat radars and imagers have been limited to operations beyond S-band due to the challenges associated with the design of wideband, compact, low-frequency antennas. Concurrently, the frequencies at VHF/UHF bands can image through clouds and foliage, and are very attractive for ice, water and biomass sensing. There is thus a need to develop wideband antennas that can operate at VHF/UHF bands which are low cost, light-weight, and packable. In this paper, we present a CubeSat deployable Tightly Coupled Dipole Array (TCDA) that achieves VSWR<3 from 80 MHz - 600 MHz, and VSWR>4 from 65 MHz - 600 MHz. While the array itself is 1.2m in length, 10cm in width and 1.5mm in thickness, it can be folded and stowed in a compact volume of 0.40U. We detail the design of the array, demonstrate the folding mechanism, and validate its performance through measurements.
... We also used an ultra-high-frequency (UHF) pulsed chirp radar developed by the Center for the Remote Sensing of Ice Sheets (CReSIS), operating over 300 MHz of bandwidth centered at 750 MHz, with a range resolution of 43 cm in ice Lewis and others, 2015). A 600-900 MHz linear FM chirp is transmitted at a 50 kHz pulse repetition frequency and directly captured using a 1 giga-sample per second analogue-to-digital converter. ...
... Radar profiles at 750 and 7 MHz along X-X' (Fig. 2) reveal a highly reflective and spatially continuous layer (hereafter referred to as Layer A) at both frequencies. Internal horizons are visible at depths below Layer A at 7 MHz, but only rarely at 750 MHz, likely because of the high sensitivity of UHF systems to scattering (Lewis and others, 2015) and steeply dipping layers (Holschuh and others, 2014). Numerous stacks of diffraction hyperbolas extend up to 300 m above the bed, where total ice thickness is ∼500 m. ...
Crary Ice Rise formed after the Ross Ice Shelf re-grounded ~1 kyr BP. We present new ice-penetrating radar data from two systems operating at center frequencies of 7 and 750 MHz that confirm the ice rise is composed of a former ice shelf buried by subsequent accumulation. Stacks of englacial diffraction hyperbolas are present almost everywhere across the central ice rise and extend up to ~350 m above the bed. In many cases, bed reflections beneath the diffraction hyperbolas are obscured for distances up to 1 km. Waveform modeling indicates that the diffraction hyperbolas are likely caused by marine ice deposits in former basal crevasses and rifts. The in-filling of rifts and basal crevasses may have strengthened the connection between the ice rise and the surrounding ice shelf, which could have influenced local and regional ice dynamics. Three internal reflection horizons mark the upper limit of disturbed ice and diffraction hyperbolas in different sections of the ice rise, indicating at least three stages of flow stabilization across the ice rise. A surface lineation visible in MODIS imagery corresponds spatially to deepening and strong deformation of these layers, consistent with the characteristics of former grounding lines observed elsewhere in Antarctica.
... Fujita et al. [48] demonstrated that reflecting horizons in the ice column shallower than 1000 m could reasonably be attributed to changes in density. Similarly, Lewis et al. [49] showed a strong correlation between the trend in the standard deviation of ice core density measurements and the relative magnitude of layer reflection coefficients derived from AR surveys. On a regional scale, high-resolution radar systems designed to investigate the near surface have shown clear evidence of continuous layering over horizontal scales of many kilometers in both Antarctica and Greenland [49]- [51]. ...
... Similarly, Lewis et al. [49] showed a strong correlation between the trend in the standard deviation of ice core density measurements and the relative magnitude of layer reflection coefficients derived from AR surveys. On a regional scale, high-resolution radar systems designed to investigate the near surface have shown clear evidence of continuous layering over horizontal scales of many kilometers in both Antarctica and Greenland [49]- [51]. ...
... If, instead, firn is predominately a stratified medium, the observed subsurface power profile is produced by the interference of quasi-specular reflections at interfaces between layers of different densities [49]. To test this hypothesis, we employ a 1-D layered dielectric medium model where the permittivity of each layer is directly proportional to its density following the empirical relationship in (4) where ρ is the layer density in g/cm 3 [60]: ...
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.
... The aquifer detections in spring 2010-2014 and 2017 were done using the Accumulation Radar (our preferred option), with a center frequency of 750 MHz and frequency bandwidth from 565 to 885 MHz. This radar system is used to generate echograms with a vertical sample interval (referred to as bin size) of ∼0.3 m in firn ( Lewis et al., 2015;Paden et al., 2014a). The surveys in spring 2015 and 2016 did not have the Accumulation Radar on board, and we instead used the Multichannel Coherent Radar Depth Sounder (MCoRDS) ( Paden et al., 2014b). ...
... The firn aquifer locations were obtained for each spring using the same methodology as in Miège et al. (2016). We merged the available firn aquifer data for 2010-2014 (total aquifer area of 21,900 km 2 ; Miège et al., 2016) with the more recent data from 2015. The 2010 polygon extents increased the total area to 29,268 km 2 . ...
Plain Language Summary
The Greenland ice sheet encloses vast amounts of liquid water beneath its surface, within a layer of old compacted snow referred to as firn. These water reservoirs, named firn aquifers, are a peculiar feature with potentially widespread impacts on ice sheet temperature, hydrology, and contribution to sea level. Monitoring aquifer areas is therefore critical in assessing the vulnerability of the Greenland ice sheet to climate change. Current knowledge on aquifer locations relies on aircraft flights of NASA Operation IceBridge, surveying only narrow stretches of the ice sheet along its flight lines. Satellite observations are necessary to provide full coverage. Here, we present the first satellite‐based map of Greenland's firn aquifers, using radar observations from the Sentinel‐1 constellation. The detection with Sentinel‐1 relies on a strong and prolonged absorption of the emitted radar signal by the liquid water within the firn, yielding low values in the measured return signal. Very good agreement is found when comparing Sentinel‐1 aquifer locations with those of Operation IceBridge. The total aquifer area estimated by Sentinel‐1 is 54,800 km 2. The long‐term continuity provided by the satellite observations offers great potential for the monitoring of changes in aquifer location and area related to climate change.
... In the wideband echogram, three distinct bands representing different firn densifications are present. Lewis and others (2015) demonstrated that the radar reflection coefficient in the top 100 m follows the same trend as the standard deviation of the ice core permittivity profile (Lewis and others, 2015). The first band extends from the surface to an approximate depth of 20 m, and it is apparent in the echogram due to the overall reduction in return power. ...
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.
