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<p>This is an example line of NSF COLDEX MARFA ice penetrating radar data (CLX/MKB2o/R66a) that has been processed to provide azimuthal information about radar echos from below, and to the front and back of the aircraft. The input was 1 meter slow time resampled coherent range record with phase intact. The data were pulse compressed and an azimuth fast Fourier transform was used to convert to azimuth angles in 1 km chunks, then slices at -19°, +19˚ and nadir were selected for these numpy arrays. These can be displayed as an RGB image with Blue = nadir, red = forward and green = rear</p> <p>The nadir slice should dominate specular echos, as seen with englacial reflecting horizons; where this trades to more balanced returns across all three channels, scattering dominates, as with rough bed rock or volume scattering. A gmt text file contains information about where this transition occurs in the ice column.</p> <p>Details in delay Doppler processing can be found in <a href="http://pds-geosciences.wustl.edu/mro/mro-m-sharad-5-radargram- v1/mrosh_2001/document/rgram_processing.pdf">Campbell et al., 2014</a>; the idea for using this approach for looking at englacial structure was discussed by <a href="https://doi.org/10.5194/egusphere-egu23-2856">Arenas-Pingarrón, Á. et al., 2023</a>. Details of HiCARs/MARFA focused processing can be found in <a href="http://dx.doi.org/10.1109/TGRS.2007.897416">Peters et al., 2007</a>.</p> 

<p><b> Introduction </b> <br> The National Science Foundations Center for Oldest Ice Exploration (<a href="https://www.coldex.org">NSF COLDEX</a>) is a Science and Technology Center working to extend the record of atmospheric gases, temperature and ice sheet history to greater than 1 million years. As part of this effort, NSF COLDEX has been searching for a site for a continuous ice core extending through the mid-Pleistocene transition. Two seasons of airborne survey were conducted from South Pole Station across the southern flank of Dome A. </p> <p><b> 2022-2023 Field Season </b> <br> In the 2022-20223 field season (CXA1), and using a BT-67 Basler, NSF COLDEX conducted 13 full flights and one weather abort from South Pole Station toward the southern flank of Dome C; as well as 1 survey flight toward Hercules Dome in support of the Hercules Dome Drilling project. Three test flights were conducted from McMurdo Station. Instrumentation included the <a href="https://doi.org/10.18738/T8/J38CO5">60 MHz MARFA ice penetrating radar </a> from the University of Texas Institute for Geophysics, a <a href="https://doi.org/10.1109/IGARSS53475.2024.10640448">UHF ice penetrating radar </a> from the Center for Remote Sensing and Integrated Systems; an GT-2 Gravimeter, and LD-90 laser altimeter and an G-823 Magnetometer. </p> <p><b> Basal specularity content </b> <br> These basal specularity content were derived from comparing 1D and 2D focused MARFA data (<a href="http://doi.org/10.1109/TGRS.2007.897416">Peters et al., 2007</a>). By comparing bed echo strengths for different focusing apertures, and accounting for the ranges and angles involved, we can derive the "specularity content" of the bed echo, a proxy for small scale bed roughness and a good indicator for subglacial water pressure in regions of distributed subglacial water (<a href="https://doi.org/10.1109/LGRS.2014.2337878">Schroeder et al., 2014, IEEE GRSL </a>, <a href="https://doi.org/10.1016/j.epsl.2019.115961">Dow et al., 2019, EPSL </a>) and smooth deforming bed material (<a href="http://doi.org/10.1002/2014GL061645">Schroeder et al., 2014, GRL</a>, <a href="http://dx.doi/org/10.1098/rsta.2014.0297">Young et al., 2016, PTRS</a>. Specularity data are inherently noisy, so these products have been smoothed with a 1 km filter.</p> 

<p><b> Introduction </b> <br> The National Science Foundations Center for Oldest Ice Exploration (<a href="https://www.coldex.org">NSF COLDEX</a>) is a Science and Technology Center working to extend the record of atmospheric gases, temperature and ice sheet history to greater than 1 million years. As part of this effort, NSF COLDEX has been searching for a site for a continuous ice core extending through the mid-Pleistocene transition. Two seasons of airborne survey were conducted from South Pole Station across the southern flank of Dome A. </p> <p><b> 2023-2024 Field Season </b> <br> In the 2023-2024 field season (CXA2), and using a BT-67 Basler, NSF COLDEX conducted 17 flights from South Pole Station toward the southern flank of Dome C. Three test flights were conducted from McMurdo Station. Instrumentation included the <a href="https://doi.org/10.18738/T8/J38CO5">60 MHz MARFA ice penetrating radar </a> from the University of Texas Institute for Geophysics, a <a href="https://doi.org/10.1109/IGARSS53475.2024.10640448">UHF ice penetrating radar </a> from the Center for Remote Sensing and Integrated Systems; an GT-2 Gravimeter, and LD-90 laser altimeter and an G-823 Magnetometer. </p> <p><b> Basal specularity content </b> <br> These basal specularity content were derived from comparing 1D and 2D focused MARFA data (<a href="http://doi.org/10.1109/TGRS.2007.897416">Peters et al., 2007</a>). By comparing bed echo strengths for different focusing apertures, and accounting for the ranges and angles involved, we can derive the "specularity content" of the bed echo, a proxy for small scale bed roughness and a good indicator for subglacial water pressure in regions of distributed subglacial water (<a href="https://doi.org/10.1109/LGRS.2014.2337878">Schroeder et al., 2014, IEEE GRSL </a>, <a href="https://doi.org/10.1016/j.epsl.2019.115961">Dow et al., 2019, EPSL </a>) and smooth deforming bed material (<a href="http://doi.org/10.1002/2014GL061645">Schroeder et al., 2014, GRL</a>, <a href="http://dx.doi/org/10.1098/rsta.2014.0297">Young et al., 2016, PTRS</a>. Specularity data are inherently noisy, so these products have been smoothed with a 1 km filter.</p> 

Code for processing data and plotting figures for the paper "Dome A basal ice truncated at an extensive geologic dichotomy in the South Pole Basin of East Antarctica". 

<p>NSF COLDEX performed two airborne campaigns from South Pole Station over the Southern Flank of Dome A and 2022-23 and 2023-24, searching for a potential site of a continuous ice core that could sample the mid-Pleistocene transition. Ice thickness data extracted from the MARFA radar system has allow for a new understanding of this region.</p> <p>Here we generate crustal scale maps of ice thickness, bed elevation, specularity content, subglacial RMS deviation and fractional basal ice thickness with 1 km sampling, and 10 km resolution. We include both masked and unmasked grids.</p> <p> The projection is in the SCAR standard ESPG:3031 polar stereographic projection with true scale at 71˚S.</p> <p>These geotiffs were generated using performed using GMT6.5 (<a href="https://doi.org/10.1029/2019GC008515">Wessel et al., 2019</a>) using the pygmt interface, by binning the raw data to 2.5 km cells, and using the <a href="https://github.com/sakov/nn-c"> nnbathy </a> program to apply natural neighbor interpolation to 1 km sampling. A 10 km Gaussian filter - representing typical lines spacings - was applied and then a mask was applied for all locations where the nearest data point was further than 8 km. </p> Ice thickness, bed elevation and RMS deviation @ 400 m length scale (<a href="http://dx.doi.org/10.1029/2000JE001429">roughness</a>) data includes the following datasets: <ul> <li> UTIG/CRESIS <a href="https://doi.org/10.18738/T8/J38CO5">NSF COLDEX Airborne MARFA data</a></li> <li> British Antarctic Survey <a href="https://doi.org/10.5285/0f6f5a45-d8af-4511-a264-b0b35ee34af6">AGAP-North</a></li> <li> LDEO <a href="https://doi.org/10.1594/IEDA/317765"> AGAP-South </a></li> <li> British Antarctic Survey <a href="https://doi.org/10.5270/esa-8ffoo3e">Polargap</a></li> <li> UTIG Support Office for Airborne Research <a href="https://doi.org/10.15784/601588">Pensacola-Pole Transect (PPT) </a></li> <li> NASA/CReSIS <a href="https://doi.org/10.5067/GDQ0CUCVTE2Q"> 2016 and 2018 Operation Ice Bridge </a> </li> <li> ICECAP/PRIC <a href="https://doi.org/10.15784/601437"> SPICECAP Titan Dome Survey </a> </ul> <p>Specularity content (<a href="https://doi.org/10.1109/LGRS.2014.2337878">Schroeder et al. 2014</a>) is compiled from <a href="https://doi.org/10.18738/T8/KHUT1U"> Young et al. 2025a </a> and <a href="https://doi.org/10.18738/T8/6T5JS6"> Young et al. 2025b</a>.</p> <p>Basal ice fractional thickness is complied from manual interpretation by Vega Gonzàlez, Yan and Singh. </p> <p>Code to generated these grids can be found at <a href="https://github.com/smudog/COLDEX_dichotomy_paper_2025"> at github.com </a></p> 

These transect projected radargrams were collected as part of the Center for Oldest Ice Exploration (COLDEX) Science and Technology Center (https://www.coldex.org) in the 2022/23 (CXA1) and 2023/24 (CXA2) airborne field seasons. The raw 3 TB data is deposited at the USAP data center at https://doi.org/10.15784/601768. The set of images in this archive was designed for easy, non expert, access to radargrams, organized according to survey design. <p> The science goal was to characterize the ice sheet between Antarctica's Dome A and Amundsen Scott South Pole Station, to locate sites of interest for the drilling of an ice core with ages spanning the mid-Pleistocene. The radar was deployed on Balser C-FMKB, and flown at ranges of up to 800 km from South Pole Station at velocities of 90 m/s and typical altitude above ground of 600 m. Other instruments included a UHF array system provided by the University of Kansas, a gravity meter, a magnetometer, a laser altimeter, and multiple global navigation satellite systems receivers. The radar data is used for finding ice thickness, bed character, englacial structure and surface assessment. <p> <b>Dataset organization</b> Transects are provided a P/S/T nomenclature, organized by the Project they are flying in, the acquisition System (typically named after the aircraft) and the Transect within the Project. <p> Transects were collected in preplanned systems with the following parameters (examples below): <p> <i>The CLX radials</i> (CLX/MKB##/R###), attempting to emulate flow lines from Dome A and radiating (in the EPSG:3031 polar stereographic projection) from easting 965 km northing 385 km, with a separation of 0.25 degrees. <p> <i>The CLX corridor</i> (CLX/MKB##/X###) rotated from the EPSG:3031 polar stereographic projection at -150 degrees and separated by 10 km in the Y direction and 3.75 km in the X direction <p> <i>The CLX2 corridor</i> (CLX2/MKB##/X###) rotated from the EPSG:3031 polar stereographic projection at -150 degrees and separated by 2.5 km in its Y direction and 2.5 km in its X direction <p> <i>The NPXE radials</i> (NPXE/MKB##/R###) radiating (in the EPSG:3031 polar stereographic projection) from easting 0 km and northing 0 km (ie South Pole), with a separation of 2 degrees. <p> <i>The SAD corridor</i> (SAD/MKB##/X###|Y####) designed to characterize the Saddle region near South Pole approximately perpendicular to the flow lines, rooted from the EPSG:3031 polar stereographic projection at -73.8 degrees and separated by 2.5 km in its Y direction and 2.5 km in the its X direction <p> <i>Untargeted transit lines</i> used the name of the expedition (CXA1) as the project, and used the flight and the increment within the flight to name the Transect (eg (CXA1/MKB2n/F10T02a). <p> <b>Processing</b> These images were processed using the CReSIS Synthetic Aperture Radar Processor (CSARP), as part of the Open Polar Radar Effort. Data were processed using pulse compression and matched filter approach for focusing optimized for producing data with 25 m along track sampling. Radio Frequency Interference was partially removed. See the Open Polar Radar server for more detail. <p> <b>Data format</b> Radar data is provided in three formats: <p> <i>Browse</i> data in PNG format are provided with marked axis depth projected, correcting for the velocity of ice, and projected along track into consistent project coordinates. Turns are trimmed off. Long transects are projected to ~30x vertical exaggeration, shorter transects have constant size. <p> <i>Image</i> data in grayscale JPEG format are provided without ornamentation. but are depth projected, correcting for the velocity of ice, and projected along track into consistent project coordinates. Turns are trimmed off. All images have a constant vertical scale of 1.69 m/pixel and horizontal scale of 25 m per pixel. The minimum black value corresponds to -140 dB, and the maximum white value corresponds to 0 dB, for a resolution of ~0.5 dB. Use of this data for radiometric interpretation has not been validated. <p> <i>Metadata</i> is provided in in comma delimited csv format. Columns included: <p> CSARP record (the number of record or trace in the original flight based processing<br> UNIX time [s] (seconds from midnight January 1, 1970, with no leap seconds) <br> Longitude [degrees] (WGS-84) <br> Latitude [degrees] (WGS-84) <br> Aircraft Elevation [m] (WGS-84) <br> Surface Echo Delay [s] (time delay between surface echo and transmission) <br> Roll [degrees] (right wing down positive) <br> Pitch [degrees] (nose down positive) <br> Heading [degrees] (right of North) <br> EPSG 3031 Easting [m] (projected coordinate) <br> EPSG 3031 Northing [m] (projected coordinate) <br> displayed_distance [km] (x-axis distance) <br> surface_elevation [m] (radar estimate surface elevation, WGS-84)<br> blanking [px] (sampled (blanked above surface return)<br> Elevation of image top [m] (WGS-84 elevation of the top of the projected image) <br> Elevation of image bottom [m] (WGS-84 elevation of the bottom of the projected image) <br> <p> A summary csv file is provided with transect name, start and end points in geographic and projected coordinates, and projection. <p> <b>Acknowledgements</b> This work was supported by the Center for Oldest Ice Exploration, an NSF Science and Technology Center (NSF 2019719). We thank the NSF Office of Polar Programs, the NSF Office of Integrative Activities, and Oregon State University for financial and infrastructure support, and the NSF Antarctic Infrastructure and Logistics Program, and the Antarctic Support Contractor for logistical support. Additional support was provided by the G. Unger Vetlesen Foundation and the NSF-sponsored Open Polar Radar project (NSF 2126503 & 2127606). 

This dataset contains the basal ice unit thickness as measured by the NSF COLDEX MARFA ice-penetrating radar survey, which mainly focuses on the southern flank of Dome A. The "basal ice unit" is hereby defined as the bottom portion of the ice sheet where no clear and traceable englacial reflection is detected by the radar sounder. Raw radar data can be found at: https://doi.org/10.15784/601768. The basal ice unit is mapped using the DecisionSpace Geosciences 10ep software package. This dataset provides three data products: • Thickness of the basal ice unit • Thickness of the stratigraphic ice unit above the basal ice unit • The shape of the basal ice unit boundary, where rapid basal ice unit thinning is observed in the middle of the South Pole Basin. 

Abstract A major subglacial lake, Lake Snow Eagle (LSE), was identified in East Antarctica by airborne geophysical surveys. LSE, contained within a subglacial canyon, likely hosts a valuable sediment record of the geological and glaciological changes of interior East Antarctica. Understanding past lake activity is crucial for interpreting this record. Here, we present the englacial radiostratigraphy in the LSE area mapped by airborne ice-penetrating radar, which reveals a localized high-amplitude variation in ice unit thickness that is estimated to be ∼12 ka old. Using an ice-flow model that simulates englacial stratigraphy, we investigate the origin of this feature and its relationship to changes in ice dynamical boundary conditions. Our results reveal that local snowfall redistribution initiated around the early Holocene is likely the primary cause, resulting from a short-wavelength (∼10 km) high-amplitude (∼20 m) ice surface slope variation caused by basal lubrication over a large subglacial lake. This finding indicates an increase in LSE water volume during the Holocene, illustrating the sensitivity in volume of a major topographically constrained subglacial lake across a single glacial cycle. This study demonstrates how englacial stratigraphy can provide valuable insight into subglacial hydrological changes before modern satellite observations, both for LSE and potentially at other locations. 

These transect organized radargrams were collected as part of the Center for Oldest Ice Exploration (COLDEX) Science and Technology Center (https://www.coldex.org) in the 2023/24 (CXA2) airborne reconnaissance field season. The raw 3 TB data is deposited at the USAP data center at https://doi.org/10.15784/601768. Flight organized data with additional processing by the University of Kansas to remove electromagnetic interference can be found at the Open Polar Radar server (https://www.openpolarradar.org). The science goal was to characterize the ice sheet between Antarctica's Dome A and Amundsen Scott South Pole Station, to locate sites of interest for the drilling of an ice core with ages spanning the mid-Pleistocene. The radar was deployed on Balser C-FMKB, and flown at ranges of up to 800 km from South Pole Station at velocities of 90 m/s and typical altitude above ground of 600 m. Other instruments included a UHF array system provided by the University of Kansas, a gravity meter, a magnetometer, a laser altimeter, and multiple global navigation satellite systems receivers. The radar data is used for finding ice thickness, bed character, englacial structure and surface assessment. Dataset organization Transects are provided a P/S/T nomenclature, organized by the Project they are flying in, the acquisition System (typically named after the aircraft) and the Transect within the Project. Transects were collected in preplanned systems with the following parameters: CLX radials (CLX/MKB##/R###), attempting to emulate flow lines from Dome A and radiating (in the EPSG:3031 polar stereographic projection) from easting 965 km northing 385 km, with a separation of 0.25 degrees. CLX corridor (CLX/MKB##/X###) rotated from the EPSG:3031 polar stereographic projection at -150 degrees and separated by 10 km in the Y direction and 3.75 km in the X direction CLX2 corridor (CLX2/MKB##/X###) rotated from the EPSG:3031 polar stereographic projection at -150 degrees and separated by 2.5 km in its Y direction and 2.5 km in its X direction NPXE radials (NPXE/MKB##/R####) primarily designed to survey the Upper Byrd Glacier Catchment, constitute spokes radiating from South Pole separated by 2 degrees, in the EPSG:3031 polar stereographic projection Untargeted transit lines used the name of the expedition (CXA2) as the project, and used the flight and the increment within the flight to name the Transect (eg (CXA2/MKB2n/F10T02a). Processing These data represent range compressed VHF radargrams as collected and analyzed in the field. The data are from the MARFA radar system, a 60 MHz ice penetrating radar system that has operated in several different guises over the years. MARFA operates with a 1 microsecond chirp with a design bandwidth of 15 MHz, allowing for ~8 meter range resolution. The record rate after onboard stacking is 200 Hz. High and low gain channels are collected from antennas on each side of the aircraft. In ground processing, the data were stacked 10x coherently to reduce range delayed incoherent surface scattering, and then stacked 5 times incoherently to improve image quality. In this preliminary processing, the effective resolution of deep scattering is several hundred meters due to range ambiguities at depth. Data format These data collection represents georeferenced, time registered instrument measurements (L1B data) converted to SI units. The data format are netCDF3 files, following the formats used for NASA/AAD/UTIG's ICECAP/OIB project at NASA's NSIDC DAAC (10.5067/0I7PFBVQOGO5). Metadata fields can be accessed using the open source ncdump tool, or c, python or matlab modules. A Keyhole Metadata Language (KML) file with geolocation for all transects is also provided. See https://www.loc.gov/preservation/digital/formats/fdd/fdd000330.shtml for resources on NetCDF-3, and https://nsidc.org/data/IR2HI1B/versions/1 for a description of the similar OIB dataset. Acknowledgements This work was supported by the Center for Oldest Ice Exploration, an NSF Science and Technology Center (NSF 2019719). We thank the NSF Office of Polar Programs, the NSF Office of Integrative Activities, and Oregon State University for financial and infrastructure support, and the NSF Antarctic Infrastructure and Logistics Program, and the Antarctic Support Contractor for logistical support. Additional support was provided by the G. Unger Vetlesen Foundation. 

These laser altimetry data were collected as part of the 2023-24 <a href="https://www.coldex.org">NSF COLDEX </a> CXA2 airborne campaign targeting the southern flank of East Antarctica's Dome A. In this Level 2 product, we have used the laser range to the surface and complementary aircraft position data to calculated the ice surface elevation, which is an important constraint on ice flow. Complementary radar, gravity, magnetics and imagery were also collected. <p> <i>Data format:</i>Data are formatted as text files with a header and the following tab delimited format columns. Data are in the same format as similar <a href="https://doi.org/10.5067/JV9DENETK13E"> IceBridge ILUTP2 altimetry data. </a> <p> Field 1: Year (UTC)<br> Field 2: Day of year (UTC)<br> Field 3: Second of day (UTC)<br> Field 4: Longitude Angle (deg) (WGS-84) <br> Field 5: Latitude Angle (deg) (WGS-84)<br> Field 6: Laser Derived Surface Elevation (m) (WGS-84)<br> <p> Missing values have been replaced by "nan". The effective footprint of the laser data is 25 m along track by 1 meter across track. Some cloud filtering was performed. <p> <i>Uncertainties</i>: A comparison of this laser altimetry dataset north of 87.5˚S with the <a href="https://doi.org/10.7910/DVN/EBW8UC"> REMAv2 </a>100 m mosaic digital terrain model indicate a median bias of 17 cm and a root mean squared (RMS) difference of 20 cm. Intersections between profiles within this survey, on the Antarctic Plateau but away from South Pole Station, have RMS differences of 6.8 cm. <p> <i>Datum: </i>WGS-84 ellipsoid; ITRF 2008 <p> <i>Geolocation: </i>Positioning and orientation for CXA1 came from loosely coupled joint PPP/inertial solutions using a Novatel OEM-4 GPS receiver and an iMAR FSAS IMU. <p> <i>Pointing bias: </i> roll: 0.340 degrees; pitch: -0.505 degrees <br> Pointing angle (pointing bias) is the angular offset of the downward-pointing laser boresight respect to the vehicle body frame's vertical (Z) axis. This estimated angle is derived by comparing measurements at crossovers. Pointing angle is provided in the vehicle body frame, using the laser origin for the rotation node. A positive pitch rotation indicates that the laser beam intersects the ground forward of the z-axis. A positive roll rotation indicates that the laser beam intersects the ground left of the z-axis. Pointing biases were found using the minimization of cross over difference method from <a href="http://dx.doi.org/10.3189/2015JoG14J048">Young et al., 2015</a>. <p> <i>Level arm: </i>X: 0 m; Y: 0.2 m; Z: -0.22 m <br> The lever arm is the position of the laser origin relative to the aircraft position solution, estimated using crossover-error minimization. Lever arm is provided in the vehicle body frame, with +X is forward, +Y is right, and +Z is down. Lever arm was measured after installation in the field. <p> <i>GNSS_antenna: </i>AeroAntenna AT1675-17W-TCNF-000-RG-36-NM <br> The coordinate system for the laser-gps lever arm is X forward, Y right, and Z down, from the center of position.