DOI: 10.5285/7c12898d-7e55-458c-ba7d-ecec8252f3b5
title: Bed, surface elevation and ice thickness measurements derived from radar data acquired during the Thwaites Glacier airborne survey (2019/2020)

description: As part of the International Thwaites Glacier Collaboration (ITGC) 4432 km of new radar depth sounding data was acquired over the Thwaites Glacier catchment by the British Antarctic Survey. Data was collected using the PASIN polametric radar system, fitted on the BAS aerogeophysical equipped survey aircraft VP-FBL. The survey operated from Lower Thwaites Glacier camp, and focused on collecting data in regions of ice &gt;1.5 km thick between 70 and 180 km from the grounding line. Additional profiles from the coast to the Western Antarctic Ice Sheet (WAIS) divide and over the eastern shear margin were also flown. Ice thicknesses between 418 and 3744 m were measured, with a minimum bed elevation of -2282 imaged. This dataset contains the navigation, surface elevation, ice thickness, and bed elevation data from the Thwaites Glacier 2019/20 season in the form of a CSV file. The Thwaites 2019/20 aerogeophysical survey was carried out as part of the BAS National Capability contribution to the NERC/NSF International Thwaites Glacier Collaboration (ITGC) program. Data processing was supported by the BAS Geology and Geophysics team.
description: The PASIN airborne radar system was run in polametric mode for the 2019/20 Thwaites field campaign, meaning the 4 antennas in each wing array were orientated at 90 degrees to each other. The belly antenna initially added an additional 4 antenna array operating in receive mode only. The belly antenna ceased to function after the initial input flight (T05). The PASIN system transmitted 5 separate pulses from the wing arrays as follows; Port 4 microseconds chirp; Starboard 4 microseconds chirp; Port 4 microseconds chirp 180deg phase shift; Starboard 4 microseconds chirp 180deg phase shift; Port 1 microseconds chirp. Data for every antenna and pulse is recorded separately, in 20 second segments. For picking the along-track bed elevation, only data from the port antenna array was used. Data from all the port antennas was combined, and the 4 microseconds and microseconds 180deg phase shift pulses were combined to enhance the array gain and minimise coherent noise. The use of the same transmit/receive array theoretically minimises the power loss due to cross polarisation of the opposite wing. Along-track SAR focusing was applied to the combined port wing data. The final data was output as a port to port (P2P) segy file. The segy files for each flight were then imported into the Promax seismic processing package. Down trace 'time' is simply the sample number across the 64 microseconds window, digitised at 120 MHz (i.e. max sample number = 7680 = 64 microseconds). Weighted trace mixing across 5 traces was applied to improve the signal to noise ratio of the data and down-trace automatic gain control was applied to reveal low amplitude returns from deeper reflectors. An initial window ~100 samples above the bed reflection was manually defined (top mute). An automated first break pick algorithm was then run to locate the precise bed return below the top mute. Subsequent manual picking removed un-realistic spikes, and selected the most physically appropriate bed surface in cases where multiple reflections were seen close to the bed. Generally the shallowest reflector was assumed to be the bed, as off-axis reflectors would likely appear later (deeper) in the section. In some cases strong reflectors which appeared deeper were chosen, with shallower week reflectors assumed to reflect entrained debris, accreted ice, or un-compensated refraction hyperbole close to the bed. The PASIN radar system does not resolve the ice surface well. Range to surface from coincident LIDAR data, or calculated from an accurate DEM is therefore preferred. However, to estimate ice thickness and hence correct bed elevation, the location of the surface reflector in the radargram must be known. To calculate the theoretical surface pick location from the LIDAR or DEM range to ground these measurements must be calibrated. To do this the ice surface location for a single flight (T05) was picked from the Port to Starboard (P2S) radar dataset. Use of this dataset avoided the problem of the transmit pulse and switching period overlapping with the surface reflection. The location of the surface reflector was picked in Promax following a similar approach to the bed, with an additional bottom mute defined ~100 samples below the surface reflection. The Promax surface pick was then plotted against the LIDAR range to ground and a linear trend fit to the data. The resulting slope and offset was used to calculate the theoretical location of the surface in all the subsequent radargrams from either LIDAR or DEM derived range to ground. Where possible the range to ground value was from LIDAR data, or interpolated from the mean Lidar elevation within ~700 m. Where no LIDAR data was available within ~1400 m horizontally the REMA DEM was used, with a smooth interpolation between the surface elevation data sources. To calculate ice thickness the difference between the bed pick and theoretical surface pick was calculated, the range in samples converted to microseconds, and the thickness in meters calculated assuming a radar velocity in ice of 168 m/microseconds. An additional 10 m correction was added to account for the assumed different radar velocity within the un-compacted firn layer. Survey locations and aircraft elevation were interpolated from 10Hz coupled Precise Point Positioning (PPP) GNSS/INS solutions processed one month after data acquisition to ensure best accuracy of satellite orbit definitions and atmospheric corrections. LIDAR data was extracted from the nadir point value from a Riegl Q240i-80 system. Note no correction for aircraft attitude has been applied, which increases the uncertainty in surface elevation. Where no LIDAR data was available the surface elevation from the REMA 8m DEM was used. DATA FORMAT: The dataset includes an intermediate processing product (in .csv), with attributes as follows: Line_ID: Flight name e.g. T05 Trace: Radar segy file trace number Longitude_decimal_degrees: Longitude in decimal degrees (WGS84 EPSG:4326) Latitude_decimal_degrees: Latitude in decimal degrees (WGS84 EPSG:4326) DateTime_YYYY-MM-DD_HH:MM:SS.SSS: Date (YYYY/MM/DD) and Time UTC (HH:MM:SS.SSS) of trace separated by 'T' Surface_elevation_WGS84_m: Surface Elevation (m) relative to WGS84 Ellipsoid Ice_thickness_m = Ice thickness (m) calculated for a radar velocity in ice of 168 microseconds and applying a 10-m firn, as follows: ((tice_time/120)*(168/2))+10 Bed_elevation_WGS84_m: Bedrock Elevation (m) relative to WGS84 Ellipsoid PriNumber: Radar counter in segy header (arbitrary units) Surface_pick: Calibrated position of surface reflection in radargram, calculated as: ((Elev-SurfElev_fill)*0.8048) + 227.4 Bed_pick: Edited pick of bed reflection position down trace from Promax (microseconds *120) Aircraft_elevation_WGS84_m: Elevation (m) of aircraft IMU relative to WGS84 Ellipsoid Campaign_name: Name of the campaign Owner: Owner of the data Funding: Funding providers Note: missing data values where no surface/bed pick/elevation exist are reported as '-9999'.
description: Data was collected using the BAS aerogeophysicaly equipped twin otter VP-FBL. The radar system was the PASIN polametric radar. ** Antenna configuration: 8 folded dipole elements: 4 transmitters (port side) 4 receivers (starboard side) Antenna gain: 11 dBi (with 4 elements) Transmit power: 1 kW into each 4 antennae Maximum transmit duty cycle: 10% at full power (4 x 1 kW) ** Radar receiver configuration: Receiver vertical sampling frequency: 22 MHz (resulting in sampling interval of 45.4546 ns) Receiver coherent stacking: 25 Receiver digital filtering: -50 dBc at Nyquist (11 MHz) Effective PRF: 312.5 Hz (post-hardware stacking) Sustained data rate: 10.56 Mbytes/second
description: Analysis of 52 crossover points within the survey area indicates a standard deviation for the bed elevation of ~22m, which is in-line with the values suggested for previous radar surveys. LIDAR-derived surface elevation has a crossover error of ~10 m. The relatively high value is attributed to the use of LIDAR nadir range to ground un-corrected for aircraft roll, pitch and yaw. - Line spacing: ~15 km (designed to inter-leave with previous surveys) - Trace spacing (post-processed data): ~24 m - Vertical resolution: ~8.4 m - Radar centre frequency: 150 MHz - Radar bandwidth: 12 MHz - Radar Receiver vertical sampling frequency: 22 MHz - Absolute GPS positional accuracy: ~0.1 m (relative accuracy is one order of magnitude better). Banking angle was limited to 10 deg during aircraft turns to avoid phase issues between GPS receiver and transmitter.