DOI: 10.5285/6be0a803-78d4-4ca9-be02-3838866763c3
title: Processed airborne radio-echo sounding data from the POLARGAP survey covering the South Pole, and Foundation and Recovery Glaciers, East Antarctica (2015/2016) - VERSION 2

description: During the austral summer of 2015/16, a major international collaboration funded by the European Space Agency (ESA) and with in-kind contribution from the British Antarctic Survey, the Technical University of Denmark (DTU), the Norwegian Polar Institute (NPI) and the US National Science Foundation (NSF), acquired ~38,000 line km of aerogeophysical data. The primary objective of the POLARGAP campaign was to carry out an airborne gravity survey covering the southern polar gap of the ESA gravity field mission GOCE, beyond the coverage of the GOCE orbit (south of 83.5degS), however aeromagnetics and ice-penetrating radar data were also opportunistically acquired. This survey covers the South Pole and Recovery Lakes, as well as parts of the Support Force, Foundation and Recovery Glaciers.

Our Twin Otter aircraft was equipped with dual-frequency carrier-phase GPS for navigation, radar altimeter for surface mapping, wing-tip magnetometers, an air-sea gravity meter, and a new ice-sounding radar system (PASIN-2).

We present here the full radar dataset consisting of the deep-sounding chirp and shallow-sounding pulse-acquired data in their processed form, as well as the navigational information of each trace, the surface and bed elevation picks, ice thickness, and calculated absolute surface and bed elevations. This dataset comes primarily in the form of NetCDF and georeferenced SEGY files. To interactively engage with this newly-published dataset, we also created segmented quicklook PDF files of the radar data.

This is Version 2 of the dataset. This version differs from Version 1, as follows:
1. The variables "fast_time" and "UTC_time_layerData" have been updated due to errors found. The error in the fast_time variable related to an error in the sampling frequency of the system, which should have been 24 MHz instead of 22MHz. This has been updated. The error in the "UTC_time_layerData" related to a rounding issue which affected the precision of this variable. This has been updated.
2. The units in the "surface_pick_layerData" and the "bed_pick_layerData" variables should have been "samples relative to the BAS radar system", instead of "microseconds". This has been corrected.
3. The metadata in this DMS entry and in the NetCDF files has also been updated. Mainly, the sampling frequency has been modified from 22 MHz to 24 MHz to reflect the radar system characteristics. This also affected the value provided for the radar system resolution and sampling interval, which have both been updated in the metadata.
4. The SEGY sampling interval value (byte numbers: 117-118 (SI)) has also been updated to reflect the change in sampling frequency mentioned above.

All other variables remain unchanged. Note that these changes do not affect the radar data or the associated radar-derived data in the files.
description: ** Instrumentation and Processing:
Radar data were collected using the new bistatic PASIN-2 radar echo sounding system operating in full polarimetric mode and mounted on the BAS Twin Otter aircraft "VP-FBL" and operating with a centre frequency of 150 MHz and using a 4-microseconds, 13 MHz bandwidth linear chirp (deep sounding).

Chirp compression was applied using a Blackman window to minimise sidelobe levels, resulting in a processing gain of 10 dB. The chirp data was processed using a coherent averaging filter (commonly referred to as unfocused Synthetic Aperture Radar (SAR) processing) with Doppler beam sharpening to enhance the signal to clutter ratio of the bed echo and improve visualisation. The received chirp of 4 microseconds, 13 MHz bandwidth data was compressed, filtered, and decimated from the original trace acquisition rate of 156.25 Hz to 2Hz, equivalent to ~30m in along-track spacing. The chirp data is best suited to assess the bed and internals in deep ice conditions. The coherent pulse-data (0.1-microseconds) was processed using a coherent averaging filter. This data is best used to assess the internal structure and bed in shallow ice conditions. Note that the chirp and pulse products for this survey are not the same size (see below for more details).

The bed reflector was first automatically depicted on the chirp data using a semi-automatic picker in the PROMAX software package. All the picks were afterwards checked and corrected by hand if necessary. The picked travel time was then converted to depth using a radar wave speed of 168 m/microseconds and a constant firn correction of 10 m. Where possible, the ice surface location within the radargrams was calculated using lidar measurements of surface elevation. In areas where lidar data was not available (principally due to terrain clearance, cloud or where the lidar was not used), the location of the surface reflection was picked directly from the radargram and a regression, local to the data gap, was used to fit the radar range to terrain clearance. The origin of the elevation measurement can be found in the 'surface_source_layerData' variable in this NetCDF, and is composed of either: 0 = LIDAR, 1 = interpolated LIDAR, 2 = radar. Bed elevation was integrated with a high precision kinematic dual-frequency GPS position solution to provide the final point data set of ice thickness and bed elevation relative to WGS84.

Please note: An uncontrolled shutdown of the radar system at the end of flight line P22 resulted in damage to all connected amplifiers in the radar receiver. This issue was repaired in the field using spare amplifier channels and a reconfiguration of the radar system. As a result, there is no line P23 and another pulse was used to pick the ice surface after flight line P22. There is also no polarised data from flight P24 onwards.

For the polarised data (flights P03-P22), the file name convention for the chirp and pulse radar variables in the NetCDF relate to the orientation and set up of the antennae:
PPVV: Port to Port, Vertical to Vertical, 4us Chirp
SSHH: Starboard to Starboard, Horizontal to Horizontal, 4us Chirp
SPHV: Starboard to Port, Horizontal to Vertical, 1us Pulse 

** Coordinates and Positions:
The coordinates provided in the NetCDF for the surface and bed elevation for each radar trace are in longitude and latitude (WGS84, EPSG: 4326). The navigation attributes for the radar data in the NetCDF are in projected X and Y coordinates (Polar Stereographic, EPSG: 3031), as follows:

Latitude of natural origin: -71
Longitude of natural origin: 0
Scale factor at natural origin: 0.994
False easting: 0
False northing: 2082760.109

The coordinates in the SEGY data are also in projected X and Y coordinates (Polar Stereographic, EPSG: 3031), although note that these are in integer format due to the SEGY limitations (see section below).

Positions are calculated for the phase centre of the aircraft antenna. All positions (Longitude, Latitude and Height) are referred to the WGS1984 ellipsoid.

** Dataset:
Please note: Due to the unstable nature of SEGY-formatted data and its uncertain long-term future, as well as the issues documented below, we also provide the full radar data in NetCDF format. The dataset provided here consists of three parts: a NetCDF file per flightline, three SEGY files per flightline (two chirp and one pulse) for the polarised flights (P03-P22) or two SEGY files per flightline (one chirp and one pulse) for the non-polarised flights (P24-P36), and one quicklook PDF file per flightline. These are described in more details below.

- NetCDF: The NetCDF files contain the processed deep-sounding chirp and shallow-sounding pulse-acquired data in their processed form, as well as the associated metadata, navigational information (in both EPSG: 3031 and WGS84 EPSG: 4326), and the associated radar-related information for each trace (e.g. surface/bed elevation and picks, ice thickness, aircraft altitude, range to surface, time of trace) which are provided as separate attributes in the NetCDF file. The navigational position of each trace comes from the surface files, and the processed GPS files when no surface information was provided or when duplicates were found in the surface file (see Quality section above). Note that for these, interpolation of the navigational data might have been required to match closely the Coordinated Universal Time (UTC) of each trace in the surface files. No data is shown as "-9999" throughout the files.

Please note: Care must be taken when comparing both the chirp and pulse radar data arrays due to varying lengths. Due to pulse data being processed with a greater amount of decimation compared with the chirp data, the trace number differs between both data sets with the consequence that the pulse data contains more elements than the chirp. However note that both chirp and pulse products start and end at the same XY location. The only way to compare the chirp and pulse radar products is to use the 2x PriNumber variables ('PriNumber_chirp' and 'PriNumber_pulse') stored in the NetCDF files. The other one dimensional variables ending in "_layerData" in the NetCDF are aligned to the 'chirp_data' variable (see below for more details).

NetCDF attributes:
Polarised:
- 'traces_chirp': Trace number for the chirp radar data (x axis)
- 'traces_pulse': Trace number for the pulse radar data (x axis)
- 'fast_time': Two-way travel time (y axis) (units: microseconds)
- 'x_coordinates': Cartesian x-coordinates for the radar data (x axis) (units: meters in WGS84 EPSG:3031) 
- 'y_coordinates': Cartesian y-coordinates for the radar data (x axis) (units: meters in WGS84 EPSG:3031) 
- ''''polarised_chirp_PPVV_data': Radar data for the processed (coherent) polarised (PPVV) chirp (units: power in dBm)
- 'polarised_chirp_SSHH_data'': Radar data for theprocessed (coherent) polarised (SSHH) chirp (units: power in dBm)
- 'polarised_pulse_SPHV_data': Radar data for the processed (coherent) polarised (SPHV) pulse (units: power in dBm)
- 'PriNumber_chirp': Incremental integer reference number related to initialisation of the radar system that permits processed chirp SEGY data and picked surface and bed to be linked back to raw radar data (also known as PriNum) (units: arbitrary - integers)
- 'PriNumber_pulse': Incremental integer reference number related to initialisation of the radar system that permits processed pulse SEGY data and picked surface and bed to be linked back to raw radar data (also known as PriNum) (units: arbitrary - integers)
- 'longitude_layerData': Longitudinal position of the trace number (units: degree_east in WGS84 EPSG:4326)
- 'latitude_layerData': Latitudinal position of the trace number (units: degree_north in WGS84 EPSG:4326)
- 'UTC_time_layerData': Coordinated Universal Time (UTC) of trace (also known as resTime) (units: seconds)
- 'terrainClearanceAircraft_layerData': Terrain clearance distance from platform to air interface with ice, sea or ground (also known as resHt) (units: meters)
- 'aircraft_altitude_layerData': Aircraft altitude (also known as Eht) (units: meters relative to WGS84 ellipsoid)
- 'surface_source_layerData'': Origin of the elevation measurement with 0 = LIDAR, 1 = interpolated LIDAR, 2 = radar
- 'surface_altitude_layerData': Ice surface elevation for the trace number from radar altimeter and LiDAR (units: meters relative to WGS84 ellipsoid)
- '''surface_pick_layerData': Location down trace of surface pick (BAS system) (units: samples relative to the BAS radar system)
- 'bed_altitude_layerData': Bedrock elevation for the trace number derived by subtracting ice thickness from surface elevation (units: meters relative to WGS84 ellipsoid)
- 'bed_pick_layerData': Location down trace of bed pick (BAS system) (units: samples relative to the BAS radar system)
- 'land_ice_thickness_layerData': Ice thickness for the trace number obtained by multiplying the two-way travel-time between the picked ice surface and ice sheet bed by 168 m/microseconds and applying a 10 meter correction for the firn layer (units: meters)

Non-polarised:
- 'traces_chirp': Trace number for the chirp radar data (x axis)
- 'traces_pulse': Trace number for the pulse radar data (x axis)
- 'fast_time'': Two-way travel time (y axis) (units: microseconds)
- 'x_coordinates': Cartesian x-coordinates for the radar data (x axis) (units: meters in WGS84 EPSG:3031) 
- 'y_coordinates': Cartesian y-coordinates for the radar data (x axis) (units: meters in WGS84 EPSG:3031) 
- 'chirp_data': Radar data for the processed (coherent) chirp (units: power in dBm)
- 'pulse_data': Radar data for the processed (coherent) pulse (units: power in dBm)
- 'PriNumber_chirp': Incremental integer reference number related to initialisation of the radar system that permits processed chirp SEGY data and picked surface and bed to be linked back to raw radar data (also known as PriNum) (units: arbitrary - integers)
- 'PriNumber_pulse': Incremental integer reference number related to initialisation of the radar system that permits processed pulse SEGY data and picked surface and bed to be linked back to raw radar data (also known as PriNum) (units: arbitrary - integers)
- 'longitude_layerData': Longitudinal position of the trace number (units: degree_east in WGS84 EPSG:4326)
- 'latitude_layerData': Latitudinal position of the trace number (units: degree_north in WGS84 EPSG:4326)
- 'UTC_time_layerData': Coordinated Universal Time (UTC) of trace (also known as resTime) (units: seconds)
- 'terrainClearanceAircraft_layerData': Terrain clearance distance from platform to air interface with ice, sea or ground (also known as resHt) (units: meters)
- 'aircraft_altitude_layerData': Aircraft altitude (also known as Eht) (units: meters relative to WGS84 ellipsoid)
- 'surface_source_layerData': Origin of the elevation measurement with 0 = LIDAR, 1 = interpolated LIDAR, 2 = radar
- 'surface_altitude_layerData': Ice surface elevation for the trace number from radar altimeter and LiDAR (units: meters relative to WGS84 ellipsoid)
- 'surface_pick_layerData': Location down trace of surface pick (BAS system) (units: samples relative to the BAS radar system)
- 'bed_altitude_layerData': Bedrock elevation for the trace number derived by subtracting ice thickness from surface elevation (units: meters relative to WGS84 ellipsoid)
- 'bed_pick_layerData': Location down trace of bed pick (BAS system) (units: samples relative to the BAS radar system)
- 'land_ice_thickness_layerData': Ice thickness for the trace number obtained by multiplying the two-way travel-time between the picked ice surface and ice sheet bed by 168 m/microseconds and applying a 10 meter correction for the firn layer (units: meters)

- SEGY: The SEGY files are provided for the processed-chirp and pulse-acquired data and have been georeferenced using the navigational position of each trace from the surface files, and the processed GPS files when no surface information was provided in the surface files. Note that for these, interpolation of the navigational data might have been required to match closely the Coordinated Universal Time (UTC) of each trace in the surface files.

Please note: Care must be taken when comparing the chirp and pulse SEGY data directly. Due to issues mentioned above, the chirp and pulse data have a different amount of traces and thus different lengths. However note that both chirp and pulse products start and end at the same XY location. Thus, use the XY coordinates associated to each trace for direct comparison of the two SEGY files (if using seismic software in geospatial window), or the 2x PriNumber variables ('PriNumber_chirp' and '''PriNumber_pulse') in the NetCDF if reading in the files programmatically. To associate the XY information found in the NetCDF into each trace of the pulse radar product, one must use the nearest PriNumber from the pulse SEGY with the PriNumber from the chirp SEGY and find the associate XY values for that specific PriNumber. Note that this issue does not affect the other NetCDF 1-D variables ending in '''_layerData'' as they are aligned to the chirp data.

SEGY header description:
- byte number 1-4 and 5-8 (SEQWL and SEQWR): Trace number for the SEGY
- byte number 9-12 (FFID): PriNumber for each SEGY trace
- byte number 73-76 (SRCX): Cartesian x-coordinates for each SEGY trace (units: meters in WGS84 EPSG:3031)
- byte number 77-80 (SRCY): Cartesian y-coordinates for each SEGY trace (units: meters in WGS84 EPSG:3031)
- byte number 115-116 (NSMP): Number of samples for each SEGY trace
- byte number 117-118 (SI): Sampling interval for each SEGY trace

Note that the current version of the SEGY (Revision 1.0) does not yet allow to store double-precision floats in the "Source X/Y" trace headers and thus the X and Y positions for each trace are rounded to the nearest integer when exporting the data. This will affect the accurate position of each trace in the SEGY data, however the precise X and Y position of each trace can be obtained from the NetCDF files if necessary. When loading in the georeferenced SEGY files into seismic-interpretation software for data visualisation and analysis, the user might be warned that duplicate traces are found within the data and that this might cause "bad performance". This is caused by the rounding of the X and Y positions in the SEGY headers as explained above and should only affect the position of a relatively small amount of traces.

- Quicklook: The quicklook PDF files were produced to allow for a quick visualisation of the radar data and the position of each flightline with regards to the rest of the survey flightlines. The radar image in the PDF is from the processed chirp radar data and is split into 25-km segments for the POLARGAP survey. These segments (and the radar images associated with them) are the same as those shown on the Polar Airborne Geophysics Data Portal.
description: ** Instrument:
Radar data were collected using the new bistatic PASIN-2 (Polarimetric radar Airborne Science Instrument) radar echo sounding system operating in full polarimetric mode and mounted on the BAS Twin Otter aircraft "VP-FBL" and operating with a centre frequency of 150 MHz and using a 4-microseconds, 13 MHz bandwidth linear chirp (deep sounding). The Pulse Repetition Frequency (PRF) was 15,625 Hz (pulse repetition interval: 64 microseconds).

** 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)

** Waveform details:
Four waveforms, 4uS Tukey port, 4uS Tukey starboard, 1uS Tukey port, 1uS Tukey starboard for flight P03 to P22. 
Two waveforms for the other flights.

** Radar receiver configuration:
Receiver vertical sampling frequency: 24 MHz (resulting in sampling interval of 41.66667 ns)
Receiver coherent stacking: 25
Receiver digital filtering: -50 dBc at Nyquist (11 MHz)
Effective PRF: depends on the flight number: From flight P03 to P22: 156.25 Hz. The other flights have an effective PRF of 312.5 Hz (post-hardware stacking)
Sustained data rate: 10.56 Mbytes/second
description: - Trace spacing (post-processed data): ~30 m
- Vertical resolution: ~7 m
- Radar centre frequency: 150 MHz
- Radar bandwidth: 13 MHz
- Radar Receiver vertical sampling frequency: 24 MHz
- Absolute GPS positional accuracy: ~0.1 m (relative accuracy is one order of magnitude better). Banking angle was limited to 10 degrees during aircraft turns to avoid phase issues between GPS receiver and transmitter.

Crossover analysis of recovered bed elevation across the entire survey gives an error estimate for the PolarGAP survey of ~28 m.