. Further by downloading this data the user acknowledges that they agree with the NERC data policy (), and the following conditions:

1. To cite the data in any publication as follows:

Ferraccioli, F., Forsberg, R., Matsuoka, K., Olesen, A., Jordan, T., Corr, H., Robinson, C., & Kohler, J. (2021). Processed airborne radio-echo sounding data from the POLARGAP survey covering the South Pole, and Foundation and Recovery Glaciers, East Antarctica (2015/2016) (Version 1.0) [Data set]. NERC EDS UK Polar Data Centre. https://doi.org/10.5285/E8A29FA7-A245-4A04-8B56-098DEFA134B9

2. The user recognizes the limitations of data. Use of the data is at the users' own risk, and there is no warranty as to the quality or accuracy of any data, or the fitness of the data for your intended use. The data are not necessarily fully quality assured and cannot be expected to be free from measurement uncertainty, systematic biases, or errors of interpretation or analysis, and may include inaccuracies in error margins quoted with the data.

Methodology:

** 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, 12 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, 12 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 belo...(140)

Data collection:

** 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, 12 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: 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: 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

Data quality:

- Trace spacing (post-processed data): ~30 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 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.