DOI: 10.5285/e203926b-7a54-4c33-a5df-04df2293d7d3
title: Processed airborne radio-echo sounding data from the FISS 2016 surveys covering the Filchner and Halley Ice Shelves, and the English Coast (western Palmer Land), West Antarctica (2016/2017) - V2

description: Three separate airborne radar surveys were flown during the austral summer of 2016/17 over the Filchner Ice Shelf and Halley Ice Shelf (West Antarctica), and over the outlet glacier flows of the English Coast (western Palmer Land, Antarctic Peninsula) during the Filchner Ice Shelf System (FISS) project. This project was a NERC-funded (grant reference number: NE/L013770/1) collaborative initiative between the British Antarctic Survey, the National Oceanography Centre, the Met Office Hadley Centre, University College London, the University of Exeter, Oxford University, and the Alfred Wenger Institute to investigate how the Filchner Ice Shelf might respond to a warmer world, and what the impact of sea-level rise could be by the middle of this century.

The 2016/17 aerogeophysics surveys acquired a total of ~26,000 line km of aerogeophysical data. The FISS survey consisted of 17 survey flights totalling ~16,000 km of radar data over the Support Force, Recovery, Slessor, and Bailey ice streams of the Filchner Ice Shelf. The Halley Ice Shelf survey consisted of ~4,600 km spread over 5 flights and covering the area around the BAS Halley 6 station and the Brunt Ice Shelf. The English Coast survey consisted of ~5,000 km spread over 7 flights departing from the Sky Blu basecamp and linking several outlet glacier flows and the grounding line of the western Palmer Land, including the ENVISAT, CRYOSAT, GRACE, Landsat, Sentinel, ERS, Hall, Nikitin and Lidke ice streams.

Our Twin Otter aircraft was equipped with dual-frequency carrier-phase GPS for navigation, radar altimeter for surface mapping, wing-tip magnetometers, an iMAR strapdown gravity system, 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" has been updated due to errors found. The error in the 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. 
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 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.

The three surveys all varied slightly in terms of radar configuration, with FISS being configured as a 12-element swath radar, Halley configured as three distinct antenna groups with different gains to cater for the awkward Brunt ice, and the English Coast configured as a polarimetric radar. More information on the antennae configuration can be found in the 'Data Collection' section further down this page. Note that flights F02-18 were flown as part of the FISS survey, flights F19-23 were flown over the Halley Ice Shelf, and flights F25-31 were flown over the English Coast (western Palmer Land).

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 125 Hz to 5Hz, equivalent to ~11m 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.

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, the location of the surface reflection was picked directly from the radargram. 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 for gaps ~55 m, 2 = radar.

Note: Flights F21-22 and F31 contain no bed/surface pick or elevation information (such as surface/bed elevation/aircraft elevations, ice thickness, etc). Flights F19-20 and F30 contain bed/surface pick but no elevation data. Note also that flight F31 only contain the shallow-sounding pulse data.

** 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.

Note: The PriNumber in both chirp and pulse SEGY differs, however the trace number and time of each trace is the same for both radar variables. We have therefore provided 2x NetCDF variables for the PriNumber in case this is needed ('PriNumber_chirp', 'PriNumber_pulse''), however one can simply link the chirp and pulse radar data by using the trace number or time of trace.

** 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, two SEGY files per flightline (one chirp and one pulse), 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.

NetCDF attributes:
- 'traces': Trace number for the 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 the 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 the 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.

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 FISS 2016 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 bistatic PASIN-2 (Polarimetric radar Airborne Science Instrument 2) radar echo sounding system mounted on the BAS Twin Otter aircraft "VP-FBL" and operating with a centre frequency of 150 MHz and using a pulse-coded waveform at an effective acquisition rate of 125 Hz and a bandwidth of 13 MHz. The Pulse Repetition Frequency was 15,625 Hz (pulse repetition interval: 64 microseconds).

** Antenna configuration:
8 transmitters (4x port side and 4x starboard side)
12 receivers (8x wings and 4x belly)

Transmit power: 1 kW into each antennae
Maximum transmit duty cycle: 10% at full power

FISS: 12x channel swatch radar (8x transmit, 12x receive)
Halley: Similar to FISS but with 30dB attenuation in the receive path for the starboard antennae to accommodate for varying ice conditions around the Brunt Ice Shelf.
English Coast: Full polarimetric mode

** Waveform details
FISS: Five waveforms, 4uS Tukey port, 4us Tukey starboard, 4uS Tukey (180deg out of phase) port, 4 uS Tukey (180deg out of phase) starboard, 1 uS Tukey antenna 1 (port).
Halley: Five waveforms, 4uS Tukey port, 4uS Tukey starboard, 4 uS Tukey (180deg out of phase) port, 4 uS Tukey (180deg out of phase) starboard, 1 uS Tukey antenna 1 (port). Starboard 30dB attenuation on RX.
English Coast: Five waveforms, 4uS Tukey port, 4uS Tukey starboard, 4 uS Tukey (180deg out of phase) port, 4 uS Tukey (180deg out of phase) starboard, 1 uS Tukey antenna 1 (port). Port antennas rotated.

** Radar receiver configuration:
Receiver vertical sampling frequency: 24 MHz (resulting in sampling interval of 41.6667 ns)
Receiver coherent stacking: 25
Receiver digital filtering: -50 dBc at Nyquist (11 MHz)
Effective PRF: 125 Hz (post-hardware stacking)
Sustained data rate: 10.56 Mbytes/second
description: - Average spacing between radar traces: 11 m
- Line spacing: ~3.5-km (English Coast)
- 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.

Note: The SEGY and bed pick files for the English Coast section of the FISS 2016/17 surveys can also be found at: https://doi.org/10.5285/e07d62bf-d58c-4187-a019-59be998939cc. However note that the SEGY data found there is not georeferenced. The datasets found here have been considerably curated and improved, and thus can be considered the latest and full dataset.