DOI: 10.5285/40c2f86b-1a02-4106-934a-42769682df66
title: Ice-sounding airborne synthetic aperture radar depth profiles from Recovery Ice Stream 2016/17 and Rutford Ice Stream 2019/20 to test the RGB-Doppler-Decomposition method.

description: This data set corresponds to data acquired by the British Antarctic Survey (BAS) airborne Synthetic Aperture Radar (SAR) PASIN2 (Polarimetric Airborne Scientific INstrument, mark 2), designed for deep ice sounding and basal 3d-mapping. The data set includes the processed SAR images as depth profiles in the Recovery Ice Stream and Rutford Ice Stream, respectively downstream and upstream of the grounding line, and respectively for the 2016/17 FISS (Filchner Ice Shelf System) and the 2019/20 BEAMISH (Bed Access, Monitoring and Ice Sheet History) projects, both during the Antarctic Summer. With multiple antennas for transmission and reception at 150-MHz central frequency, and an across-track physical array, PASIN2 resolves the ambiguities for distinguishing between scatterers from port and starboard directions; however, in the two SAR images of the current dataset the port/starboard ambiguities are not resolved. On this dataset the user will be able to apply the RGB Doppler Decomposition method in the Doppler domain, interpret the results, and modify the different parameters and colours to contrast the results, all with the outcome of conducting new decompositions according to other datasets and needs. The RGB Spectral Decomposition is a generalised framework to interpret the SAR images: first, the Doppler or range spectral domains are first split into three sub-bandwidths; next, to each of the three a colour of a triplet of colours is assigned; and finally the three are superposed into one single image by the addition of the three colours. If the decomposition is applied on the Doppler spectrum, the new image contains the directional information related to the Doppler frequencies: positive frequencies when the radar approaches the target, near zero frequencies when the relative distance from radar to target is near stationary, and negative when the radar leaves it behind. If the backscattering is characterised by a very broad beamwidth the target will be gray/white, and if by a very narrow beamwidth then the target will be represented by one of the colours of the triplet.

This work has received funding from the NERC grant NE/L013444/1, project: Ice shelves in a warming world: Filchner Ice Shelf System (FISS), Antarctica.
The 2016/17 data were collected as part of the NERC grant NE/L013770/1, project: Ice shelves in a warming world: Filchner Ice Shelf System (FISS), Antarctica.
The 2019/20 data were collected as part of the BAS National Capability contribution to the NERC/NSF International Thwaites Glacier Collaboration (ITGC) program.
description: The raw data was collected by the airborne Synthetic Aperture Radar (SAR) PASIN2. It is 150-MHz coherent pulsed radar, transmitting chirped pulses with linear frequency sweep, with 13 MHz bandwidth and effective pulse repetition frequency 125 Hz. On board of a De Havilland DHC-6 Twin Otter aircraft ("VP-FBL), in ice-sounding flights the nominal speed is 60 m/s, and the terrain clearance (height above closest surface) is 300 m. PASIN2 has 12 antenna elements: 8 underwing folded-dipoles switching between transmit (TX) and receive (RX) modes, and 4 printed-dipoles being RX-only attached to the fuselage belly. The data processing is performed off-line, with a full workflow consisting of: 1) channel calibration; 2) 2D SAR imaging, based on pulse-compression for range dimension, and back-projection for along-track dimension; 3) Direction of Arrival estimation (DoA) of the remaining across-track angle, based on the non-linear MUSIC algorithm, by using several SAR images from different transmitters or receivers; and 4) 3D-mapping from the three dimensions of range, along-track and across-track angle, by correcting the real depths and across-track distances, regarding the case of incorrectly assumed vertical DoA of a single SAR image. In the here presented dataset are included two basic SAR products (without either DoA or 3D-mapping), each from a nominal surveying flight above two ice streams:
- an only range-focussed image (without along-track focussing) above the Recovery Ice Stream, flying across-flow above a subglacial channel in the Filchner Ice Shelf, as part of the FISS project in season 2016/17, Flight F11. This component of the data set was included to show that the RGB Doppler Decomposition can also be applied to range-compressed data without SAR focussing, and an application to interpret subglacial channels imagery.
- a full-focussed (range and along-track) SAR image above the Rutford Ice Stream, flying across flow in the grounded ice from the ice stream center to near its margin, as part of the BEAMISH project in season 2019/20, Flight T04. This component of the data set was included to show the ice-column imaging capabilities, including ice surface, shallow and deep ice, and bedrock.

The two SAR product outputs (one per flight section) are formatted as NetCDF (Network Common Data Format) files (.nc) for data sharing. They contain standard data and its associated metadata, including units, conventions, intervals and human-readable descriptions, among others. NetCDF file content can be explored and plotted with software such as the free stand-alone Panoply, and the content can be read and created using available interfaces in C, C++, Java, Fortran, Python, IDL, MATLAB, and R, among others. The most import global variables in the here presented netCDF files are:


SAR image coordinates:
- 'profile': dimension variable 'profile_id', with the index of the profile within the SAR image, as an integer starting at 0.
- 'radialDepthGrid': dimension variable, with the depth grid in the SAR image as the vertical location below (positive) and above (negative) surface, units 'meters', positive 'down', axis 'z'.
- 'pulse': Index of the transmitted pulse within the effective PRF of each waveform (after pulse stacking) of the whole data take (not of the SAR image), and its increment is the subsampling factor to get the SAR-image PRF from the effective PRF of each waveform, or vice versa; dimension 'profile'. To obtain the PRF of each waveform during the data take from the SAR-image PRF:
waveform PRF [Hz] = diff('pulse') x SAR-image PRF [Hz]
- 'pulseTime': time at the transmitted pulse within the effective PRF of each waveform (after pulse stacking) of the whole data take (not of the SAR image), and its increment (in seconds) is the inverse of the SAR-image PRF; units 'number of days from January 0, year 0000'; dimension 'profile'. To obtain the SAR-image PRF the 'pulseTime' needs to be converted into seconds, as the originals units are days:
SAR-image PRF [Hz] = 1/(diff('pulseTime')days x 24hours/day x 3600 seconds/hour) [Hz]

SAR image samples, complex numbers with real and imaginary components:
- 'SARimageReal': Real part of complex processed SAR image, dimensions 'profile' and 'radialDepthGrid'.
- 'SARimageImag': Imaginary part of complex processed SAR image, dimensions 'profile' and 'radialDepthGrid'.

Aircraft positioning:
- 'latitudeAircraft': Latitude of the aircraft, standard name 'Latitude', units 'degrees_north', dimension 'profile'.
- 'longitudeAircraft': Longitude of the aircraft, standard name 'Longitude', units 'degrees_east', dimension 'profile'''.
- 'heightAircraft': Height of aircraft above mean sea level (asl), standard name 'height_above_mean_sea_level', units 'meters', positive 'up', dimension 'profile'.
- 'terrainClearanceAircraft': terrain clearance, distance from the platform to air interface with ice, sea or ground; units 'meters'; dimension 'profile'.
- 'speedAircraft': aircraft speed, standard name 'platform_speed_wrt_air', units 'meters/second', dimension 'profile'.

Aircraft attitude (rotation):
- 'rollAircraft': Aircraft roll, positive according to right-hand rule pointing towards aircraft direction (along-track), standard name 'platform_roll_starboard_down', units 'degrees', dimension 'profile'.
- ''pitchAircraft': Aircraft pitch, positive according to left-hand rule pointing towards port direction, standard name 'platform_pitch_fore_up', units 'degrees', dimension 'profile'.
- 'yawAircraft': Aircraft yaw, positive according to left-hand rule pointing upwards, perpendicular to flat sea level, standard name 'platform_yaw_fore_starboard', units 'degrees', dimension 'profile''.
description: PASIN2 (Polarimetric Airborne Scientific INstrument, mark 2) radar; Matlab R2020b software. The parameter files used for processing are in the open repository https://github.com/antarctica/sar-rgb-spectral-decomposition 

Instrument
The data were collected with the British Antarctic Survey (BAS) PASIN2 (Polarimetric Airborne Scientific INstrument, mark 2), an airborne Synthetic Aperture Radar (SAR) at 150 MHz (wavelength = 2 m in vacuum) and 13 MHz bandwidth, designed for deep ice sounding and basal 3d-mapping mounted on the BAS Twin Otter "VP-FBL" aircraft. It can be operated as part of a geophysical suite with additional radars, sensors for magnetic and gravity field detection, lidar and a camera. Accurate time and positional data are obtained via geodetic GPS.

Antennas
PASIN2 has 12 antenna elements: 8 underwing-antennas switching between transmit (TX) and receive (RX) modes, and 4 pod-antennas being RX-only. The 8 TX/RX elements are folded dipoles of length 0.39wavelength, 4 at each side (port and starboard) and separated by 0.8wavelength. The 4 RX-only are end-loaded printed dipoles within a radome attached to the fuselage, with a separation of 0.5wavelength, and are referred as belly. The 12 elements are independent receivers, whereas the 4 elements in port and starboard arrays are each driven, hence achieving 24 phase centers (2TX, 12RX), non-unique. The wings, with a slope (dihedral) of σ = 3.5 degrees, behave as plane reflectors for port and starboard elements, directing the TX pattern of each array towards different directions. The antenna orientation is always within a horizontal plane, never vertical. The convention for naming the orientations is H for along-wing, and V for along-track (if a photograph of the antennas was taking above the aircraft, in the two-dimensional photograph H would have a horizontal orientation, and V vertical). The polarization must be manually changed before taking off, in port and starboard. In this dataset, the antenna polarizations for transmission and reception were always H (along-wing).

Transmitter configuration
The transmission method was time-division multiplexing with a system pulse repetition frequency (SPRF) of 15.625 kHz, and a (full cycle) receiving window of 64 us (microseconds). In the data take of this dataset, the number of pre-programmed alternating transmitted waveforms was 5, including TX side and pulse type. The transmitted signals were generated with an arbitrary waveform generator (AWG), directly tuning the carrier frequency to 150 MHz. The AWG has two independent outputs, for port and starboard, and each divides into four towards separated low pass filters, high-power amplifiers and the antenna elements. In the here presented dataset:
- Recovery Ice Stream (FISS project, season 2016/17, Flight F11) from port array transmissions.
- Rutford Ice Stream (BEAMISH project, season 2019/20, Flight T04): from starboard array transmissions.

Waveforms
The transmitted pulses were chirps with linear frequency modulation. The bandwidth is limited to 13 MHz, with duration of 1 us (microseconds) or 4 us. To filter common interference and ADC offsets, the 4us-pulses were transmitted in 2 different waveforms: one shifted 180degrees relative to the other (0-pi modulation). The cycle of 5 waveforms was: 1) port-4us-0degrees; 2) starborad-4us-0degrees; 3) port-4us180degrees; 4) starboard-4us-180degrees; and 5) port-1us-0degrees. In the off-line processing, received signals from both 0degrees and 180degrees waveforms are subtracted, cancelling the common sources and increasing the SNR by 3 dB, with a coherence interval of 128 us, or 7.7 mm for an aircraft speed 60 m/s. In the here presented dataset:
- Recovery Ice Stream (FISS project, season 2016/17, Flight F11) from port array transmissions with 4us waveforms (0 and 180degrees).
- Rutford Ice Stream (BEAMISH project, season 2019/20, Flight T04) from starboard array transmissions with 4us waveforms (0 and 180degrees).

Receiver configuration
In reception, after a low pass filter, each channel is digitised at 120 MHz rate (ADC, Pentek 71660 4-channel 16-bit digitizer on a custom FPGA), and echoes from the same waveform within 25 transmitted cycles are stacked (summed), giving an effective pulse repetition frequency (PRF) of 125 Hz as a result of dividing the system pulse repetition frequency (SPRF, = 15.625 kHz) by 25. The resulting signal is stored in one of the three separated units for port, belly and starboard sections. In the two images of the here presented dataset the reception was from an antenna not used for transmitting:
- Recovery Ice Stream (FISS project, season 2016/17, Flight F11) from the starboard receiver SC (the closest to the starboard wing tip).
- Rutford Ice Stream (BEAMISH project, season 2019/20, Flight T04) from port receiver P1 (the closest to the port wing tip).
description: The image above Recovery Ice Stream (season 2016/17, Flight F11):
- processing level: only to range-processing, thus without SAR (along-track) focussing of any kind;
- sampling of the output (back-projection) grid: 6.48 meters/pixel in range, and 16 milliseconds/pixel (62.5 Hz) in along-track;
- lightspeed in ice: Ci = 168.3751 meters/microsecond;
- spectral bandwidth: Bw = 13 MHz in pulse (range), and Dw = 50 Hz in Doppler (along-track);
- average aircraft speed: V = 60.92 m/s;
- processed along-track beamwidth (antenna aperture): 2 x arcsin(wavelength x (Dw/2) / (2 x V)) = 48.46 degrees in air, and 26.66 degrees in ice after refraction with refractive index 1.78;
- resolution (if without spectrum windowing): Ci/(2 x Bw) = 6.476 meters in range, and incremented with depth in along-track (because no SAR focusing).

The image above Rutford Ice Stream (season 2019/20, Flight T04):
- processing level: SAR focussing by back-projection;
- sampling of the output (back-projection) grid: 6.48 meters/pixel in range, and 16 milliseconds/pixel (62.5 Hz) in along-track;
- lightspeed in ice: Ci = 168.3751 meters/microsecond;
- spectral bandwidth: Bw = 13 MHz in pulse (range), and Dw = 30 Hz in Doppler (along-track);
- average aircraft speed: V = 55.18 m/s;
- processed along-track beamwidth (antenna aperture): 2 x arcsin(wavelength x (Dw/2) / (2 x V)) = 31.55 degrees in air, and 17.57 degrees in ice after refraction with refractive index 1.78;
- resolution (if without spectrum windowing): Ci/(2 x Bw) = 6.476 meters in range, and V/Dw = 1.84 meters in along-track.

The amplitude and relative phase of the antenna patterns in transmission (as an array of 4 antennas) and reception (as 12 single and independent antennas), were assessed with calibration flights above the sea surface. The patterns are assumed as invariant along each session, if the antenna configuration remains.

The transmitted radar pulses are not calibrated, as parameter drifts are not expected to affect the quality.