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:

DATA REFERENCE

Jordan, T., Robinson, C., Porter, D., Locke, C., & Tinto, K. (2020). Processed line aerogravity data over the Thwaites Glacier region (2018/19 season) [Data set]. UK Polar Data Centre, Natural Environment Research Council, UK Research & Innovation. https://doi.org/10.5285/B9B28A35-8620-4182-BF9C-638800B6679B

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:

PROCESSING:

Gravity data during the ITGC survey were collected using both a strapdown (Jordan and Becker 2018) and traditional stabilised platform approach (Valliant 1992). The initial strapdown gravity signal was recovered from raw INS and GPS data using a Kalman filtering technique implemented in the TerraPos GPS processing software. Raw data were processed in a tightly coupled manner solving for gravity, position, and sensor orientation simultaneously. Ionospheric and clock errors were solved using Precise Point Positioning (PPP) technique with accurate satellite clock and orbit corrections used in the 'Final' ephemeris data downloaded more than a month after the survey was completed. The applied Kalman filter included a gravity smoothing filter assuming a 5km gravity correlation constant and 10 mGal permitted standard deviation. In order to minimise thermal effects on the QA2000 accelerometers, the IMU was warmed up from ambient temperatures inside an insulating enclosure for at least three hours before each flight. However, very significant thermal drift of >30 mGal across a flight remained apparent when comparing the data to an independent Operation IceBridge (OIB) dataset (Cochran and Bell 2010, updated 2018). A thermal correction (C) based on the sensor internal temperature (T) was calibrated by comparison with the existing regional OIB data. The thermal correction took the form of a polynomial:

C= -0.0174T^2 + 2.2619T + 5.8554

After thermal correction long wavelength residual errors (>~30 minutes/100 km) attributed to hysteresis in the thermal response of the gravity system were still apparent. These were corrected for by a combination of statistical levelling, and fitting the long wavelength residual errors to the gravity field revealed by the OIB dataset.

The stabilised platform gravity data was collected using a LaCoste & Romberg sensor (S83) mounted in a ZLS stabilised platform. Raw gravity data was collected at 1Hz. The GPS positional data processed using TerraPos, described above, was used to calculate standard gravimetric corrections for vertical acceleration, Eotvos effect, Latitude and elevation (free air). Sensor drift was low and stable, and was calculated based on a series of still reading taken before and after every survey flight. The final free air anomaly was filtered with two passes of a B-Spline filter with smoothness 1 and tension of 0.05. Before each pass of the B-Spline filter the data was manually edited to remove spikes and other data errors at the start of line segments.

All the processing has been carried using Geosoft Oasis Montaj software.

RESOLUTION:

The line spacing for the survey is variable between 5 km and 10 km. Line data is generally designed to interleave with existing OIB data. Along-track resolution of the strapdown data is ~5 km.

DATA FORMAT:

Data are provided in ASCII file format (.xyz).

Three databases are provided with aerogravity data: one with the Strapdown processing flow, a second with the L&R processing flow, and a final simplified database with the optimal free air gravity anomalies from the strapdown system.

The strapdown data set includes the following channels:

Name of the file : 'Thw_tera_pos_grav_share.XYZ'

Flight: Flight number

Date: GPS date (YYY/MM/DD)

Time: GPS time (HH:MM:SS.SSSS)

Lat_deg: Latitude (DD.DDDDDDD)

Lon_deg: Longitude (DD.DDDDDDD)

Hght_GRS80: Elevation on the GRS80 ellipsoid (m)

X: X coordinate m Polar stereographic true scale -71

Y: Y coordinate m Polar stereographic true scale -71

Heading: Heading (degrees from Geodetic N)

Roll: Role (degrees from local level Geodetic)

Pitch: Pitch (degrees from local level Geodetic)

FAA: Initial free air gravity anomaly (mGal)

ICEBRIDGE: Reference gravity values from grid of ICEBRIDGE gravity anomalies (mGal).

IMU__C_: Internal temperature of IMU (degrees C).

Synthetic_Dicebr...(29)

Data collection:

Data was collected using the BAS twin otter VP-FBL, equipped for aerogeophysical surveys. Strapdown gravity data were recorded using an iMAR RQH-4001 inertial measurement unit (IMU) owned by Lamont-Doherty Earth Observatory. This sensor package consists of three Honeywell QA2000 accelerometers (mounted in mutually perpendicular directions), and three ring laser gyroscopes. Coincident GPS data were recorded with a NovAtel receiver. The stabilised platform gravity data were collected using a LaCoste & Romberg sensor (S83) mounted in a ZLS stabilised platform.

Data quality:

The standard deviation of the strapdown gravity crossover errors after thermal correction and levelling to OIB data was 2.2 mGal, consistent with a Root Mean Square Error (RMSE) of 1.56 mGal. The Stabilised platform gravity system did not perform as well, and the data from this system is included here for completeness.