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:

Jordan, T., Ferraccioli, F., Bell, R., Damaske, D., & Robinson, C. (2020). Antarctica's Gamburtsev Province (AGAP) Project - Airborne gravity data (2007-2009) [Data set]. UK Polar Data Centre, Natural Environment Research Council, UK Research & Innovation. https://doi.org/10.5285/8E5F910B-11D6-4A9D-BDF7-175C9B98CFB8

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:

Airborne gravity data are presented in Ferraccioli et al (2011). Only BAS data are presented here but USAP flown flight lines details can be found at http://pgg.ldeo.columbia.edu/wp/dataprojects/gambit-agap-consortium/. BAS data processing steps are outlined below, as are the full definitions of all channels.

Processing steps:

1/ Calculate observed gravity.

True spring tension (ST_real) is calculated from the posted spring tension (ST) correcting for the fact that for this survey the true spring tension approaches the posted value at 38 mGal per second.

Beam velocity (Beam_vel) is derived from raw beam position (RB) assuming a centred difference approximation.

Relative gravity (rec_grav) = (Spr_tens_real+((beam_vel)*k_fac)+CC)*scale_value, k_fac=30, meter scale value =0.9966.

Still readings (Still) are in mGal and were calculating assuming a 2nd order best fit to the approximately linear drift of the meter observed at the tie down points.

Airborne absolute gravity values (Abs_grav) = Rec_grav- Still + Base

2/ Corrections to derive free air anomalies (disturbances).

Vertical acceleration (VaccCor) is calculated as 2nd derivative of flight altitude (Height_WGS1984), with a 3 point mean filter applied after differencing to reduce short wavelength noise.

Eotvos correction (EotvosCor) follows (Harlan, 1968).

Latitude correction (LatCor) = 978.03185(1+0.005278895 sin2Lat- 0.000023462 sin4Lat) (IUGG 1967).

Free air correction (FaCor) = 0.3086*Height_WGS1984. NOTE subsequent free air values are defined as gravity disturbances in geodesy, as they are referred to the ellipsoid (Hackney and Featherstone, 2003).

Horizontal acceleration correction (HaccCor). For this survey the approximation of (Swain, 1996) was used, assuming a damping factor of 0.707, and a platform period of 4 minutes.

3/ Free air anomaly and filtering.

Free air anomaly (Free_air) = Abs_grav-VaccCor+EotvosCor+FaCor-LatCor-(0.5*HaccCor)

Filtered free air anomaly (FAA_filt) used 9 km 1/2 wavelength space domain kernel filter (Holt et al., 2006).

Final free air data (FAA_clip) was produced by manually masking turns, start and end of lines, and other regions of noisy data.

Free air anomaly data was statistically levelled to give a consistent grid. The correction applied is Level_cor and the final anomaly channel is FAA_level.

Upward continued free air anomaly (FA_4600m) was produced by upward continuing free air data from the collected flight altitude to 4600 m.

Channel description:

Basic Channels

Date UTC date (YYYY/MM/DD)

Time UTC time (HH:MM:SS.SS)

FlightID Sequential flight number and survey ID e.g. W12

Line_name Line Number e.g. LW200.1:12

Lon Longitude WGS 1984

Lat Latitude WGS 1984

x x projected meters *

y y projected meters *

Height_WGS1984 Aircraft altitude (meters) in WGS 1984, for processing see location data page

Raw gravity Channels

ST Spring Tension (meter units)

CC Cross Coupling (meter units)

RB Raw beam position (Mv)

XACC Cross axis accelerometer (Mv)

LACC Long axis accelerometer (Mv)

Still Airborne meter still reading value (mGal)

Base Absolute gravity reference, from land gravity (mGal)

Calculation Channels

St_real True Spring tension value (meter units)

Beam_vel Gravity meter beam velocity (Mv/sec)

Rec_grav Recalculated relative gravity (mGal)

Abs_grav Calculated absolute gravity (mGal)

VaccCor Vertical acceleration correction (mGal)

EotvosCor Eotvos correction (mGal)

LatCor Latitude correction (mGal)

FaCor Free air correction (mGal)

HaccCor Horizontal acceleration correction (mGal)

Free air Channels

Free_air Un-filtered free air anomaly (mGal)

FAA_filt Filtered free air anomaly data (mGal)

Filtereee FAA_clip Filtered, masked free air anomaly data (mGal)

FA_4600m Free air anomaly data...(5)

Data collection:

In total, 120, 000 line km of data were aquired from a nested survey grid with line spacing of 5km and tie lines ~ 33 km apart.

Data quality:

To obtain high quality airborne gravity data requires accurate kinematic post-processing of GPS data. Estimated cross-over errors at the intersection between lines and tie lines is better than 2 mGal, for wavelengths of 9 km.