DOI: 10.5285/3a9c8604-2bca-48c1-a40c-48a873076581
title: Airborne gravity data over Marguerite Bay collected with a Windracers Ultra UAV (2023/24 season)

description: Airborne gravity data provides insight into the regional subsurface geology and tectonic history. This dataset includes processed airborne data collected over Marguerite Bay using an iCorous strapdown gravity sensor. The gravity sensor was mounted in a Windracers Ultra UAV, serial TD-02. Gravity data was collected at a mean altitude of 500 m on nine flights between 120 and 260 km long, originating from Rothera Research Station. The flight pattern covered an area of 24 km by 75 km, with lines spaced 2 km apart. Data has an along line resolution of ~2 km. The survey targeted a tectonic break between different sectors of the Antarctic Peninsula identified in existing magnetic data. The survey was flown as part of the Innovate UK SWARM project demonstrating the utility of the Windracers Ultra as a platform for environmental science. 

This study was funded by Innovate UK through their Future flight challenge support for the "Protecting environments with unmanned aerial vehicle swarms" project (reference: 10023377). We thank BAS operations for their support and specifically the BAS air unit and ground support staff whose close cooperation and engagement with the UAV deployment made the project successful. We also thank staff at Windracers and Distributed avionics who provided remote support for UAV operations across the field season.
description: Instrumentation and Processing:
Data was collected using an iCORUS strapdown gravity sensor, including full thermal stabilisation, rented from iMAR Ltd. The sensor was mounted approximatley coincident with the center of mass of the Windracers Ultra survey UAV, directly below the primary survey GNSS antenna. The sensor was powered and allowed to reach a manually set stable temprature 48 hours prior to operation and remained powered and at the set temprature for the duration of the survey operation. A set ambiant temprature of 5 degrees Celsius was chosen, consistent with expected operational conditions. Variations of +/- 15degrees Celsius around this value were permissable within the systems heating/cooling capability and these limits were not exceded during the survey. Please note that the sensors actual stabilised internal temprature was a constant value ~20degrees Celsius above this set ambiant value. The sensor was set to colect the required Inertial Navigation System (INS) and GNSS data &gt;15 minutes before and after each survey flight. GNSS base station data for post processing was collected at either the Rothera International GNSS Service (IGS) station, or using a tempoary Javad base station reciever installed adjacent to the runway. 
Gravity values (referenced to the ellipsoid so technically gravity disturbance) were calculated using the terrapos GNSS processing software package. This implements a Kalman filtering approach to integrate INS (accelerometer and gyroscope triads) data with GNSS satellite data to simultaneously solve for platform position, velocity and attitude as well as the gravitational field variations. To conduct the processing, precise satellite ephemeris were downloaded from IGS to improve the quality of the solution. The lever arm values between the gravity sensor and the GNSS antenna must be known accurately for high quality processing. These values were initially determined by a combination of physical measurements and use of CAD images of the Windracers Ultra platform, with an accuracy of ~2 cm. Subsequently the lever arm estimate was improved by allowing the Kalman filter to optimise the lever arm values. This procedure was applied to a magnetic compensation flight, as the relatively high dynamics (roll/pitch/yaw) give a better signal for solving the lever arm. After four iterations stable, low error (&lt;0.5 cm) lever arm estimates were achieved, which were applied for all subsequent missions. 
The output gravity values were visually assessed for quality using the Geosoft software package. Offsets between recovered gravity values for different flights of up to 20 mGal were apparent, with some flights showing an apparent linear trend of up to 5 mGal between start and end. To resolve this error the output 'absolute gravity' value for the UAV during the static measurements before and after each flight was compared to the absolute gravity value reported for Rothera Hanger tie point by Rene Foresberg during the PolarGAP survey (https://earth.esa.int/eogateway/documents/20142/37627/PolarGap-2015-2016-final-report.pdf). The differences were assumed to be the offset in the gravity anomaly value. A linear trend between the start and end static offsets was imposed to account for system/processing drift. All static readings were made withing 50 m, and more typically ~20 m from the hanger tie point, so this is a relatively robust check. Minor line to line offsets remained visible in the gravity data after levelling to the hanger tie point. Simple statistical levelling, applied using two missions flown either orthogonal or oblique to the main survey as tie lines, reduced the errors further. The southern part of the survey lacked tie lines, and an additional judgement was made to reduce the amount of statistical levelling in this region, as it provided the best visual minimisation of line to line noise.
To convert the free air anomaly (disturbances) on the ellipsoid to true free air gravity anomalies (referenced to the geoid) we used the EIGEN-6C4 Geoid as reference. The geoid-ellipsoid difference (m) was converted into a free air correction (mGal) using a standard value of 0.3086 mGal/m. This value was added to the gravity disturbance. 
Two further corrections were applied to the gravity data: the Bouguer correction and the Airy Isostatic correction. These remove the impact of shallow topography/bathymetry and deep variations in crustal thickness respectively. The Bouguer correction was calculated at an observation altitude of 500 m, coincident with the survey flight altitude. Terrain elevation, ice and water thickness were taken from Bed machine Antarctica in an area extending &gt;160 km from the survey boundaries. The Bouguer correction was calculated using a prism based methodology, with densities of 2670, 1028 and 915 kg.m-3 for rock, water and ice respectively. Ice and rock above 500m altitude was assumed to give a negative gravity signature. The gravity effect from all layers was summed to give the full Bouguer correction at 500 m altitude. This correction sampled onto the flight lines and subtracted from the Free air anomaly to give the final Bouguer gravity anomaly.
The airy isostatic correction was calculated using the GMT 5.4.4 gravfft routine. All ice and water layers were converted into equivalent topographic thicknesses based on the densities above and added to the bed elevation to create a raster of "equivalent topography". Airy isostatic compensation of this load by a Moho interface at a depth of 23 km, and the associated gravity anomaly, was calculated assuming a load and mantle densities of 2670 and 3330 kg.m-3 respectively. The airy isostatic correction was subtracted from the Bouguer gravity anomaly to give the final Airy isostatic anomalies.

Data is provided as an ASCII comma separated variable file. Dummy values are denoted by *, and the data set includes the following 19 columns:
flight_id: Flight number across the season (17 to 25).
line: Line segment number (sequential 0, 1, 2, etc).
date: UTC date (YY\MM\DD)
time_UTC: UTC time (HH:MM:SS.SSSSSSSS)
longitude: Longitude (DDD.DDDDDD)
latitude Latitude (DD.DDDDDDD)
projection_x_coordinate: Projected Polar Stereographic m (standard parallel -71)
projection_y_coordinate: Projected Polar Stereographic m (standard parallel -71)
height_above_reference_ellipsoid: Platform elevation m (WGS 1984 ellipsoid).
platform_roll: Platform Roll (degrees)
platform_pitch: Platform Pitch (degrees)
platform_yaw: Platform heading (degrees from true N)
gravity_absolute: Calculated absolute gravity at platform (mGal) 
gravity_disturbance: Unlevelled free air gravity referenced to the WGS1984 ellipsoid (mGal).
FAA_anomaly_geoid: Levelled free air gravity anomaly referenced to the geoid (mGal).
Bouguer_correction: Bouguer correction (mGal).
Bouguer_anomaly: Bouguer anomaly (mGal).
airy_correction: Airy isostatic correction (mGal).
airy_anomaly: Airy isostatic anomaly (mGal).
description: Sensor(s): iCORUS strapdown gravity sensor from iMAR Ltd.
Platform(s):UAV - Windracers Ultra TD-02
description: The processed gravity data was created using a Kalman filter with a spatial corelation coefficient of 2 km, and an assumed standard deviation for the gravity data of 10 mGal. The assumed minimum along track resolution is therefore 2 km. Comparison of repeat lines shows differences with a standard deviation of 1.66 mGal. This is taken to represent the accuracy of the dataset after levelling.