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., Robinson, C., Reed, T., & Toomey, R. (2024). Airborne gravity data over Marguerite Bay collected with a Windracers Ultra UAV (2023/24 season) (Version 1.0) [Data set]. NERC EDS UK Polar Data Centre. https://doi.org/10.5285/3a9c8604-2bca-48c1-a40c-48a873076581

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:

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° C was chosen, consistent with expected operational conditions. Variations of +/- 15° 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 ~20° above this set ambiant value. The sensor was set to colect the required Inertial Navigation System (INS) and GNSS data >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 (<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 (disturban...(24)

Data collection:

Sensor(s): iCORUS strapdown gravity sensor from iMAR Ltd.

Platform(s):UAV - Windracers Ultra TD-02

Data quality:

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.