DOI: 10.5285/79e63097-f5dc-41ff-8ca5-36bc4f95a6ff
title: Strapdown aerogravity survey across the Brunt Ice Shelf, 2017

description: This data set contains strapdown aerogravity data collected during an airborne radar survey of the Brunt Ice Shelf as part of the NERC/BAS Life Time of Halley project. Unlike traditional stabilised platform gravity surveys strapdown gravity techniques, using an Inertial Measurement Unit (IMU) rigidly attached to the aircraft as the sensor, allow collection of gravity data during draped or turbulent flight. This was a key factor allowing for data collection during a dedicated radar survey. The survey was flown draped with an average height above the ice surface of 420 m, and includes 4716 km of new data. The aircraft used was the BAS aerogeophysicaly equipped twin otter VP-FBL. Data are available in both Geosoft database (.gdb) and ASCII file formats (.xyz).
The Brunt 2017 survey was carried out as part of the NERC/BAS Life Time of Halley project. Subsequent data processing was supported by the BAS Geology and Geophysics team. In addition we thank the BAS air unit and staff at Halley Research Station for their support of the 2017 field campaign.
description: Gravity data during the Brunt 2017 survey were collected using a strapdown approach.  This technique was first developed in the 1990's but advances in sensor design and data processing have only recently made this a viable method for field data collection. The strapdown gravity data was processed using an 18-state Kalman filter in conjunction with a Rauch-Tung-Striebel (RTS) smoother. Besides the 15 states of a typical IMU/GNSS integrating Kalman filter used for navigation (3-D position, velocity, attitude, and six inertial sensor biases), additional states are used to model the gravity disturbance with respect to GRS80 normal gravity. GPS coordinates were processed with a standard precise point positioning software package (Novatel Waypoint GrafNav 8.60), giving an average estimated positional error of 7.1 cm in the vertical and 3.1 cm in the horizontal. The GPS positional estimates were introduced as observations to the Kalman filter for the strapdown gravity calculation. The Kalman filter (and smoother) provides gravity estimates in a one-step evaluation, i.e. no additional low-pass filtering step is required. This method is sometimes referred to as the indirect method of strapdown gravimetry, because gravity is determined indirectly by introducing GNSS positions to the Kalman filter, rather than computing GNSS accelerations in a pre-processing step and manually combining them with the specific forces measured by the accelerometers in order to determine gravity.
In order to minimise thermal effects on the QA2000 accelerometers, the IMU was warmed up for at least two hours before each flight. However, a further thermal correction was applied to compensate reproducible thermal effects arising from internal sensor temperature changes along the flights. Details on the strapdown gravity data processing and the thermal calibration methods can be found in (Becker, 2016).
description: The standard deviation of the strapdown gravity crossover errors for the 2017 season was 2.5 mGal, consistent with a Route Mean Square Error (RMSE) of 1.8 mGal. The estimated GPS positional errors were 7.1 cm in the vertical and 3.1 cm in the horizontal.

Resolution: The line spacing for the main survey area is 5km, with regional lines spaced ~18 km apart.  The optimum along track resolution of the strapdown gravity system is approximately 100 seconds, consistent with a full wavelength spatial resolution for the 2017 Brunt survey of ~6 km (at an aircraft speed of approximately 60 m/s).
Data is provided in one of two formats: Geosoft database (.gdb) with associated .dbview file, or ASCII file format (.xyz). The data contains the following channels:
(1) "DATE" - Date (YYYY/MM/DD).
(2) "TIME_UTC" - UTC time (HH:MM:SS.SS).
(3) "Lat" - Longitude (decimal degrees, WGS84).  PPP post processed.
(4) "Lon" - Latitude (decimal degrees, WGS84).  
(5) "X" - Projected Polar Stereographic coordinate (m). Standard parallel -71.
(6) "Y" - Projected Polar Stereographic coordinate (m).
(7) "ELEV" - Flight elevation (m).  WGS84 ellipsoid. 
(8) "FAA" - Free air anomaly (mGal) referenced to GRS80 normal gravity.