The gravity signal is extracted from an inertially based system in which a small mass is suspended within a magnetic field. Tiny variations in the acceleration of the gravimeter produce small electrical signals in the sensor as the mass moves within the magnetic field.

A routine that examined horizontal acceleration was used to determine where lines began and ended for the purposes of gravity data, since gravity data is perturbed by strong maneuvering. The line numbering convention used is 1FFF.XX where FFF = flight number and XX = extension number. Each new line in a flight is denoted by an increment in the extension number. For example, 1017.04 is the 4th line detected in flight 17. In some cases lines were included even though they exceeded the horizontal acceleration limits. These lines have extension numbers greater than 50. For example, 1017.52 is the 2nd line in flight 17 that was retained despite the aircraft maneuvering during the line. The accuracy of data on these lines may be reduced, but they are retained to provide coverage in areas where no other gravity data exists.

Grid Filter Parameters

The FAG140 free Air Gravity 140s full wavelength line filter contains data sampled from a filtered grid. The degree of filtering was chosen to suit the area in question. This represents the best gravity data, as grid filtering can lower noise levels by averaging across adjacent lines if the lines are spaced tightly enough. It also produces a consistent data set by averaging together line crossings.

Line Filter Parameters

Line filter parameters for 2009-12 Antarctica data are as follows:

• 70s full wavelength line filter = 0% pass at 52.5s, 100% pass at 105s full wavelength = approx 5.2 km half-wavelength resolution at 150 m/s flying speed.
• 100s full wavelength line filter = 0% pass at 75s, 100% pass at 150s full wavelength = approx 7.5 km half-wavelength resolution at 150 m/s flying speed.
• 140s full wavelength line filter = 0% pass at 105s, 100% pass at 210s full wavelength = approx 10.5 km half-wavelength resolution at 150 m/s flying speed.

Line filter parameters for 2010-12 Greenland data are as follows:

• 70s full wavelength line filter = 0% pass at 52.5s, 100% pass at 105s full wavelength = approx 5.2 km half-wavelength resolution at 150 m/s flying speed.
• 100s full wavelength line filter = 0% pass at 75s, 100% pass at 150s full wavelength = approx 7.5 km half-wavelength resolution at 150 m/s flying speed.
• 140s full wavelength line filter = 0% pass at 105s, 100% pass at 210s full wavelength = approx 10.5 km half-wavelength resolution at 150 m/s flying speed.

Processing Steps

The following processing steps are performed by the data provider.

1. The gravimeter data are filtered and decimated to 10 Hz to match the GPS data.
2. GPS-derived accelerations are subtracted from the data.
3. The gravity is corrected for the Eotvos effect.
4. The expected gravity at the measurement latitude is subtracted.
5. The resulting anomalies are decimated to 2 Hz and low-pass filtered to suppress noise.
6. The free-air correction is applied.
7. Quality control and removal of artifacts.
8. Data is broken up into straight line segments to remove effects of horizontal accelerations during turns.
9. Final quality control.

Version History

Version 01.1 data for 2010 Greenland: All of the 2010 Greenland data for the IceBridge Sander AIRGrav L1B Geolocated Free Air Gravity Anomalies data set were replaced 01 October 2010. The data were all correct, however, the transformation to the Scientific Committee of Antarctic Research (SCAR) Polar Stereographic projection was in error. This has been corrected with the Version 01.1 data.

Version 01.2 data for 2009 Antarctica: All of the 2009 Antarctica data for the IceBridge Sander AIRGrav L1B Geolocated Free Air Gravity Anomalies data set were replaced 27 June 2011. The previous version of the data required re-processing because an error was discovered in the aircraft attitude data files that caused everything for the 2009 Antarctica flights to be rotated 180 degrees. Additionally, after 2009, gravity data processing was changed from a simple free air correction formula to a second order free air correction. Also, since 2009 there have been processing improvements that reduce noise levels, especially at shorter wavelengths (70s) on some lines. The 2009 Antarctica data files have been re-processed for second order free air correction and reduction of noise levels. These changes have been made with the Version 01.2 data.

Version 01.3 data for 2009 and 2010 Antarctica: On 21 May 2012, the 2009 and 2010 Antarctica data were replaced by V01.3. The previous data versions were organized regionally by glacier basin. V01.3 gravity data are organized by flight lines.

Version 01.4 data for 2010 and 2011 Greenland: On 26 November 2012, the 2010 and 2011 Greenland data were replaced by Version 01.4. The V01.4 data are arranged by flight, rather than broken into lines grouped by area. The last field in the data files is now a data quality flag, instead of a gridded value. Aircraft attitude data are no longer included.

Version 01.5 data for 2009, 2010, and 2011 Antarctica: On 28 February 2013, the 2009, 2010, and 2011 Antarctica data were replaced by Version 01.5. The V01.5 data are arranged by flight, rather than broken into lines grouped by area. The file naming convention has changed. As it is not used, the "grid" column has been removed from the data files. The data have been re-leveled. In a few places the data values may be slightly different than in older versions. The leveling adjustments are very small, approximately 0.1 to 0.3 mGal, and are present only for new lines during 2012 in an area where only a few lines were flown, such as the Foundation/Recovery area. The leveling adjustment is given in data column 18. It is constant offset for a particular track line and thus does not affect interpretation. This is not common and the adjustments are very small and thus do not affect interpretation. The replacement 2010 Antarctica data contain one less data file than the previous version. The file, IGGRV1B_V01_20101018.xyz, was a transit flight where the meter was being tested, and was removed because it does not apply to Antarctica.

Version 01.6 data for 2010, 2011, and 2012 Greenland: On August 20, 2013, the IGGRV1B Greenland data files were replaced for the dates: 05 May 2010 through 28 May 2010, 14 March 2011 through 16 May 2011, and 12 March 2012 through 17 May 2012. For the data captured on these days, the platformID is changed from DC-8 to P-3B.

Version 01.7 data: On June 10, 2014, all IGGRV1B Greenland and Antarctica data files from 16 October 2009 to 07 November 2012 were replaced. The file format was changed to adhere to the NASA ASCII text standard. There was no change in processing or data values.

Version 01.8 data: On July 26, 2017, the entire IceBridge IGGRV1B data set was replaced with re-leveled data to ensure consistency within the entire data set. This version uses a different leveling protocol than previous versions. Additionally, the Fall 2016 Antarctic data were added at this time.