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Data Set ID:
IGCMG2

IceBridge CMG GT-1A Gravimeter L2 Geolocated Free Air Gravity Disturbances, Version 1

This data set contains geolocated free air gravity disturbances derived from measurements taken over Antarctica using the GT-1A gravity meter S-019. The data were collected by scientists working on the Investigating the Cryospheric Evolution of the Central Antarctic Plate (ICECAP) project, which is funded by the National Science Foundation (NSF) and the Natural Environment Research Council (NERC) with additional support from NASA Operation IceBridge.

Geographic Coverage

Parameter(s):
  • Geodetics/Gravity > Gravity
Spatial Coverage:
  • N: -53, S: -90, E: 180, W: -180

Spatial Resolution:
  • 6.5 km x 6.5 km
Temporal Coverage:
  • 22 November 2012
Temporal Resolution: 12 month
Data Format(s):
  • XML
  • ASCII Text
Platform(s) BT-67
Sensor(s): CMG-GT-1A
Version: V1
Data Contributor(s): Donald Blankenship, Duncan Young, Thomas Richter, Jamin Greenbaum
Data Citation

As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.

Blankenship, D. D., D. A. Young, T. G. Richter, and J. S. Greenbaum. 2014. IceBridge CMG GT-1A Gravimeter L2 Geolocated Free Air Gravity Disturbances, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: http://dx.doi.org/10.5067/3X4CIKKSYQRU. [Date Accessed].

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Detailed Data Description

Format

Each data file is in space-delimited ASCII text format with an associated XML file. The XML files contain file creation, point latitudes and longitudes, and campaign metadata.

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File and Directory Structure

Data files are available in the https://n5eil01u.ecs.nsidc.org/ICEBRIDGE/IGCMG2.001/ directory. Files are organized into folders by year, month, and day, for example: /2012.12.05/.

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File Naming Convention

The files are named according to the following convention and as described in Table 1:

Example file names:

IGCMG2_2012339_SCT_JKB2h_Y46b_grvfld.txt

IGCMG2_2012339_SCT_JKB2h_Y46b_grvfld.txt.xml

IGCMG2_YYYYDOY_PPP_JKB2h_TTTT_grvfld.xxx

Where:

Table 1. File Naming Convention
Variable Description
IGCMG2 Short name for IceBridge CMG GT-1A Gravimeter L2 Geolocated Free Air Gravity Disturbances
YYYY Four-digit year of survey
DOY Day of year of survey
PPP Geographic area (Project)
JKB2h Host platform for timing (System)
TTTT Transect name within Project
grvfld Gravity field
.xxx indicates .txt or .xml file
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File Size

ASCII text data files range from approximately 19 KB to 1.8 MB.

XML files range from approximately 10 KB to 18 KB.

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Volume

The entire data set is approximately 25 MB.

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Spatial Coverage

Spatial coverage for this data set is Antarctica, represented by this extent:

Southernmost Latitude: 90° S
Northernmost Latitude: 53° S
Westernmost Longitude: 180° W
Easternmost Longitude: 180° E

Spatial Resolution

Small features are progressively suppressed due to low pass filtering to create final output. The filter used has a half amplitude point at 150 second wave period corresponding to about a 6.5 km wide gravity feature.

Projection and Grid Description

WGS-84 ellipsoid; International Terrestrial Reference Frame (ITRF) 2008

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Temporal Coverage

These data were collected from 23 November 2012 to the present as part of the ICECAP and Operation IceBridge funded campaigns.

Temporal Resolution

Temporal resolution is seasonal. IceBridge campaigns are conducted on an annual repeating basis. Arctic and Greenland campaigns are conducted during March, April, and May, and Antarctic campaigns are conducted during October and November. Campaigns for this data set typically extend from November to early January.

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Parameter or Variable

Parameter Description

Data parameters are described in Table 2.

Table 2. Free Air Gravity Disturbance Parameter Description
Parameter Description Units
YEAR Year of survey Year, UTC
DOY Day Of Year of survey Day, UTC
SOD Second of day of survey Seconds, UTC
LON Longitude Angle Degrees WGS-84
LAT Latitude Angle Degrees WGS-84
GRV Free Air Gravity Disturbance mGal
AC_ELEVATION Aircraft Elevation Meters

Sample Data Record

Shown below are the first ten data records from the file: IGCMG2_2012339_SCT_JKB2h_Y46b_grvfld.txt.

sample data record
Figure 1. Sample Data Record
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Software and Tools

The data files can be read by any software capable of reading ASCII text.

Quality Assessment

This data set has not been analyzed sufficiently to assign quantitative accuracy values. A related set obtained on the same field season in West Antarctica exhibited noise with a RMS amplitude of 2.2 to 2.4 milligal (via analysis of crossovers, and without leveling). The East Antarctic data set was gathered under more varied conditions. Smooth flights with few altitude changes will have comparable or better accuracy.

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Data Acquisition and Processing

Theory of Measurements

This data set was created using the current standard techniques of use for stabilized platform airborne scalar gravimeters. The gravimeter consists of a high sensitivity accelerometer which is kept vertical via a gimbaled platform. Carrier phase GPS is recorded and post processed to determine the accelerations on the gravimeter which are due to aircraft motions. The vertical accelerations due to aircraft motions are subtracted from the vertical accelerations observed by the gravimeter to create a residual signal which consists of the measured acceleration due to gravity contaminated with noise and deterministic non-gravitational accelerations. This residual is corrected for known meter characteristics and other non-gravitational deterministic effects (Eotvos, etc.) then low pass filtered to produce the best estimate of the actual gravitational acceleration values.

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Data Acquisition Methods

The GT-1A gravimeter instrument was operated according to Canadian Microgravity published procedures.

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Derivation Techniques and Algorithms

The data reduction algorithms are embedded within the vendor's reduction software. A Kalman filter in the software is used to estimate the gravity result from recorded meter data including vertical and horizontal accelerations.

Gravity values recorded by the meter were referenced using a Lacoste and Romberg portable gravity meter (G-399) to conduct regular gravity ties between the aircraft parking site and a gravity measurement point on the north side of the Casey Station Science Building, where a value of 982379.929 mGal had been previously recorded by Geoscience Australia.

Table 3. Gravity Tie Measurements during 2011 at Casey Station (scale factor of 10.04547 for first 2 digits and 1.04543 for next 2 digits for G-399)

Date

Location

Reading (counts)

Reading (mGals)

Science avg (mGal)

Science - Aircraft (mGal)

Nov. 20, 2013

Science

5770.5

6,022.163

6022.455535

98.8389026

Aircraft

5676.24

5,923.617

Science

5771.06

6,022.748

Nov. 22, 2013

Science

5771.03

6,022.717

6022.899843

99.10548045

Aircraft

5676.41

5,923.794

Science

5771.38

6,023.083

Nov. 28, 2013

Science

5771

6,022.686

6022.94166

99.45048395

Aircraft

5676.12

5,923.491

Science

5771.49

6,023.198

Nov. 30, 2013

Science

5771.51

6,023.219

6023.276198

99.37728825

Aircraft

5676.51

5,923.899

Science

5771.62

6,023.334

Dec. 1, 2013

Science

5771.51

6,023.219

6023.286652

99.30410495

Aircraft

5676.59

5,923.983

Science

5771.64

6,023.355

Dec. 1, 2013

Science

5771.61

6,023.323

6023.422558

99.36682795

Aircraft

5676.66

5,924.056

Science

5771.8

6,023.522

Dec. 2, 2013

Science

5771.65

6,023.365

6023.427785

99.3197816

Aircraft

5676.71

5,924.108

Science

5771.77

6,023.491

Trajectory and Attitude Data

The aircraft trajectory data is available in the position and height data included in the data records. Attitude data is available in the input data files listed in the Processing Steps section.

Processing Steps

Input data: IceBridge CMG 1A Dynamic Gravity Meter Time-Tagged L1B Vertical Accelerations (IGCMG1B) and IceBridge GPS/IMU L1B Primary Position and Attitude Solution (IPUTG1B).

Processing: GTGRAV v45 proprietary Gravimetric Technologies code.

Data were processed using differential dual carrier phase GPS solutions to estimate aircraft vertical and horizontal accelerations.

An (effectively) 150 second filter was used to smooth the resulting data.

Data were automatically edited based on horizontal and vertical acceleration thresholds.

The final result is the disturbance from the GRS-80 conventional series formula for the global gravity field corrected with the 1967 international free air correction formula:

GRS80 conventional normal gravity = 9.780327*(1 + 0.00530 sin2 (LAT) − 0.0000058 sin2 (2*LAT)) m/s2

Free Air Correction = (0.308768 − 0.000440*sin2(LAT) − 0.000000144*AC_ELEVATION)*AC_ELEVATION

No continuation or leveling has been applied to the data.

Missing values have been replaced by "nan."

Error Sources

Environmental conditions: the precise accelerations and attitudes recorded by the gravimeter are subject to degradation by aircraft maneuvering, turbulence and shocks; and to a lesser extent by smooth changes in altitude and turns. The best results are from smooth, straight, level flight as can be determined by examination of the GPS trajectory data and instrument records of accelerometer saturation events. The resolution of gravity data is fundamentally limited by the distance from the source of the gravity anomaly; in the case of this survey that was determined by the thickness of the ice, the depth of the sea water, and the height of the aircraft (typically 200 to 800 meters).

Positioning limitations: The GPS data were processed using a base at Casey Station as the reference for differential GPS processing. Observations at long distances from this base are expected to be degraded somewhat. In particular, the November 22 and 23, 2012 flights between Davis and Casey Station were flown at more than 1000 km from the GPS base.

Hardware issues: The GT-1A instrument was running new development software for polar operations that may have introduced errors. Power was lost to the instrument due to blizzards at Casey Skiway prior to the November 22, 2012 and November 26 2012 flight, the instrument was still warming up during these flights.

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Sensor or Instrument Description

Gravity was measured using GT-1A gravity meter S-019 leased from CMG Operations of Perth, Australia. The GT-1A system is a three axis stabilized gravimeter, with a ±500 Gal dynamic range primary vertical accelerometer. Accelerations are sampled on board at 300 Hz. These are averaged over 16 sample intervals, for an effective sample rate of 18.75 Hz. The system has demonstrated accuracy on the order of 0.5 mGal in airborne surveys with spatial resolutions of a few km.

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References and Related Publications

Contacts and Acknowledgments

Donald D. Blankenship, Duncan A. Young, Thomas G. Richter, Jamin S. Greenbaum
University of Texas at Austin
Institute for Geophysics
Austin, TX, 78759-8500

Acknowledgments: 

Support: NASA ICEBRIDGE (NNX11AD33G); AAD Project 4077. Funding: NASA's Operation Ice Bridge (grant NNX11AD33G) to the University of Texas at Austin.

Document Information

Document Creation Date

04 June 2014

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