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IceBridge BGM-3 Gravimeter L2 Geolocated Free Air Anomalies, Version 1
This data set contains free air anomalies measurements taken over Antarctica using the BGM-3 Gravimeter. The data were collected by scientists working on the Investigating the Cryospheric Evolution of the Central Antarctic Plate (ICECAP) project funded by the National Science Foundation (NSF) and the Natural Environment Research Council (NERC) with additional support from NASA Operation IceBridge.
|Data Contributor(s):||Donald Blankenship, Duncan Young, Thomas Richter, Jamin Greenbaum|
|Metadata XML:||View Metadata Record|
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. 2011, updated 2014. IceBridge BGM-3 Gravimeter L2 Geolocated Free Air Anomalies, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: http://dx.doi.org/10.5067/8DJW56PKY133. [Date Accessed].
Detailed Data Description
The data files are in space-delimited ASCII text format, with a header offset by # leading characters, corresponding to the NASA Aerogeophysical ASCII data standard. Each data file is paired with an associated XML file. The XML files contain location, platform, and instrument metadata.
Data are available on the FTP site in the
https://n5eil01u.ecs.nsidc.org/ICEBRIDGE/IGBGM2.001/. Within this directory, the folders are organized by date, for example
/2011.12.20/. Each folder contains .txt and associated .xml files.
Files are named according to the following convention and as described in Table 1:
||File name prefix indicating IceBridge BGM-3 Gravimeter L2 Geolocated Free Air Anomalies|
||Four-digit year of survey|
||Day of year of survey|
||Geographic track line|
||Indicates ASCII text file (.txt), or XML file (.xml)|
The data files range from approximately 21 KB to 842 KB.
XML files range from approximately 10 KB to 54 KB.
The entire data set is approximately 68 MB.
These data were primarily collected over the Wilkes Subglacial Basin and Aurora Subglacial Basin in East Antarctica.
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
Smaller features are progressively suppressed due to low pass filtering to create final output. The filter used has a half amplitude point at 185 second wave period corresponding to about a 7.9 km wide gravity feature. For much of the survey, the distance to the gravity anomaly sources due to ice thickness precludes significantly smaller observable features.
Projection and Grid Description
WGS-84 ellipsoid; ITRF 2008
These data were collected from 08 January 2009 to 12 December 2011 as part of ICECAP, NSF, NERC, and Operation IceBridge funded campaigns.
The BGM-3 instrument was not flown during the Antarctica 2012 campaign.
Note: 2008 campaign data were acquired pre-IceBridge.
ICECAP campaigns were conducted on an annual basis. East Antarctic campaigns for this data set typically extend from November to early January.
The BGM-3 Gravimeter L2 Geolocated Free Air Anomalies data files contain fields as described in Table 2.
|YEAR||Year of survey||UTC|
|DOY||Day of Year of survey||UTC|
|SOD||Second of day||UTC|
|LON||Longitude||Decimal degrees WGS-84|
|LAT||Latitude||Decimal degrees WGS-84|
|GRV||Free Air Gravity Disturbance (WGS-84)||mGal|
|AC-ELEVATION||Aircraft elevation||Meters WGS-84|
Sample Data Record
Figure 1 shows the first ten data records from the data file
Software and Tools
No special tools are required for the ASCII text files.
The RMS of random error implied by crossover statistics is about 3.6 milligal. This is without the application of leveling algorithms.
Data Acquisition and Processing
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.
Gravity was measured using BGM-3 gravity meter 203 on loan from the National Geospatial Intelligence Agency's Geodetic Survey division, St. Louis, MO.
The output of the gravimeter consists of accelerations measured in real time. These accelerations are recorded at a rate of 1 Hz by the external data recording system which time stamps the data as they are acquired and recorded.
Dual frequency carrier phase GPS data were recorded separately for use in the gravity data processing.
Trajectory and Attitude Data
The aircraft trajectory data is available in the position and height data included in the data records. These data were derived from post-processed carrier phase GPS records. Aircraft attitude data is available in the IPUTG1B data set listed above.
Data were processed using dual carrier phase GPS solutions to estimate aircraft vertical and horizontal accelerations.
Horizontal tilts of the stabilized platform were estimated using the equations of motion of the platform and horizontal accelerations derived from GPS positions. These tilts were used to derive tilt corrections to the vertical accelerations measured by the gravimeter.
A finite impulse response low-pass filter with half amplitude frequency point of 0.0054 Hz (185 seconds) was used to smooth the resulting data.
Data were automatically edited based on maximum allowed platform tilt and maximum allowed unfiltered acceleration variance thresholds. Very little manual editing was performed on the results which passed automatic editing.
The final result is the disturbance from the WGS-84 model for the global gravity field.
Free Air Correction used the following formula: (0.3087691) − (0.0004398*sin(LAT)2 * AC_ELEVATION) − (7.2125e-8 * (AC_ELEVATION2)).
No continuation or leveling has been applied to the data.
Missing values have been replaced by "nan".
On 02 July, 2013, the V01 2009, 2010, and 2011 Antarctica data were replaced by V01.1. V01.1 data files include UTC timestamps and re-ordered fields.
In Antarctica 2009, the primary platform failed after ICP2/F06 (J316/2009) and was replaced for ICP2/F12 (J331/2009).
In Antarctica 2010, the gravity meter suffered from drift issues, and the sensor cable was replaced post season.
Full leveling corrections have not yet been performed for the Antarctica 2010 dataset, and a further release is expected.
Gravity was measured using BGM-3 gravity meter 203, on loan from the National Geospatial Intelligence Agency's Geodetic Survey division, St. Louis, MO.
The BGM-3 is a two-axis stabilized platform system which uses an inertial grade accelerometer as the vertical acceleration sensor. Integrated acceleration data is recorded at 1 Hz by an external data acquisition device.
References and Related Publications
Contacts and Acknowledgments
Donald D. Blankenship, Duncan Young, Thomas G. Richter, Jamin S. Greenbaum
University of Texas at Austin
Institute for Geophysics
Austin, TX, 78759-8500
Funding: NASA's Operation Ice Bridge (grant NNX11AD33G) to the University of Texas at Austin. 2008 campaign data were acquired pre-IceBridge.
Document Creation Date
04 February 2015