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Data Set ID:
IGBGM1B

IceBridge BGM-3 Gravimeter L1B Time-Tagged Accelerations, Version 1

This data set contains vertical acceleration values for Antarctica using the BGM-3 Gravimeter. The data were collected by scientists working on the the International Collaborative Exploration of the Cryosphere through Airborne Profiling (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: Not Specified
Temporal Coverage:
  • 31 December 2008 to 24 December 2011
(updated 2014)
Temporal Resolution: Varies
Data Format(s):
  • XML
  • ASCII Text
Platform(s) BT-67
Sensor(s): BGM-3
Version: V1
Data Contributor(s): Donald Blankenship, Scott Kempf, Duncan Young, Thomas Richter, Martin Siegert, Jason Roberts, Tas van Ommen, Emmanuel Lemeur
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., S. D. Kempf, D. A. Young, T. G. Richter, M. J. Siegert, J. L. Roberts, T. van Ommen, and E. Lemeur. 2012, updated 2014. IceBridge BGM-3 Gravimeter L1B Time-Tagged Accelerations, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: http://dx.doi.org/10.5067/5CHZMTE5DLU8. [Date Accessed].

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Detailed Data Description

Format

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.

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File and Directory Structure

Data are available on the FTP site in the https://n5eil01u.ecs.nsidc.org/ICEBRIDGE/IGBGM1B.001/ directory. Within this directory are folders organized by date, for example /2011.12.13/.

Data stored in the /2008.12.31/ folder were collected as part of the 2009 campaign.

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File Naming Convention

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

IGBGM1B_2011346_TOT_JKB2e_X15b_accel.txt 
IGBGM1B_2011346_TOT_JKB2e_X15b_accel.txt.xml

IGBGM1B_YYYYDOY_AAA_JKBnx_XXXX_accel.xxx

Where:

Table 1. Naming Convention
Variable Description
IGBGM1B File name prefix indicating IceBridge BGM-3 Gravimeter L1B Time-Tagged Accelerations
YYYY Four-digit year of survey
DOY Day of year of survey
AAA Geographic area
JKBnx Host platform
XXXX Geographic track line
accel Acceleration values
.xxx Indicates ASCII text file (.txt), or XML file (.xml)
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File Size

The data files range from approximately 16 KB to 1.5 MB.

XML files range from approximately 10 KB to 56 KB.

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Volume

The entire data set is approximately 204 MB.

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Spatial Coverage

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: 90° E
Easternmost Longitude: 180° E

Spatial Resolution

Not applicable to these raw instrument outputs.

Projection and Grid Description

WGS-84 ellipsoid; ITRF 2008

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Temporal Coverage

These data were collected from 31 December 2008 to 24 December 2011 as part of ICECAP, NSF, NERC, and Operation IceBridge funded campaigns. Data collected in late December 2008 are part of the 2009 campaign.

Temporal Resolution

ICECAP campaigns were conducted on an annual basis. East Antarctic campaigns for this data set typically extend from November to early January.

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Parameter or Variable

Parameter Description

The gravimeter L-1B time-tagged accelerations files contain fields as described in Table 2.

Table 2. File Parameter Description
Parameter Description Units
YEAR Year of survey UTC
DOY Day of Year of survey UTC
SOD Second of day UTC
SEQ Packet sequence number n/a
LON Longitude Decimal degrees WGS-84
LAT Latitude Decimal degrees WGS-84
AC-ELEVATION Aircraft elevation at CG antenna Meters WGS-84
ROLL Roll, right wing down positive Degrees
PITCH Pitch, nose up positive Degrees
HEADING Heading, w.r.t north Degrees
VERT_ACCELERATION Vertical acceleration milliGals

Sample Data Record

A sample of the data file: IGBGM1B_2010321_ICP3_JKB2b_TF014a_accel.txt is shown in Figure 1.

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

No special tools are required for the ASCII text files.

Quality Assessment

Quality depends on the flight conditions for each data transect and must be assessed on a flight by flight basis. Smooth flight with small trajectory changes will produce good data.

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Data Acquisition and Processing

Theory of Measurements

This data set has four data types:

  1. Timestamps: time in UTC
  2. Aircraft positions: geodetic location of the reference GPS antenna
  3. Aircraft attitudes: three angles to describe the aircraft orientation in reference to direction North and the local vertical.
  4. Force on a constrained vertical sensor converted to equivalent acceleration of an unconstrained mass reported as Vertical Acceleration. Vertical Acceleration is the purpose of this data set. The other fields are auxiliary data.

A sensitive accelerometer is held vertical by a 2-axis platform which counters aircraft pitch and roll. The measured force on the accelerometer is reported as the Vertical Acceleration by the device. This is the sum of gravitational force and force due to inertial accelerations in the vertical direction.

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Data Acquisition Methods

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.

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Derivation Techniques and Algorithms

Raw instrument outputs are recorded with timestamps then minimally processed to convert from the native instrument output format to ASCII formatted numerical values.

Trajectory and Attitude Data

Trajectory and attitude data from GPS and an IMU external to the instrument are prepended to instrument data to create the data records in these files.

Processing Steps

The raw instrument output format is captured at 1 Hz and time stamped. The raw format is converted to ASCII numeric values and recorded. This raw numeric data consists of a value which is multiplied by a Scale Factor and added to a Bias to obtain the acceleration values in milligals, which is the value recorded in these data files. The header for each file has entries for the Scale Factor and Bias used.

The time stamps are slightly corrected by a few milliseconds to remove jitter from the acquisition system.

Version History

On 02 May, 2012, V01 2011 Antarctica data were replaced by V01.1. In V01.1, additional fields were added containing low-precision real-time position and orientation data resampled to match the times of the existing geophysical samples.

On 26 April 2013, the 2009, 2010, and 2011 Antarctica data were replaced. The format of 2009 and 2010 Antarctica data was revised to that of the 2011 Antarctica data, ensuring the time stamps are UTC year, day, and time, and adding georeferencing fields including real time position, roll, pitch and heading. For some campaigns V01.2 includes more flights than V01.1.

Error Sources

Instrumental Errors

he gravity instrumentation must be stabilized in temperature over a period of time related to the length of time it has been off and uncontrolled. For most of these data the instrument was well stabilized. For data from the 2010-11 season, the instrument had large issues with drift.

Environmental Conditions

The precise accelerations recorded by the gravimeter are subject to degradation by aircraft maneuvering, turbulence and shocks, and to a lesser extent by smooth changes in altitude. The best results are from smooth, straight, level flight as can be determined by examination of the GPS trajectory.

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Sensor or Instrument Description

Instrument name: Bell Aerospace (now Lockheed-Martin) BGM–3 number #203.

Type: Two–axis stabilized platform scalar gravimeter.

Resolution: 1 Hz in time.

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References and Related Publications

Contacts and Acknowledgments

Donald D. Blankenship, Thomas G. Richter, Scott D. Kempf, Duncan A. Young
University of Texas at Austin
Institute for Geophysics
Austin, TX, 78759-8500, USA

Martin J. Siegert
University of Bristol
12 Berkeley Square University Road
Clifton, Bristol
BS8 1SS England

Jason L. Roberts and Tas van Ommen
Australian Antarctic Division
Kingston, Tasmania 7050, Australia
and
Antarctic Climate & Ecosystems Cooperative Research Centre
Hobart, Tasmania 7001, Australia

Emmanuel Le Meur
University of Grenoble Alpes, LGGE, UMR5183
38041 Grenoble, France

Note: Data file header contains personnel information for each transect.

Acknowledgments: 

NASA's Operation Ice Bridge (grant NNX09AR52G, contract NNG10HP06C (American Recovery and Reinvestment Act), and grant NNX11AD33G) to the University of Texas at Austin. NSF's International Polar Year activities (grant ANT-0733025) to the University of Texas at Austin, the Antarctic Ecosystems and Climate Collaborative Research Center at the University of Tasmania, and the UK's NERC grant NE/D003733/1 to University of Edinburgh. See data file header for funding information for each transect.

Document Information

Document Creation Date

10 June 2014

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