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IceBridge Sander AIRGrav L1B Geolocated Free Air Gravity Anomalies

This data set contains Greenland and Antarctic gravity measurements taken from the Sander Geophysics Airborne Inertially Referenced Gravimeter (AIRGrav) system. The data were collected as part of NASA Operation IceBridge funded campaigns.

Operation IceBridge products may include test flight data that are not useful for research and scientific analysis. Test flights usually occur at the beginning of campaigns. Users should read flight reports for the flights that collected any of the data they intend to use. Check IceBridge campaign Flight Reports for dates and information about test flights.

Table of Contents

  1. Contacts and Acknowledgments
  2. Detailed Data Description
  3. Data Access and Tools
  4. Data Acquisition and Processing
  5. References and Related Publications
  6. Document Information

Citing These Data

The following example shows how to cite the use of this data set in a publication. For more information, see our Use and Copyright Web page.

Cochran, James R., and Robin E. Bell. 2010, updated 2013. IceBridge Sander AIRGrav L1B Geolocated Free Air Gravity Anomalies, [indicate subset used]. Boulder, Colorado USA: NASA DAAC at the National Snow and Ice Data Center. http://nsidc.org/data/iggrv1b.html.

Overview

Platform

NASA DC-8

NASA P-3B

Sensor

Sander Geophysics Airborne Inertially Referenced Gravimeter (AIRGrav)

Spatial Coverage

Greenland and Antarctica

Spatial Resolution

5 to 7 kilometers

Temporal Coverage

16 October 2009 to the present

Temporal Resolution

Seasonal

Parameters

Gravity

Data Format

Geosoft space-delimited ASCII text files

Metadata Access

View Metadata Record

Get Data

FTP

1. Contacts and Acknowledgments

Investigator(s) Name and Title

James R. Cochran, Robin E. Bell
Lamont-Doherty Earth Observatory
109C Oceanography
61 Route 9W - PO Box 1000
Palisades, New York 10964-8000 USA

Technical Contact

NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449  USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
e-mail: nsidc@nsidc.org

Acknowledgements

Data collection was supported by NASA grants NNX09AR49G, NNG10HP20C, and NNX10AT69G. We thank Stefen Elieff, Kevin James, Sean O'Rourke and Eric Renaud from Sander Geophysics; and Kirsteen Tinto, Indrani Das and Timothy Creyts from Lamont-Doherty Earth Observatory who participated in data collection.

2. Detailed Data Description

The AIRGrav data contain gravity measurements including acceleration data in three orthogonal directions.

Format

Data files are in Geosoft ASCII format.

File and Directory Structure

Data files are organized on the FTP site, ftp://n4ftl01u.ecs.nasa.gov/SAN2/ICEBRIDGE_FTP/, as described in Figure 1.

directory structure

Figure 1. Directory Structure

File Naming Convention

Gravity data files are named using the conventions shown below. File name variables are described in Table 1:

IGGRV1B_20100322.xyz
IGGRV1B_YYYYMMDD.xyz

Where:

Table 1. Gravity Data File Naming Convention
Variable Description
IGGRV1B Short name for IceBridge Sander AIRGrav L1B Geolocated Free Air Gravity Anomalies
YYYY Four-digit year
MM Two-digit month
DD Two-digit day
.xyz Indicates ASCII text data file

File Size

Data files range from approximately 5 MB to 30 MB.

Volume

The entire data set is approximately 3.7 GB.

Spatial Coverage

Spatial coverage for the IceBridge Sander AIRGrav Level-1B Geolocated Free Air Gravity Anomalies campaigns include the Greenland and Antarctic areas. In effect, this represents the two coverages noted below.

Arctic / Greenland:
Southernmost Latitude: 60° N
Northernmost Latitude: 90° N
Westernmost Longitude: 180° W
Easternmost Longitude: 180° E

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

Spatial Resolution

Spatial resolution depends on the airplane speed and the length of the filter. This is typically 5 to 7 km, or aircraft speed * 70 to 140 seconds. Narrow features can be detected, but generally with reduced amplitude.

Projection and Grid Description

Projection: Polar Stereographic.

Grid: 5 to 7 km spatial grid depending on data line spacing.

Temporal Coverage

16 October 2009 to the present.

Temporal Resolution

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.

Parameter or Variable

The IceBridge Sander AIRGrav Level-1B Geolocated Free Air Gravity Anomalies data set contains gravity measurements. Gravity data are measured as accelerations along three orthogonal axes, and are reduced to free Air Gravity anomaly values by removing the accelerations due to aircraft motion.

Parameter Description

2009-2012 Antarctica Gravity Data File Parameters

The Antarctica gravity data files contain fields as described in Table 2.

Table 2. 2009-2012 Antarctica Gravity Data Parameter Description
Col Name Format Units Description
1 LAT F15.7 Degrees Latitude decimal degrees WGS-84
2 LONG F15.7 Degrees Longitude decimal degrees WGS-84
3 DATE I9 YYYYMMDD Date
4 DAY I5 DDD Day of year
5 FLIGHT I10 1FFF.XX Line number
6 TIME F11.2 Seconds UTC seconds past midnight (rolls over)
7 FIDUCIAL F11.2 Seconds UTC seconds past midnight (continuous)
8 PSX F15.2 Meters Polar stereographic X WGS-84, SCAR projection (not included in 2010 and 2011 Greenland data)
9 PSY F15.2 Meters Polar stereographic Y WGS-84, SCAR projection (not included in 2010 and 2011 Greenland data)
10 UPSX F15.2 Meters Universal Polar stereographic X WGS-84
11 UPSY F15.2 Meters Universal Polar stereographic Y WGS-84
12 WGSHGT F11.2 Meters Height WGS-84 (height above GRS80 ellipsoid)
13 FX F15.2 mGal Gravimeter X acceleration
14 FY F15.2 mGal Gravimeter Y acceleration
15 FZ F15.2 mGal Gravimeter Z acceleration
16 EOTGRAV F15.2 mGal Eotvos and latitude corrected gravity, unfiltered
17 FACOR F11.2 mGal Free air correction
18 INTCOR F11.2 mGal Intersection levelling correction
19 FAG070 F11.2 mGal Free Air Gravity, 70s full wavelength line filter
20 FAG100 F11.2 mGal Free Air Gravity, 100s full wavelength line filter
21 FAG140 F11.2 mGal Free Air Gravity, 140s full wavelength line filter
22 FLTENVIRO I11 N/A -1 = no data, 0 normal conditions, 1 = disturbed conditions

Flight Environment (FLTENVIRO), Column 22, indicates data availability. At present, it is a simple flag based on what was used for the earlier by-line data deliveries:
-1 = no data (data that was between lines)
0 = normal conditions (lines that had extension numbers < 50 previously)
1 = disturbed conditions (lines that had extension numbers > 50 previously)

Additional Notes:

  • Position channel is the location of the gravimeter, not the GPS antenna.
  • NULL values are represented by "NaN".

2010-2012 Greenland Gravity Data File Parameters

The 2010-2012 Greenland gravity data files contain fields as described in Table 3.

Table 3. 2010-2012 Greenland Gravity Data Parameter Description
Col Name Format Units Description
1 LAT F15.7 Degrees Latitude decimal degrees WGS-84
2 LONG F15.7 Degrees Longitude decimal degrees WGS-84
3 DATE A9 N/A Date
4 DAY A5 N/A Day of year
5 FLT I10 N/A Flight number
6 TIME F11.2 Seconds UTC seconds past midnight (rolls over)
7 FIDUCIAL F11.2 Seconds UTC seconds past midnight (continuous)
8 UPSX F15.2 Meters Universal Polar stereographic X WGS-84
9 UPSY F15.2 Meters Universal Polar stereographic Y WGS-84
10 WGSHGT F11.2 Meters Height WGS-84 (height above GRS80 ellipsoid)
11 FX F15.2 mGal Gravimeter X acceleration
12 FY F15.2 mGal Gravimeter Y acceleration
13 FZ F15.2 mGal Gravimeter Z acceleration
14 EOTGRAV F15.2 mGal Eotvos and latitude corrected gravity, unfiltered
15 FACOR F11.2 mGal Free air correction
16 INTCOR F11.2 mGal Intersection levelling correction
17 FAG070 F11.2 mGal Free Air Gravity, 70s full wavelength line filter
18 FAG100 F11.2 mGal Free Air Gravity, 100s full wavelength line filter
19 FAG140 F11.2 mGal Free Air Gravity, 140s full wavelength line filter
20 FLTENVIRO I11 N/A -1 = no data, 0 normal conditions, 1 = disturbed conditions

The flight environment indicates data availability. At present, it is a simple flag based on what was used for the earlier by-line data deliveries:
-1 = no data (data that was between lines)
0 = normal conditions (lines that had extension numbers < 50 previously)
1 = disturbed conditions (lines that had extension numbers > 50 previously)

Additional Notes:

  • Position channel is the location of the gravimeter, not the GPS antenna.
  • NULL values are represented by "NaN".

3. Data Access and Tools

Get Data

Data are available via FTP.

Software and Tools

NSIDC provides MATLAB readers that read IGGRV1B aircraft gravity data files. The data files also may be opened by any ASCII text reader.

4. Data Acquisition and Processing

Data Acquisition Methods

The gravimeter is located as near the airplane center of mass as possible. Simultaneously acquired gravimeter output GPS data are recorded on hard disks on the plane. Following the flight this data is downloaded onto a PC for processing.

Derivation Techniques and Algorithms

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.

Table 4. Grid Filters Used for 2009-2011 Antarctica
Area Grid Filter
Coats / Filchner 7000 m half-wavelength spatial filter
Pine Island / Thwaites 6000 m half-wavelength spatial filter
Abbott 7000 m half-wavelength spatial filter
Larson 7000 m half-wavelength spatial filter
Pine Island (high altitude) 7000 m half-wavelength spatial filter
Peninsula (high altitude) 5000 m half-wavelength spatial filter

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.

Sensor or Instrument Description

The gravity instrument is a Sander AIRGrav designed for airborne applications. The AIRGrav system consists of a three-axis gyro-stabilized, Schuler-tuned inertial platform on which three orthogonal accelerometers are mounted. The primary gravity sensor is the vertical accelerometer that is held within 10 arc-seconds (0.0028 degree) of the local vertical by the inertial platform, monitored through the complex interaction of gyroscopes and two horizontal accelerometers (Sander et al. 2004). An advantage of the AIRGrav system over other airborne gravimeters is that it has been shown to be capable of collecting high-quality data during draped flights (Studinger et al. 2008). The gravimeter records accelerations arising from variations in the Earth's gravity field and accelerations experienced by the airplane. These accelerations are recorded at 128 Hz. Aircraft accelerations are obtained utilizing differential GPS measurements. Gravity Data rate is 2 Hz.

5. References and Related Publications

Sander, S., M. Argyle, S. Elieff, S. Ferguson, V. Lavoie, and L. Sander. 2004. The AIRGrav airborne gravity system, in Airborne Gravity 2004 - Australian Society of Exploration Geophysicists Workshop, edited by R. Lane, pp. 49-53, Geoscience Australia, http://www.ga.gov.au/image_cache/GA16642.pdf.

Studinger, M., R. E. Bell, and N. Frearson. 2008. Comparison of AIRGrav and GT-1A airborne gravimeters for research applications, Geophysics, 73: 151-161.

Related Data Collections

Related Web Sites

IceBridge Data Web site at NSIDC (http://nsidc.org/data/icebridge/index.html).

IceBridge Web site at NASA (http://www.nasa.gov/mission_pages/icebridge/index.html).

ICESat/GLAS Web site at NASA Wallops Flight Facility (http://glas.wff.nasa.gov/).

ICESat/GLAS Web site at NSIDC (http://nsidc.org/daac/projects/lidar/glas.html).

Improving GLAS data using GRACE (http://nsidc.org/data/icesat/grace.html).

6. Document Information

Acronyms and Abbreviations

The acronyms used in this document are listed in Table 5.

Table 5. Acronyms and Abbreviations
Acronym Description
AIRGrav Airborne Inertially Referenced Gravimeter
ASCII American Standard Code for Information Interchange
CIRES Cooperative Institute for Research in Environmental Science
FTP File Transfer Protocol
L1B Processing Level 1B
NASA National Aeronautics and Space Administration
NSIDC National Snow and Ice Data Center
PEN-LVIS High altitude flight over the Antarctic Peninsula
PIG Pine Island Glacier
PIG-LVIS High altitude flight over Pine Island Glacier
SCAR Scientific Committee of Antarctic Research
URL Uniform Resource Locator
UTC Universal Time Code
WGS-84 World Geodetic System 1984

Document Creation Date

16 March 2011

Document Revision Date

05 October 2011

23 May 2012

16 August 2012

27 November 2012

28 February 2013

14 June 2013

Document URL

http://nsidc.org/data/docs/daac/icebridge/iggrv1b/index.html