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Data Set ID:
ILNSA1B

IceBridge Narrow Swath ATM L1B Elevation and Return Strength, Version 2

This data set contains spot elevation measurements of Greenland, Arctic, and Antarctic sea ice acquired using the NASA Airborne Topographic Mapper (ATM) narrow-swath instrumentation. The data were collected as part of Operation IceBridge funded aircraft survey campaigns.

Version Summary:

Version 2 data are in HDF5 format beginning with the 2013 Arctic campaign.

Version 1 data are in Qfit binary format for 2012 and earlier campaigns.

  • Beginning with the 2013 Greenland campaign, the data file format is HDF5.
  • Qfit data files for all previous campaigns are to be replaced with HDF5 files.
  • The data set title (longname) changes from "IceBridge Narrow Swath ATM L1B Qfit Elevation and Return Strength" to "IceBridge Narrow Swath ATM L1B Elevation and Return Strength".

Geographic Coverage

Parameter(s):
  • Sea Ice > Sea Ice Elevation
Spatial Coverage:
  • N: -53, S: -90, E: 180, W: -180

  • N: 90, S: 60, E: 180, W: -180

Spatial Resolution:
  • 2 m x 2 m
Temporal Coverage:
  • 20 March 2013 to 7 November 2016
(updated 2017)
Temporal Resolution: Varies
Data Format(s):
  • HDF5
  • XML
Platform(s) C-130, DC-8, HU-25C, P-3B
Sensor(s): ATM, Narrow Swath ATM
Version: V2
Data Contributor(s): Michael Studinger
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.

Studinger, M. S. 2014, updated 2017. IceBridge Narrow Swath ATM L1B Elevation and Return Strength, Version 2. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: http://dx.doi.org/10.5067/CXEQS8KVIXEI. [Date Accessed].

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Detailed Data Description

Format

The 2013 data are in HDF5 format. The fundamental form of the ATM topography data is a sequence of laser footprint locations acquired in a swath along the aircraft flight track.

HDF5 Data

The root group in the HDF5 file contains individual parameters for the latitude, longitude and elevation of the laser footprint. The root group also contains two subgroups as described in Table 2. Each data file is paired with an associated XML file. The XML files contain location, platform, and instrument metadata.

Note: Currently ILNSA1B data for 2011 through 2012 are in qfit format stored separately as ILATM1B V01. Beginning with the 2013 Arctic campaign, all data are provided in HDF5 format. In the near future, qfit data from all campaigns prior to Spring 2013 will be replaced with HDF5 data and added to V02. For details on the qfit data, see the V01 documentation.

Note: For sub-sampled ATM data, see the IceBridge ATM L2 Icessn Elevation, Slope, and Roughness data set.

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File and Directory Structure

Data are available on the FTP site in the https://n5eil01u.ecs.nsidc.org/ICEBRIDGE/ILNSA1B.002/ directory. Within this directory, the folders are organized by date, for example /2016.11.07/.

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File Naming Convention

Files are named according to the following conventions and as described in Table 1:

ILNSA1B_20130425_113628.ATM4CT3.h5
ILNSA1B_20130425_113628.ATM4CT3.h5.xml

ILNSA1B_YYYYMMDD_HHMMSS.ATM4CT3.xxx

Where:

Table 1. File Naming Convention
Variable Description
ILNSA1B Short name for IceBridge Narrow Swath ATM L1B Elevation and Return Strength
YYYY Four-digit year of survey
MM Two-digit month of survey
DD Two-digit day of survey
HH Two-digit hours, beginning of file time
MM Two-digit minutes, beginning of file time
SS Two-digit seconds, beginning of file time
ATM4C Airborne Topographic Mapper instrument identification
T3 ATM transceiver designation
.xxx

Indicates file type:
.h5 = HDF5 data file
.xml = XML metadata file

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File Size

Elevation measurement HDF5 files range from approximately 107 KB to 26 MB.

XML files range from approximately 4 KB to 25 KB.

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Volume

The entire data set is approximately 56 GB.

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Spatial Coverage

Spatial coverage for the narrow swath ATM campaign includes Arctic, Greenland, and Antarctic sea ice.

Arctic and Greenland Sea Ice:
Southernmost Latitude 59° N
Northernmost Latitude: 90° N
Westernmost Longitude: 180° W
Easternmost Longitude: 180° E

Antarctic:
Southernmost Latitude: 90° S
Northernmost Latitude: 63° S
Westernmost Longitude: 180° W
Easternmost Longitude: 180° E

Spatial Resolution

The ATM surface elevation measurements have been acquired from a conically scanning LIDAR system. Coupled with the motion of the aircraft in flight, the resulting array of laser spot measurements is a tight spiral of elevation points. The surface elevation measurements generally consist of a pattern of overlapping roughly elliptical patterns on the surveyed surface, forming a swath of measurements along the path of the aircraft.

The angular swath width of the ATM narrow scan instrument is approximately 2.7 degrees off-nadir (5.4 degrees full angle). At a nominal altitude above ground of 450 m, that scan angle will yield a swath on the ground roughly 45 m wide.

Resolution varies with altitude flown, aircraft groundspeed, and scanner configuration for the LIDAR. For the narrow swath data, at a typical altitude of 450 m above ground level, an aircraft groundspeed of 250 knots, a laser pulse rate of 3 kHz, and a scan width of 2.7 degrees off-nadir, the average point density is one laser shot per 2 m2 within the swath. However, the sampling of laser shots in the laser swath is not evenly distributed.

Projection and Grid Description

Data are given in geographic latitude and longitude coordinates. Data coordinates are referenced to the WGS84 ellipsoid. Reference frame is prescribed by the International Terrestrial Reference Frame (ITRF) convention in use at the time of the surveys. For more on the reference frame, see the ITRF 2008 specification Web site.

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Temporal Coverage

These data were collected as part of Operation IceBridge funded campaigns beginning 20 March 2013 to 07 November 2016.

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.

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Parameter or Variable

The Narrow Swath ATM L1B Elevation and Return Strength data set includes sea ice elevation measurements, and relative transmitted and return reflectance.

The ATM times are rounded to 0.001 seconds. The ATM instrument operates at a sampling rate of 3 kHz. When rounding to 0.001 seconds, three points will appear with the same time stamp.

Parameter Description

HDF5 File Parameter Description

Parameters contained in HDF5 Narrow Swath ATM data files are described in Table 2.

Table 2. HDF Group and Contents Description

Group

Parameter

Description

Units

/ancillary_data/ reference_frame ITRF designation of reference frame Text name
Min_latitude Minimum value of latitude for this file Degrees
Min_longitude Minimum value of longitude for this file

Degrees

Max_latitude

Maximum value of latitude for this file

Degrees

Max_longitude

Maximum value of longitude for this file

Degrees

Header_text Raw data (in human readable form) containing comments or processing history of the parameter data. None
Header_binary Raw data (in binary form) containing comments or processing history of the parameter data. None
/(root) latitude Laser spot latitude Degrees
longitude Laser spot longitude Degrees
elevation Laser spot elevation above ellipsoid Meters
/instrument_parameters/ azimuth Scanner azimuth angle Degrees
gps_pdop GPS dilution of precision (PDOP) Dimensionless
pitch Pitch angle Degrees
roll Roll angle Degrees
rcv_sigstr Received (reflected) signal strength dimensionless relative values (or data numbers, DN)
xmt_sigstr Transmitted (start pulse) signal strength dimensionless relative values (or data numbers, DN)
pulse_width Laser received pulse width at half height, number of digitizer samples at 0.5 nanosecond per sample Count
rel_time Relative time measured from start of file Seconds
time_hhmmss GPS time packed, example: 153320.100 = 15 hours 33 minutes 20 seconds 100 milliseconds. Seconds

Sample HDF5 Data Record

Below is an illustration of elevation, latitude, and longitude values from a sample of the ILNSA1B_20130425_113628.ATM4CT3.h5 data file as displayed in the HDFView tool.

sample data record

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Software and Tools

The following external links provide access to software for reading and viewing HDF5 data files. Please be sure to review instructions on installing and running the programs.

HDFView: Visual tool for browsing and editing HDF4 and HDF5 files.

Panoply netCDF, HDF and GRIB Data Viewer: Cross-platform application. Plots geo-gridded arrays from netCDF, HDF and GRIB datasets.

For additional tools, see the HDF-EOS Tools and Information Center.

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Data Acquisition and Processing

A laser altimeter measures range from the instrument to a target by measuring the elapsed time between emission of a laser pulse and detection of laser energy reflected by the target surface. Range to the target is calculated as half the elapsed emission/return time multiplied by the speed of light. Target range is converted to geographic position by integration with platform GPS and attitude or Inertial Measurement Unit (IMU) information.

Data Acquisition Methods

The ATM instrument package includes suites of LIDAR, GPS and attitude measurement subsystems. The instrument package is installed onboard the aircraft platform and calibrated during ground testing procedures. Installation mounting offsets, the distances between GPS and attitude sensors and the ATM LIDARs, are measured using surveying equipment. One or more ground survey targets, usually aircraft parking ramps, are selected and surveyed on the ground using differential GPS techniques. Prior to missions, one or more GPS ground stations are established by acquiring low rate GPS data over long time spans. Approximately one hour prior to missions both the GPS ground station and aircraft systems begin data acquisition. During the aircraft flight, the ATM instrument suite acquires LIDAR, GPS and attitude sensor data over selected targets, including several passes at differing altitudes over the selected ground survey calibration sites. The aircraft and ground systems continue to acquire data one hour post-mission. Instrument parameters estimated from the surveys of calibration sites are used for post-flight calculation of laser footprint locations. These parameters are later refined using inter-comparison and analysis of ATM data where flight lines cross or overlap.

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Derivation Techniques and Algorithms

Each ATM surface elevation measurement corresponds to one laser pulse. The measurements have not been re-sampled. The transmitted laser pulse and the received backscatter pulse from the ground surface are photodetected and captured by a waveform digitizer. Post-flight processing of the waveforms yields the time of flight between transmitted and received signals. This time of flight value is converted to a distance compensated for speed of light through atmosphere. GPS data is processed post-flight to yield the position of the aircraft at 0.5 second intervals. The scan azimuth of the LIDAR scanner mirror together with the aircraft attitude determine the pointing angle of the LIDAR. Aircraft position, pointing angle of the LIDAR, and range measured by the LIDAR are used to compute position of laser footprint on the ground.

Processing Steps

The following processing steps are performed by the data provider.

  1. Preliminary processing of ATM LIDAR data through the cvalid program, applying calibration factors to convert time of flight to range, scan pointing angles, and interpolate attitude to each LIDAR measurement.
  2. Processing of GPS data into aircraft trajectory files using double-differenced dual-frequency carrier phase-tracking.
  3. Determination of all biases and offsets: heading, pitch, roll, ATM-GPS [x,y,z] offset, scanner angles, range bias.
  4. Processing of the LIDAR and GPS data with all biases and offsets through the qfit program, resulting in output files containing a surface elevation (ellipsoid height) and a geographic location in latitude and east longitude, with ancillary parameters noted in Table 2.

Version History

Version 2.0 of the ILNSA1B data. Beginning with the 2013 Arctic campaign, all data are provided in HDF5 format. In the near future, qfit data from all campaigns prior to Spring 2013 will be replaced with HDF5 data.

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Sensor or Instrument Description

The ATM is an airborne LIDAR instrument used by NASA for observing the Earth's topography for several scientific applications, foremost of which is the measurement of changing Arctic and Antarctic icecaps and glaciers. The ATM instrument is a scanning airborne laser that measures surface elevation of the ice by timing laser pulses transmitted from the aircraft, reflected from the ground and returning to the aircraft. This laser pulse time-of-flight information is used to derive surface elevation measurements by combining measurement of the scan pointing angle, precise GPS trajectories and aircraft attitude information. The ATM measures topography as a sequence of points conically scanned in a swath along the aircraft flight track at rates up to 5000 measurements per second. The sampling frequency for the data is 3 kHz.

The ATM instruments are developed and maintained at NASA's WFF in Virginia, USA. During Operation IceBridge, the ATM has been installed aboard the NASA P-3 aircraft based at WFF, or the NASA DC-8 aircraft based at Dryden Air Force Base in Palmdale, California. During previous campaigns, the ATM has flown aboard other P-3 aircraft, several de Havilland Twin Otters (DHC-6), and a C-130. The ATM has been used for surveys flown in Greenland nearly every year since 1993. Other uses have included measurement of sea ice, verification of satellite radar and laser altimeters, and measurement of sea-surface elevation and ocean wave characteristics. The ATM often flies in conjunction with a variety of other instruments and has been participating in NASA's Operation IceBridge since 2009.

The ATM project has been acquiring lidar data over ice and snow regions since 1993. There have been many instrument upgrades over the years to ensure that the NASA ATM systems collect the most accurate lidar elevations possible. The ATM project normally installs and operates two lidars on the aircraft platform (P-3 or DC-8). From 2009 to 2010, data were provided to NSIDC only from the ATM 4B2T that collects wide scan lidar data. In 2011, a new ATM transceiver scanner assembly designated as ATM 4BT4 replaced the ATM 4BT2. The ATM 4BT2 and 4BT4 data are in the IceBridge ATM L1B Elevation and Return Strength data set.

The second lidar system on the aircraft, designated ATM 4CT3, was operated in the past as a backup to the ATM 4BT2 lidar instrument, or was modified to test alternate lidar system improvements. In 2011, the 4CT3 instrument was modified by replacing the original scanner motor assembly, which contained a 22-degree off-nadir mirror, with a newer scanner motor assembly containing a 2.7-degree off-nadir mirror. ATM 4CT3 laser power was reduced and data were collected using the narrow swath scanner. Analysis of the 2011 ATM 4CT3 low altitude data combined with the wider swath ATM 4BT4 data captured at the same time, shows great promise in helping sea ice scientists measure sea surface elevations over open leads. The current ATM 4CT3 narrow swath data are provided for sea ice missions only. The instrument is not used for land ice missions.

Note: CAMBOT images and .cam files containing aircraft position and attitude corresponding to the ATM qfit data can be found in the IceBridge CAMBOT L1B Geolocated Images data set.

Table 3 provides information on ATM transceivers used during IceBridge missions and the resultant filename designations.

Table 3. ATM System Designations by IceBridge Campaign
Year Campaign Wide ATM System* 
(xx) = Full Scan Angle (degrees)
Narrow ATM System* 
(x) = Full Scan Angle (degrees)
2009 Greenland 4BT2 (30) n/a
2009 Antarctica 4CT3 (44) n/a
2010 Greenland (DC-8) 4CT3 (44) n/a
2010 Greenland (P-3) 4BT2 (30) n/a
2010 Antarctica 4BT2 (30) n/a
2011 Greenland 4BT4 (30) 4CT3 (5)**
2011 Antarctica 4BT2 (30) 4CT3 (5)**
2012 Greenland 4BT4 (30) 4CT3 (5)**
2012 Antarctica 4BT4 (30) 4CT3 (5)**
2013 Greenland 4BT4 (30) 4CT3 (5)**
2014 Arctic 4BT4 (30) 4CT3/5AT3 (5)**
2014 Antarctic 5BT4 (30) 5AT3 (5)**
2015 Arctic 5AT3 (30) 5BT5 (5)
2015 Fall Arctic 5BT5 (5) *** n/a
2016 Greenland 5AT2 (n/a) n/a

* The ATM system designation is noted in the filename for each data file.
** Data are provided for sea ice missions only.

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References and Related Publications

Contacts and Acknowledgments

William Krabill
NASA/Wallops Flight Facility (WFF)
Code 614.1
Hydrospheric & Biospheric Sciences Laboratory
Wallops Island, VA 23337

Acknowledgments: 

The ATM project team would like to acknowledge the dedicated NASA P3 and DC8 flight crews, whose efforts allowed the safe and efficient collection of this data over some of the most isolated and extreme regions on this planet.

Document Information

Document Creation Date

17 January 2014

Document Revision Date

09 June 2014

14 January 2015

08 Febuary 2016

21 June 2017

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