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IceBridge Narrow Swath ATM L1B Qfit Elevation and Return Strength, Version 1

This data set contains spot elevation measurements of Greenland, Arctic, and Antarctic sea ice acquired using the NASA Airborne Topographic Mapper (ATM) 4CT3 narrow scan instrumentation. The data were collected as part of Operation IceBridge funded aircraft survey 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

We kindly request that you cite the use of this data set in a publication using the following citation example. For more information, see our Use and Copyright Web page.

Krabill, William B. 2011, updated 2012. IceBridge Narrow Swath ATM L1B Qfit Elevation and Return Strength, [indicate subset used]. Boulder, Colorado USA: NASA DAAC at the National Snow and Ice Data Center. http://nsidc.org/data/ilnsa1b.html.

Overview

Platform

NASA DC-8 and P-3B

Sensor

Airborne Topographic Mapper (ATM) 4CT3

Spatial Coverage

Greenland, Arctic, and Antarctic sea ice

Spatial Resolution

Nominal swath width of 45 m, with an average spatial resolution of 1 laser shot for every 2 m2 inside the swath.

Temporal Coverage

16 March 2011 to 08 November 2012

Temporal Resolution

Seasonal

Parameters

Sea Ice Elevation Measurements
Surface Return Amplitude Relative Un-calibrated Values

Data Format

qfit 32 bit little-endian binary files

Metadata Access

View Metadata Record

Get Data

FTP

1. Contacts and Acknowledgments

Investigator(s) Name and Title

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

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

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.

2. Detailed Data Description

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. The narrow swath ATM data are sea ice elevation measurements only, and do not include land ice. The data are stored in little-endian binary qfit output files organized as 32 bit, 4 byte, binary integer words scaled to retain the precision of the measurements. The beginning of each file contains a header of one or more records followed by a data segment in which there is one record per laser shot.

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

File Naming Convention

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

ILNSA1B_20120314_124441.atm4cT3.qi

YYYYMMDD_HHMMSS.atm4cT3.qi

Where:

Table 1. File Naming Convention
Variable Description
ILNSA1B Short name for IceBridge Narrow Swath ATM L1B Qfit 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
.qi indicates qfit output file

File Size

Elevation measurement *.qi files range from approximately 1 MB to 54 MB.

Volume

The entire data set is approximately 99 GB.

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 qfit 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.

Temporal Coverage

These data were collected as part of Operation IceBridge funded campaigns beginning 16 March 2011 to 08 November 2012.

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

The ATM qfit 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

Parameters contained in qfit ATM data files are described in Table 2. Column numbers 1 to 12 in Table 3 represent columns left to right in the data. Columns are not numbered in the data files.

Table 2. Parameter Description, Units with Scale Factor, and Range
Column Description Units with Scale Factor Range
1 Relative Time measured from start of file Seconds 10-3 Greater than 0
2 Laser Spot Latitude Degrees 10-6 -90.0 to 90.0
3 Laser Spot East Longitude Degrees 10-6 0.0 to 360.0
4 Elevation of the laser spot above ellipsoid Meters 10-3 any real value
5 Start Pulse Signal Strength (relative) Dimensionless relative values (or data numbers, DN) positive integer value
6 Reflected Laser Signal Strength (relative) Dimensionless relative values (or data numbers, DN) positive integer value
7 Scan Azimuth Degrees 10-3 0.0 to 360.0
8 Pitch Degrees 10-3 -90.0 to +90.0
9 Roll Degrees 10-3 -90.0 to +90.0
10 GPS Dilution of Precision (PDOP) times 10 Dimensionless Greater than 0
11 Laser received pulse width at half height, number of digitizer samples at 0.5 nanosecond per sample. Count Greater than 0
12 GPS time packed, example: 153320100 = 15 hours 33 minutes 20 seconds 100 milliseconds. Seconds of the day in GPS time. As of 01 January 2009 GPS time = UTC + 15 seconds. 000000000 to 235959999

Sample Data Record

Below is an ASCII format excerpt of the ILNSA1B_20110322_145022.atm4cT3.qi data file converted from the binary. The twelve fields in each record correspond to the columns described in Table 2.

sample data record

3. Data Access and Tools

Get Data

Data are available via FTP

Software and Tools

The sea ice elevation measurement files contain qfit binary data. The qfit format was developed for use at Wallops Flight Facility (WFF). NSIDC provides a C qfit data reader that reads a binary qfit file and outputs a text file, an IDL qfit data reader that reads qfit data into an IDL array, and a MATLAB reader that reads qfit data files. LAStools can read and write NASA ATM qfit format.

4. 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.

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.

Note: The 23 October 2012 data (20121023) were collected at a higher aircraft altitude than the other survey dates, so it has a slightly different accuracy. The accuracy of the elevation measurements is on decimeter-level, instead of the centimeter-level.

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.

Error Sources

The ATM 4cT3 lidar used to create the ILNSA1B data suffered a laser failure before reaching the first data line on the 20120317 mission. Three qfit files have been delivered, which contain elevation measurements just prior to the laser failure. The laser could not be repaired in the field, so a spare laser was installed on the 4cT3 instrument back in Thule, Greenland on 03/20/2012. The 4cT3 data set resumes with data collected on 03/21/2012.

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 qfit data are in the IceBridge ATM L1B Qfit 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)**

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

5. References and Related Publications

Kwok, R., G. F. Cunningham, S. S. Manizade, and W. B. Krabill. 2012. Arctic sea ice freeboard from IceBridge acquisitions in 2009: Estimates and comparisons with ICESat. Journal of Geophysical Research 117: C02018. doi:10.1029/2011JC007654.

Related Data Collections

Related Web Sites

  • Airborne Topographic Mapper Web site at NASA Wallops Flight Facility (http://atm.wff.nasa.gov/).
  • Description of ATM QFIT Output Data (revised 2009-feb-13).
  • Description of DEM Generation, Dry Valleys, Antarctica (http://rsl.geology.buffalo.edu/research/DEM/home.htm).
  • 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).
  • ITRF 2008 Specification Web site (http://itrf.ensg.ign.fr/ITRF_solutions/2008/ITRF2008.php).

6. Document Information

Acronyms and Abbreviations

The acronyms used in this document are listed in Table 4.

Table 4. Acronyms and Abbreviations
Acronym Description
ASCII American Standard Code for Information Interchange
ATM Airborne Topographic Mapper
CIRES Cooperative Institute for Research in Environmental Science
ECS NASA Earth Observing System Data and Information System (EOSDIS) Core System
FTP File Transfer Protocol
GPS Global Positioning System
ICESat Ice, Cloud, and land Elevation Satellite
IMU Inertial Measurement Unit
INS Inertial Navigation System
ITRF International Terrestrial Reference Frame
L1B Level-1B
LIDAR LIght Detection And Ranging
NASA National Aeronautics and Space Administration
NSIDC National Snow and Ice Data Center
P-3 NASA Lockheed P-3 Orion aircraft
T2 ATM 15-degree off-nadir scanner
T3 ATM 23-degree off-nadir scanner
URL Uniform Resource Locator
UTC Universal Time Code
WFF Wallops Flight Facility
WGS 84 World Geodetic System 1984

Document Creation Date

09 January 2012

Document Revision Date

11 June 2012

04 September 2012

11 June 2013

02 January 2014

Document URL

http://nsidc.org/data/docs/daac/icebridge/ilnsa1b/index.html