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Data Set ID:
BLATM1B

Pre-IceBridge ATM L1B Qfit Elevation and Return Strength, Version 1

This data set contains spot elevation measurements of Arctic, Greenland, and Antarctic sea ice and ice surface acquired using the NASA Airborne Topographic Mapper (ATM) instrumentation.

Geographic Coverage

Spatial Coverage:
  • N: 90, S: 60, E: 180, W: -180

  • N: -53, S: -90, E: 180, W: -180

Spatial Resolution:
  • Varies x Varies
Temporal Coverage:
  • 23 June 1993 to 30 October 2008
Temporal Resolution: Varies
Parameter(s):
  • Glaciers/Ice Sheets > Glacier Elevation/Ice Sheet Elevation
Platform(s) DC-8, P-3B
Sensor(s): ATM
Data Format(s):
  • Binary
Version: V1
Data Contributor(s): William Krabill

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.

Krabill, W. B. 2012. Pre-IceBridge ATM L1B Qfit Elevation and Return Strength, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: http://dx.doi.org/10.5067/8Q93SAT2LG3Q. [Date Accessed].

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Detailed Data Description

Format

ATM data is generally distributed in the output format of the processing program, qfit, which combines airborne laser ranging data and aircraft attitude from the INS with positioning information from a processed kinematic differential GPS trajectory. Qfit output files, which usually have names ending in a .qi extension, are organized as 32-bit (4-byte) binary 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. For surveys prior to and including spring 2010, qfit processing was performed on an Apple PowerPC processor (and Sun/Motorola before that). Accordingly, data from spring 2010 and earlier were written in big-endian byte order.

The files are organized into fixed-length logical records. The beginning of the file contains a header of one or more records followed by a data segment, in which there is one record per laser shot. It is not necessary to interpret the header to use the laser data. The first word of the header (and the file) is a 32-bit binary integer giving the number of bytes in each logical record. Commonly qfit files have 12 words per record and this integer will be the number 48. The remainder of the initial logical record is padded with blank bytes (in this case 44 blank bytes). 10-word and 14-word formats have also been used, as described below in the Parameters section.

The remainder of the header is generally a series of logical records containing the processing history of the file. In these logical records, the initial word contains a 32-bit binary integer with a value between -9000000 and -9000008. The remaining bytes in each header record is filled with a string of ASCII characters containing information on file processing history. In this case, the byte offset (as a longword integer) from the start of file to the start of laser data will be the second word of the second record of the header. A simple method for removing the header records is to eliminate records that begin with a negative value since the first word of records in the data segment is always a positive number.

In the data segment of the file, the information contained in words 2-9 of the output record pertains to the laser pulse, its footprint, and aircraft attitude. The last word of each record is always the GPS time of day when the laser measurement was acquired.

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File and Directory Structure

Data files are available on in the ftp://n5eil01u.ecs.nsidc.org/SAN2/PRE_OIB/BLATM1B.001/ directory. Files are organized into folders by year, month, and day, for example: /1993.06.27/ through /2008.10.30/.

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File Naming Convention

Files are named according to various conventions from 1993 to 2008. File name examples are shown in Table 1.

Year and ATM Instrument Example File Names
Table 1. File Name Examples by Campaign Year and ATM Instrument
1993 BLATM1B_930627aoltm_t2f2_c
1994 BLATM1B_940528aoljr_160236
1995_atm1 BLATM1B_950519atm1_125126
1996_atm1 BLATM1B_960523atm1_145648sr
1997_atm1 BLATM1B_970425atm1_185451sr
1998_atm1 BLATM1B_980627atm1_135623sr.gapF
1999_atm2 BLATM1B_990513atm2_131725jr.lutFx
2000_atm2 BLATM1B_000529atm2_153655jr.lutFx
2001_atm3 BLATM1B_010520atm3_110524jr.lutFx
2002_atm2 BLATM1B_20021122atm2_161135jr
2002_atm3 BLATM1B_20020528atm3_132805jr.lutFx
2003_atm3 BLATM1B_20030320atm3_203456jr.lutFx
2004_atm2 BLATM1B_20041118atm2_171917jr.lutF
2005_atm4 BLATM1B_20050505_183352
2006_atm4 BLATM1B_20060531_171650
2007_atm4b BLATM1B_20070925_123435.atm4bT2.rangeExample
2008_atm4 BLATM1B_20081030_152251.ATM4BT2.rangeExample

Variables for the naming conventions listed in Table 1 are described in Table 2:

Examples:
BLATM1B_950519atm1_125126
BLATM1B_YYMMDDatminst_HHMMSS

BLATM1B_960523atm1_145648sr
BLATM1B_990513atm2_131725jr.lutFx
BLATM1B_YYMMDDatminst_HHMMSSaa.aaaaa

BLATM1B_20070925_123435.atm4bT2.rangeExample
BLATM1B_YYYYMMDD_HHMMSSatminst.rangeExample

Variable Description
Table 2. File Naming Convention
BLATM1B Short name for Pre-IceBridge ATM L1B Qfit Elevation and Return Strength
YY or YYYY YY = Two-digit year, YYYY = Four-digit year
MM Two-digit month
DD Two-digit day
atminst ATM instrument version identification, for example: aoltm, aoljr, atm1, atm1(sr), atm1, atm1, atm2(jr), atm2, atm3, atm4, atm4aT3, atm4bT2, atm4cT3, atm4bT2. (aol refers to the early ATM, the terrain mapping configuration of the Airborne Oceanographic Lidar, sometimes called the AOL-TM.)
HH Two-digit hours, beginning of file time
MM Two-digit minutes, beginning of file time
SS Two-digit seconds, beginning of file time
aa jr - junior, sr = senior
aaaaa

gapF: QA filtering step to clean up erroneous data due to an artifact of data acquisition.

lutF: Method used with leading-edge range determination that included the application of a lookup table, derived from a ground calibration, to filter out very saturated signals and apply a range adjustment to the received laser pulse, based on its integrated energy.

lutFx: Similar to lutF, except the adjustment was applied to the transmitted laser pulse, in addition to the received pulse.

rangeExample Range determination module that utilized leading-edge tracking.
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Volume

The entire data set is approximately 573 GB.

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Spatial Coverage

The focus of the ATM surveys has been Greenland, but they also include Iceland, Svalbard, Canada, Alaska, Antarctica (West/Peninsula, and McMurdo region), and several sea ice regions. In effect, this represents the coverage noted below.

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

Antarctica:
Southernmost Latitude: 90° S
Northernmost Latitude: 53° 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. Resolution varies with the altitude flown and the scanner configuration for the LIDAR. At a typical altitude of 500 m above ground level, a laser pulse rate of 5 kHz, and a scan width of 22.5 degrees off-nadir, the average point density is one laser shot per 10 m2 within the swath.

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 23 June 1993 to 30 October 2008.

Temporal Resolution

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 Pre-IceBridge ATM L1B Qfit Elevation and Return Strength data set includes glacier, ice sheet, and 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 5 kHz. When rounding to 0.001 seconds, five points will appear with the same time stamp.

Parameter Description

The data formats and parameters are described in Tables 3, 4, and 5.  The format is designated by the logical record length given in the first word of the data file.

Column Description Units with Scale Factor
Table 3. 12-word qfit Format (in use since 2006) Parameter Description
1 Relative Time measured from start of file Seconds 10-3
2 Laser Spot Latitude Degrees x 1,000,000
3 Laser Spot Longitude Degrees x 1,000,000
4 Elevation Meters 10-3
5 Start Pulse Signal Strength (relative) Dimensionless relative values (or data numbers, DN)
6 Reflected Laser Signal Strength (relative) Dimensionless relative values (or data numbers, DN)
7 Scan Azimuth Degrees x 1000
8 Pitch Degrees x 1000
9 Roll Degrees x 1000
10 GPS Dilution of Precision (PDOP) times 10 Dimensionless
11 Laser received pulse width Count, digitizer samples
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.
Column Description Units with Scale Factor
Table 4. 10-word qfit Format (in use prior to 2006) Parameter Description
1 Relative Time measured from start of file Seconds 10-3
2 Laser Spot Latitude Degrees x 1,000,000
3 Laser Spot Longitude Degrees x 1,000,000
4 Elevation Meters 10-3
5 Start Pulse Signal Strength (relative) Dimensionless relative values (or data numbers, DN)
6 Reflected Laser Signal Strength (relative) Dimensionless relative values (or data numbers, DN)
7 Scan Azimuth Degrees x 1000
8 Pitch Degrees x 1000
9 Roll Degrees x 1000
10 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.

Between 1997 and 2004 some ATM surveys included a separate sensor to measure passive brightness. In the 14-word format, words 10-13 pertain to the passive brightness signal, which is essentially a relative measure of radiance reflected from the earth's surface within the vicinity of the laser pulse. Some records which have passive data without valid laser data will contain zeros in place of the laser latitude, longitude, and elevation. The horizontal position of the passive footprint is determined relative to the laser footprint by a delay formulated during ground testing. The elevation of the footprint is synthesized from surrounding laser elevation data. NOTE: The passive data is not calibrated and its use, if any, should be qualitative in nature. It may aid the interpretation of terrain features. The measurement capability was engineered into the ATM sensors to aid in the identification of the water/beach interface acquired with the instrument in coastal mapping applications.

Column Description Units with Scale Factor
Table 5. 14-word qfit Format (in use since between 1997 and 2004) Parameter Description
1 Relative Time measured from start of file Seconds 10-3
2 Laser Spot Latitude Degrees x 1,000,000
3 Laser Spot Longitude Degrees x 1,000,000
4 Elevation Meters 10-3
5 Start Pulse Signal Strength (relative) Dimensionless relative values (or data numbers, DN)
6 Reflected Laser Signal Strength (relative) Dimensionless relative values (or data numbers, DN)
7 Scan Azimuth Degrees x 1000
8 Pitch Degrees x 1000
9 Roll Degrees x 1000
10 Passive Signal (relative) Dimensionless relative values
11 Passive Footprint Latitude Degrees x 1,000,000
12 Passive Footprint Longitude Degrees x 1,000,000
13 Passive Footprint Synthesized Elevation Millimeters
14 GPS Time packed example: 153320100 = 15h 33m 20s 100ms

Sample Data Record

Below is an ASCII format excerpt of the 20080627_134422.atm4cT3.rangeExample.qi data file converted from the binary. The 12 fields in each record correspond to the columns described in Table 3.

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Software and Tools

The elevation measurement files contain qfit binary data. The qfit format was developed for use at Wallops Flight Facility (WFF). NSIDC provides the "qi2txt" 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.

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Data Acquisition and Processing

Theory of Measurements

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.

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

Prior to 2008 surveys, the GPS trajectory was edited to restrict PDOP<9 in order to limit GPS errors to be less than roughly 5 cm. The output survey data would therefore have occasional gaps where the PDOP>9. Some applications of ATM data have less stringent accuracy requirements that would be better served by preserving the data in these gaps. Starting in 2008, the PDOP limit was changed to 20, which could allow occasional GPS errors up to about 15 cm. The PDOP value is carried in the qfit output and can be used to edit data for applications requiring greater precision. Any file in the 10-word format, or files in the 12-word format processed prior to January 2009, will have PDOP limited.

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Data Acquisition Methods

The ATM deployments included two lidar systems whenever the aircraft platform would allow. The redundancy reduced the risk of hardware failures and provided a means of validating modifications to the lidar. The data were processed from both instruments up to a point, but then efforts concentrated on refining and delivering data from a single lidar. Each campaign therefore has a "primary" lidar, and sometimes a "secondary" lidar. This data set contains only data from the primary lidar.

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 processing program, qfit, combines airborne laser ranging data and aircraft attitude from the Inertial Navigation System (INS) with positioning information from a processed kinematic differential Global Positioning System (GPS) trajectory.

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 3.
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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 ATM instruments evolved over the timespan of the data set 1993-2008. The nomenclature reflects some of these changes. The early "ATM" was the terrain mapping configuration of the Airborne Oceanographic Lidar, sometimes called the AOL-TM. Names have included aoljr (i.e. junior), atmsr (i.e. senior), atmjr, ATM1, ATM2, ATM3, and the several versions of ATM4. The ATM instruments are developed and maintained at NASA's WFF in Virginia, USA.

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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

Acknowledgements: 

The 2002, 2004, and 2008 Antarctic campaigns were made possible through a collaboration between NASA/ATM, the Chilean Centro de Estudios Científicos (Center for Scientific Studies) aka CECS, and the Armada de Chile (Chilean Navy).

Document Information

DOCUMENT CREATION DATE

05 August 2016

DOCUMENT REVISION DATE

Questions? Please contact:

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