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IceBridge ATM L2 Icessn Elevation, Slope, and Roughness, Version 2
This data set contains resampled and smoothed elevation measurements of Arctic and Antarctic sea ice, and Greenland, Antarctic Peninsula, and West Antarctic region land ice surface acquired using the NASA Airborne Topographic Mapper (ATM) instrumentation. The data were collected as part of Operation IceBridge funded aircraft survey campaigns.
Version 2 ILATM2 data are in Comma Separated Value (CSV) ASCII format. A header has been added to the Version 2 data format. In Version 1, the ILATM2 data set consisted of pairs of files for each data segment: one data file and one summary file. The header of the Version 2 data file contains information that had appeared in the summary file, making the summary file redundant.
|Platform(s)||C-130, DC-8, HU-25A, HU-25C, P-3B, WP-3D Orion|
|Data Contributor(s):||Michael Studinger|
|Metadata XML:||View Metadata Record|
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 ATM L2 Icessn Elevation, Slope, and Roughness, 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/CPRXXK3F39RV. [Date Accessed].
Detailed Data Description
NOTE: See the Errors and Limitations section for information on narrow-swath ATM data substituted for wide-swath data during the 14 April 2015 campaign.
The ATM L2 Icessn Elevation, Slope, and Roughness data files are in Comma Separated Value (CSV) ASCII format.
The header of each CSV data file contains information for: Input filename, Number of segments, Nadir block width, Output interval, Smoothing interval, Trajectory file used, International Terrestrial Reference Frame. Also included are headings for each field in the data file: UTC_Seconds_Of_Day, Latitude(deg), Longitude(deg), WGS84_Ellipsoid_Height(m), South-to-North_Slope, West-to-East_Slope, RMS_Fit(cm), Number_Of_ATM_Measurments_Used, Number_Of_ATM_Measurements_Removed, Distance_Of_Block_To_The_Right_Of_Aircraft(m), Track_Identifier
Each data file is paired with an associated XML file. The XML files contain location, platform, and instrument metadata.
Data are available on the HTTPS site in the
https://n5eil01u.ecs.nsidc.org/ICEBRIDGE/ILATM2.002/ directory. Within this directory, the folders are organized by date, for example
Icessn files are named according to the following convention and as described in Table 1:
||File name prefix indicating ATM L2 data|
||Start time: two-digit hours|
||Start time: two-digit minutes|
||Start time: two-digit seconds|
||Alongtrack values smoothed in icessn process|
||Nadir block plus 3 off-nadir blocks per time stamp|
||50% overlap between successively smoothed blocks|
||Indicates file type, e.g.: CSV (.csv), XML (.xml)|
CSV files range from approximately 10 KB to 813 KB.
XML file size range from approximately 10 KB to 36 KB.
The entire data set is approximately 11 GB.
Spatial coverage for the IceBridge ATM campaigns includes the Arctic, Greenland, Antarctica, and surrounding ocean areas. 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
Southernmost Latitude: 90° S
Northernmost Latitude: 53° S
Westernmost Longitude: 180° W
Easternmost Longitude: 180° E
The ATM surface elevation measurements have been re-sampled at a variable along-track time interval, typically 0.5 seconds, which at aircraft survey speed is a distance along the flight track of approximately 50 meters. The distance can vary with aircraft speed. Each set of along-track records contains a fixed 80 m across-track nadir platelet as well as three or five additional platelets that together span the entire swath of the ATM scan. This data set contains data from one of two different ATM sensors depending on the year the data were collected. The ATM 15-degree off-nadir scanner (T2) has an off-nadir scan angle of 15 degrees and these data were smoothed into three across-track platelets. The ATM 23-degree off-nadir scanner (T3) has an off-nadir scan angle of 23 degrees and is smoothed into five across-track platelets. The across-track widths of these three to five additional platelets can vary with aircraft altitude.
Projection and Grid Description
No projection. Data are georeferenced to WGS-84.
These data were collected as part of Operation IceBridge funded campaigns from 31 March 2009 to 15 September 2016.
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.
The ATM L2 Icessn Elevation, Slope, and Roughness data set includes glacier, ice sheet, and sea ice elevation measurements, slope measurements, and roughness measurements.
Icessn data files contain parameters as defined in Table 2.
|UTC_Seconds_Of_Day||Time at which the aircraft passed the mid-point of the block.||Seconds of the day in UTC||0 to 86400|
|Latitude||Latitude of the center of the block.||Degrees||-90.0 to +90.0|
|Longitude||East longitude of the center of the block.||Degrees||0.0 to 360.0|
|WGS84_Ellipsoid_Height||Height above WGS84 ellipsoid of the center of the block.||Meters||-100.0 to 10000.0|
|South-to-North_Slope||South to North slope of the block.||Dimensionless||any real value|
|West-to-East_Slope||West to East slope of the block.||Dimensionless||any real value|
|RMS_Fit||RMS fit of the ATM data to the plane.||Centimeters||Greater than 0.0|
|Number_Of_ATM_Measurments_Used||Number of points used in estimating the plane parameters.||Count||Greater than 0|
|Number_Of_ATM_Measurements_Removed||Number of points removed in estimating the plane parameters.||Count||Greater than or equal to 0|
|Distance_Of_Block_To_The_Right_Of_Aircraft||Distance of the center of the block from the centerline of the aircraft trajectory (starboard = positive, port = negative).||Meters||real valued|
|Track_Identifier||Track identifier (numbered 1...n, starboard to port, and 0 = nadir).||Number||0, 1, 2, …|
Sample Data Record
Below is an excerpt from data file
Software and Tools
NSIDC provides a MATLAB reader that reads ATM icessn data files from the Operation IceBridge Airborne Topographic Mapper instrument. The elevation measurement files and the summary files also may be opened by any text editor or word processing program that reads ASCII text files..
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.
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.
The ATM surface elevation measurements have been re-sampled into an icessn format which smoothes the data and reduces the data volume. Users desiring unsampled data should use the ATM Level-1B format data.
The fundamental form of ATM topography data is a sequence of laser footprint locations acquired in a swath along the aircraft flight track. The icessn program condenses the ATM surface elevation measurements by fitting a plane to blocks of points selected at regular intervals along track and several across track. The block size and spacing can be specified, but a few typical values are used. The along-track distance smoothed is the distance which the aircraft moves in a fixed interval, 0.5 seconds for P-3 and DC8 aircraft, and 1.0 seconds for DHC-6 Twin-Otter. The data output interval is half of the smoothing interval so that there is 50 percent overlap between successively smoothed blocks. For each along-track position/time, there are multiple blocks spaced evenly across-track to span the swath width. Typically the number of blocks is five for the T3 scanner and three for the T2 scanner. There is an additional block located at aircraft nadir with a width typically set to 80 m. If a single profile is desired, the nadir profile can be selected from the full data set.
The two slopes estimated are used to estimate surface elevations at points other than the center point through the use of the following algorithm shown in Equation 1 and described in Table 3:
|ht(phi,lambda) = ht(phi0,lambda0)
+ SNslope * (phi - phi0) * 6378137 * pi/180
+ WEslope * (lambda - lambda0) * cos(phi0) * 6378137 * pi/180
||height in meters at coordinates (phi, lambda)|
||latitude at location of interest (radians)|
||longitude at location of interest (radians)|
||latitude at center of tile (radians)|
||WGS84 ellipsoid semi-major axis (meters)|
||longitude at center of tile (radians)|
||south/north slope of the tile|
||west/east slope of the tile|
Note: The multiple across-track planes at a given along-track position will have the same time tag.
The following processing steps are performed by the data provider.
- Preliminary processing of ATM LIDAR data through the cvalid program, applying calibration factors to convert time of flight to range, scan pointing angles and interpolated attitude to each LIDAR measurement.
- Processing of GPS data into aircraft trajectory files using double-differenced dual-frequency carrier phase-tracking.
- Processing of the cvalid program output combined with the GPS trajectory data through the qfit program, resulting in an output file containing a surface elevation (ellipsoid height) and a geographic location in latitude and east longitude with other ancillary parameters.
- Processing of the qfit output through the icessn program which averages the qfit surface elevation data into a small number of blocks or surface planes.
Beginning with the 2013 Arctic campaign, all new data are provided in Version 2 CSV format. Data from 2012 and earlier campaigns remained in the previously-used Fixed-width, space-delimited ASCII format. On 10 August 2015, Version 1 fixed-width space-delimited ASCII text files for 2009.03.31 to 2012.11.08 were replaced with .csv files and were included with ILATM2 Version 2 data. The Version 2 data set now includes all ILATM2 data from 2009.03.31 to present.
A header was added to the Version 2 ILATM2 data format. In Version 1, the ILATM2 data set consisted of pairs of files for each data segment: one data file, for example ILATM2_YYYYMMDD_HHMMSS_50pt_smooth_nadir3seg, and one summary file, ILATM2_YYYYMMDD_HHMMSS_50pt_icessn_summary. The new header of the Version 2 CSV data file contains information that had appeared in the summary file, making the summary file redundant.
Error and Limitations
Uncertainty in slope estimates:
The uncertainty of the WGS84_Ellipsoid_Height, South-to-North_Slope, and West-to-East_Slope can be attributed to both physical topographical features and measurement error. The uncertainty in the WGS84_Ellipsoid_Height can be reasonably approximated with the RMS_Fit value. The uncertainty in the South-to-North_Slope and West-to-East_Slope can both be reasonably approximated with the equation:
|Slope Sigma (m/m) = RMS_Fit / sqrt(500 * Number_Of_ATM_Measurements_Used)||(Equation 2)|
||RMS fit of the ATM data to the plane in meters|
||Number of points used in estimating the plane parameters|
Note: The RMS_Fit value in the ILATM2 data file is in centimeters and must be converted to meters to be used in the equation above.
Attenuated ATM signal 12 and 13 April 2010:
During collection of IceBridge ATM Greenland data on 12 and 13 April 2010, hydraulic oil progressively leaked from the forward landing gear on the DC8 aircraft. The oil was blown back along the bottom of the fuselage and across the nadir window through which the ATM was transmitting and receiving the laser signal. The ATM signal was attenuated, and data in part of the scan is missing as a result. The problem developed during the flight and worsened through time. The ATM still acquired more than half of the shots throughout the scan. The net effect of this problem is to decrease the number of shot returns logged, the same as if the laser power was reduced. To the user this will appear as a reduced point density on the ground. This issue will not affect the accuracy of the data. In the Antarctic 2010 campaign, fuel leakage degraded the signal in a similar fashion.
Skipped glaciers and supplemented data for April 28, 2012:
The April 28, 2012 flight traversed the notoriously turbulent regions of Greenland's southeast glaciers. During the flight, two planned glaciers were skipped due to concern about expected severe turbulence. The survey data spans roughly 11:15 to 18:20 UTC. On the approach to Ikerssuaq glacier at 16:56:19.5 (GPST=60994.5 secs), both the ATM T3 and T4 instruments quit recording data within 0.1 second of each other. T3 resumed at 16:57:05.3 whereas T4 did not resume for the rest of that day's flight. Following this event, the flight followed the Ikerssuaq flow line, then traversed straight west across the icesheet back to the Kangerlussuaq airport. The data gap spans 46 seconds, from the fjord up to about 500 m elevation on the Ikerssuaq glacier. The T4 data quit during the creation of the file 20120428_165532.ATM4BT4.F1.qi. The Level-1B T4 data were supplemented by the these Level-1B narrow swath files of T3 data from the latter part of the survey:
The above files can be found with the IceBridge ATM L1B Qfit Elevation and Return Strength data in the /2012.04.28/ folder. For details on the ATM 4BT3 and 4BT4 instruments, see the Sensor Or Instrument Description section below, and the IceBridge Narrow Swath ATM L1B Qfit Elevation and Return Strength data set documentation.
ATM wide scan instrument failure 14 April 2015 - ATM Narrow Scan instrument fills data gap:
On 14 April 2015, the ATM wide-scan instrument suffered a failure in the scanning mechanism and did not collect data for that mission. However, the ATM narrow-scan instrument was operating normally and has been used to fill in the gap in ILATM2 data for this mission.
The ILATM2 averaged product is derived from the ILATM1B wide-scan elevation point cloud, when possible. The ILATM2 data splits the swath into three segments relative to the direction of flight: port-side, center, and starboard-side. From these strips, the data is further separated into 0.5-second-long tiles. With a nominal Above Ground Level (AGL) of 450 m, a swath width of 240 m, and a groundspeed of 100 m/s; this creates tiles that are 80 m wide and 50 m long. There is also a fourth tile created, called the nadir file, which is positioned directly below the aircraft. This tile is also nominally 80 m wide and 50 m long.
The ATM narrow-scan swath width is only about 40 m wide, so generating tiles equally-sized to the ATM wide-swath is not possible. For best results, it was decided to separate the lidar swath in half: port-side and starboard-side. For consistency, the along-track distance of the tiles is still 0.5 seconds. Therefore, for this delivery, the 14 April 2015 tiles are roughly 20 m wide and 50 m long. The nadir tile is also included, which is still nominally 80 m wide and 50 m long like the wide-swath ATM, and for straight-and-level flight includes the entire swath.
Fall 2015 Campaign
For the Fall 2015 ATM data, some ATM elevations are adjusted slightly (~10 cm) to compensate for a systematic anomaly related to the ATM scanner azimuth. The overall mean elevation is not changed, but some elevations around the scan are adjusted upward or downward as a function of scanner azimuth. For further details on the adjustment method, see Yi et al. 2015.
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 instrument measures topography as a sequence of points scanned in a swath along the aircraft flight track. The sampling frequency for the ATM is 5 kHz.
The ATM instruments are developed and maintained at NASA's Wallops Flight Facility (WFF) in Virginia, USA. During Operation IceBridge, the ATM has been installed aboard the NASA P3-B aircraft based at WFF, or the NASA DC8 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.
References and Related Publications
Contacts and Acknowledgments
Cryospheric Sciences Laboratory
NASA Goddard Space Flight Center
Greenbelt, Maryland USA
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 Creation Date
29 January 2014
Document Revision Date
9 June 2014
14 January 2014
06 February 2015
30 June 2015
10 August 2015
08 January 2016
08 June 2016
20 December 2016
02 March 2017
22 March 2017