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.

Krabill, William B. 2010, updated current year. IceBridge ATM L2 Icessn Elevation, Slope, and Roughness, [list dates of data used]. Boulder, Colorado USA: NASA Distributed Active Archive Center at the National Snow and Ice Data Center. Digital media. http://nsidc.org/data/ilatm2.html.

Overview

Platform	NASA DC-8 and P-3B
Sensor	Airborne Topographic Mapper (ATM)
Spatial Coverage	Antarctica, Greenland, and Arctic/Antarctic oceans
Spatial Resolution	80 m sample width 40 m spacing along track
Temporal Coverage	31 March 2009 to the present
Temporal Resolution	Seasonal
Parameters	Elevation Measurements Slope Measurements Roughness Measurements
Data Format	Fixed-width, space-delimited ASCII text files
Metadata Access	View Metadata Record
Data Access	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 ATM L2 Icessn Elevation, Slope, and Roughness data files are in fixed-width, space-delimited ASCII text format.

File Naming Convention

File Size

Elevation measurement files are approximately 26 KB to 350 KB.
Summary files are approximately 7 KB each.

Volume

The entire data set is approximately 5 GB.

Spatial Coverage

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

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

Spatial Resolution

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.

Temporal Coverage

These data were collected as part of Operation IceBridge funded campaigns from 31 March 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 ATM L2 Icessn Elevation, Slope, and Roughness data set includes glacier, ice sheet, and sea ice elevation measurements, slope measurements, and roughness measurements.

Parameter Description

Sample Data Record

3. Data Access and Tools

Data Access

Data are available via FTP.

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.

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

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 Qfit 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 4:

.

ht(phi,lambda) = ht(phi0,lambda0) + SNslope * (phi - phi0) * 6378137 * pi/180 + WEslope * (lambda - lambda0) * cos(phi0) * 6378137 * pi/180

(Equation 1)

Where:

Table 4. Surface Elevations Estimate Algorithm Variables

Variables	Description
ht	height in meters at coordinates (phi, lambda)
phi	latitude at location of interest (radians)
lambda	longitude at location of interest (radians)
phi0	latitude at center of tile (radians)
6378137	WGS84 ellipsoid semi-major axis (meters)
lambda0	longitude at center of tile (radians)
SNslope	south/north slope of the tile
WEslope	west/east slope of the tile

Note: The multiple across-track planes at a given along-track position will have the same time tag.

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 interpolated attitude to each LIDAR measurement.
2. Processing of GPS data into aircraft trajectory files using double-differenced dual-frequency carrier phase-tracking.
3. 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.
4. 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.

Error Sources

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.

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:

20120428_165449.atm4cT3.F1.qi
20120428_170030.atm4cT3.F1.qi
20120428_170524.atm4cT3.F1.qi
20120428_171019.atm4cT3.F1.qi
20120428_171514.atm4cT3.F1.qi
20120428_172008.atm4cT3.F1.qi
20120428_172503.atm4cT3.F1.qi
20120428_172957.atm4cT3.F1.qi
20120428_173452.atm4cT3.F1.qi
20120428_173947.atm4cT3.F1.qi
20120428_180815.atm4cT3.F1.qi
20120428_181219.atm4cT3.F1.qi
20120428_181619.atm4cT3.F1.qi

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, and the IceBridge Narrow Swath ATM L1B Qfit Elevation and Return Strength data set documentation.

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 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.

5. References and Related Publications

Related Data Collections

Related Web Sites

6. Document Information

Acronyms and Abbreviations

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

Table 5. 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
INS	inertial navigation system
L2	Level 2
LIDAR	LIght Detection And Ranging
NASA	National Aeronautics and Space Administration
NSIDC	National Snow and Ice Data Center
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

10 January 2011

Document Revision Date

06 April 2011

07 March 2011

14 July 2011

22 August 2012

Document URL

http://nsidc.org/data/docs/daac/icebridge/ilatm2/index.html