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.

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. The scan azimuth of the lidar scanner mirror together with the aircraft attitude determine the pointing angle of the lidar. GPS aircraft position, pointing angle of the lidar, and range measured by the lidar are used to compute position of laser footprint on the ground.

Trajectory and Attitude Data

Aircraft position is determined by Global Navigation Satellite System (GNSS) systems that incorporate NAVSTAR Global Positioning System (GPS) and, for later campaigns, the Globalnaya Navigatsionnaya Sputnikovaya Sistema (GLONASS). Carrier phase measurements are logged by an antenna and receiver on the aircraft. In post-flight processing, these measurements are combined with similar measurements from static ground stations to produce a kinematic differential solution of the aircraft trajecotry at 0.5 second intervals, and more recently at 0.1 second intervals.

Aircraft attitude is logged from a commercial Intertial Navigation System (INS), also known as an Inertial Measurement Unit (IMU).

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 4.

Version History

• Version 1: The data for 2009 through 2012 are stored in qfit format as ILATM1B Version 1.
• Version 2: Beginning with the 2013 Arctic campaign, all data are provided in HDF5 format.

Error Sources

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.

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 T4 data is being supplemented by the these 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 Narrow Swath ATM L1B Qfit Elevation and Return Strength (ILNSA1B) 2012 Greenland data. For details on the ATM 4BT3 and 4BT4 instruments, see the Sensor Or Instrument Description section, and the ILNSA1B data set documentation.

No data for 14 April 2015:
On 14 April 2015, the ATM wide-scan instrument suffered a failure in the scanning mechanism and did not collect data for that mission.

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).

Fall 2016 Campaign
On 07 November 2016, the ATM wide-scan lidar (ATM6aT6) suffered a laser system failure and did not collect any usable ILATM1B data for that flight. However, the ATM narrow-scan lidar (ATM5bT5) operated normally. Generally, data from the narrow-scan instrument are only provided for sea ice flights, where the smaller off-nadir angle is beneficial for lead detection. In this case however, ILNSA1B data were made available for the 07 November 2016 flight as a substitute for the missing ILATM1B data.