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 the 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 for 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.

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:

Where:

Note: The RMS_Fit value 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 flight on 28 April 2012 traversed the notoriously turbulent regions over 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 folder for 28 April 2012. 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).

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. Consequently, there are no ILATM2 data for this date.

Fall 2018 and Spring 2019 Campaigns (10/10/2018 to 05/16/2019):
As compared to most other Operation IceBridge ATM laser altimetry data sets, this particular data set has certain limitations in accuracy that result from a recently identified problem related to the application of the solid Earth tide correction in the data processing stage. This error can cause long-wavelength errors in elevations that are less than decimeter in magnitude and which vary in both space and time. The error wavelength is typically hundreds of kilometers, so it should not significantly affect most analyses of this data set, but it can be smaller because it depends on the number and position of base stations used for the trajectory solution and on other factors including moon phase. The error only affects the 2018 DC-8 Antarctic and 2019 P-3 Arctic Spring ATM data sets published at NSIDC DAAC. Resolution of this error is in progress, and a future version of this data set will eliminate it. The user should consider the elevation issue in any scientific interpretation or other use of the data set. Users are requested to report their findings about data quality to NSIDC User Services, to be forwarded to the ATM team, for information and comment before publication or reporting elsewhere.