The Airborne Topographic Mapper (ATM) is a conically-scanning laser altimeter that measures the surface topography of a swath of terrain directly beneath the path of the aircraft. Figure 1 depicts the ATM in operation measuring a swath of ice in Greenland that has been previously surveyed. ATM surveys can be used for change detection, with laser swaths re-surveyed after a few years, and differences between the two surveys yielding estimates of elevation change during the interim. Comparison of surveys in 1993-4 and 1998-9 produced the first comprehensive assessment of the mass balance of the Greenland ice sheet (Krabill et al., 1999, 2000). ATM surveys can also be used for comparison with satellite altimeter measurements for calibration/validation (e.g. Martin et al., 2005) or for determining elevation change. The ATM has collected high quality topographic data from a wide variety of platforms, including the NASA P3, a Chilean Navy P3, a US Navy P3, the NASA DC8, the NCAR C-130, and a half-dozen Twin Otters.
The accuracy of ATM surveys has been assessed previously (Krabill et al. 1995, 2002). In this paper we consider the accuracy which can be and has been achieved using updated instrumentation and techniques, with the primary application the surveying of Greenland and Antarctic ice sheets in recent years. Section 2 describes the instrumentation currently used. Section 3 describes the techniques used for analyzing ATM ramp passes at the base airport, and crossing passes at the science target sites, to determine accuracy in ATM measurements. Section 4 describes the procedures used for producing aircraft trajectories, compares trajectories computed using two entirely different techniques, and assesses trajectory accuracy for several ATM campaigns. Section 5 analyzes the performance of INS systems on these campaigns and computes the trajectory height errors due to INS errors. Section 6 assesses the various contributions to horizontal survey errors. Section 7 then summarizes the overall error levels which would be expected from ATM surveys.
The full ATM accuracy report can be downloaded here ATM Accuracy report 2012