Ideally, the response of the Advanced Topographic Laser Altimeter System (ATLAS) instrument on the Ice, Cloud, and land Elevation Satellite–2 (ICESat-2) observatory to returns from a flat target at a fixed distance would be a single peak. However, ICESat-2 profiles from very flat surfaces display extra features below the presumed Earth surface. This paper identifies the multiple reflections within the ATLAS receiver that create those extra pulses. We describe their sources, compare their measured position to optical ray trace predictions, and discuss their relative amplitude. We then explore the possibility of using afterpulses to extract surface elevation information from highly saturated returns, in which the first return pulse is distorted and displaced by nonlinear effects in the detector.

The Advanced Topographic Laser Altimeter System (ATLAS) is the sole science instrument on NASA’s ICESat-2 mission. It was designed to measure elevation simultaneously along six tracks on the earth’s surface with centimeter-level vertical precision, demanding a picosecond-level precision in photon time of flight. To ensure this precision requirement was met, we developed for the Level-1B ATLAS data product a careful verification procedure. To quantify the amount of acceptable error we needed to understand the effects of the various floating-point precisions at all steps of the calculations and the limitations of the programming languages used for the production software (Fortran) and for the verification software (Python). For example, we found in the 64-bit photon time-of-flight calculations that differences even at 13 or 14 decimal places often revealed that an incorrect calibration value had been selected. Without first characterizing the acceptable error, such small errors could be overlooked and could propagate into critical inaccuracies in the science products. We describe our methods and lessons learned in order to inform future remote sensing verification efforts.