NASA developed the Multiple Altimeter Beam Experimental Lidar (MABEL) airborne instrument to serve as a testbed for the Advanced Topographic Laser Altimeter System (ATLAS) aboard NASA's Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2). The goal of the MABEL team was to address measurements that can only be addressed by field measurements—issues related to ice, snow, water, and clouds. MABEL was carried aboard NASA's ER-2 research aircraft at an operational altitude of approximately 20 km.