An instrument for altimetry, and in mass-balance studies for the measurement of elevation change by repeated altimetry, that uses pulses of laser radiation, for example at 532 nm (green) or 1024 nm (near infrared) wavelengths. There are both profiling and scanning laser altimeters. A profiling system is nadir-pointing, while a scanning system uses a rotating mirror, or a series of sensors arrayed in a parallel (push broom) configuration, to obtain a swath rather than a linear profile of measurements. The Ice, Cloud and land Elevation Satellite (icesat, 20032010) measured surface elevations with approximately 70 m footprint and 170 m along-track spacing. Adjacent tracks are separated by a few to a few tens of kilometres, the lesser separations being found at the polar extremities of the orbit. Sources of error include sensor saturation, atmospheric scattering effects, and inaccurate knowledge of the laser pointing angles. Aircraft altimeters have footprints of 1 m or smaller and along-track spacing on the order of 1 to 3 m, and are less affected by atmospheric and pointing errors. Laser altimeters are unable to obtain measurements through clouds. Laser is an acronym standing for light amplification by stimulated emission of radiation. A related term, lidar (light detection and ranging), applies more generally to the measurement of scattered light from distant targets.