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 (n
ear 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.
The measurement of surface elevation (altitude) with a laser altimeter. Particularly when used to measure elevation change, laser altimetry has become a leading source of data for the measurement of m
ass balance by the geodetic method. If, for logistical or financial reasons, it is not possible to survey the whole glacier by airborne laser altimetry, it is necessary to extrapolate to obtain a glacier-wide geodetic mass balance.
The most recent time (15 000 to 80 000 years ago) during which continental glaciers covered subpolar regions and existed at elevations as much as 1000 m lower than today; corresponding to periods in w
hich oxygen isotopes from marine sediment cores indicate that global sea level was 50-150 m lower and global temperature 5-10C lower than today.
The period during the last ice age when the glaciers and ice sheets reached their maximum extent, approximately 21 ka ago. This period has been widely studied because the radiative forcings and bounda
ry conditions are relatively well known.
The most recent time (115 000 to 125 000 years ago) during which global temperatures were as high as or higher than in the postglacial, when continental glaciers were limited to the Arctic and Antarct
ic, and sea levels were near current positions.
With reference to the effect of energetic particles on spacecraft microcircuits, a serious type of single event upset in which the microcircuit is either permanently stuck or cannot be reset without b
eing turned off and on.