Laser Altimeter Measurements Research Articles

Progressive increase in ice loss from Greenland

Laser altimeter measurements over Greenland show increasing thickening rates above 2000 m, reflecting increasing snowfall in a warming climate. But near‐coastal thinning rates have increased substantially since the mid 1990s, and net mass loss more than doubled from an average of 4–50 Gt yr−1 between 1993/4 and 1998/9 to 57–105 Gt yr−1 between 1998/9 and 2004. This increasing trend is very similar to findings from independent mass‐budget studies, but differs widely from ERS radar altimeter results. This may result from limitations associated with the large ERS footprint over sloping and undulating surfaces that typify fast, narrow glaciers where thinning is most pronounced.

Glacier retreat, mass‐balance and thinning: sermilik glacier, south greenland

This study documents thinning and retreat of the South Greenland ice margin and discusses possible reasons in the light of mass‐balance and change of dynamic conditions. Analyses of satellite images have shown that the glacier tongue of Sermilik glacier disintegrated within the past 15 years. Furthermore, the observed thinning close to the Sermilik glacier front was as much as 120 m water equivalent during this period. This figure was derived by comparing surface elevation data from a digital elevation model (1985) and laser altimeter measurements from the year 2000, showing surface elevation changes along a flow line of Sermilik glacier. Mass‐balance data from in situ measurements performed at a centre flow line of the glacier are presented. These data are compared to results from remote sensing analyses of the study area. Net ablation reconstruction over the last 41 years from positive‐degree‐day modelling, at various locations along the Sermilik glacier massbalance transect, shows an increase during the past decades. These analyses indicate that only 55% of the total thinning in this area can be explained by mass‐balance changes. The remaining 45% of the thinning is attributed to changes in the dynamic behaviour of the glacier, such as an increase of creep towards the end of the twentieth century. The significant thinning along the Qagssimiut lobe can also be explained as a combination of mass‐balance changes and changes in ice dynamic behaviour.

Fluid dynamical implications of anastomosing slope streaks on Mars

The Mars Global Surveyor mission has imaged slope streaks, some of which have formed in periods as short as 109 days. These features are one of the most currently active surface processes on Mars. Some slope streaks have flow‐like morphologic characteristics, which include anastomosing patterns influenced by small topographic barriers. In order to understand what processes gave rise to these specific features, we applied viscoplastic flow numerical modeling techniques to simulated Martian surfaces. We simulate Martian surfaces with observed slope streaks by using Mars Orbiter Laser Altimeter measurements to obtain large‐scale slope measurements and a photoclinometry technique on Mars Orbiter Camera images to obtain meter‐scale topographic information. Our numerical simulations of slow‐moving plastic flows show that a fluid rheology and a short formation period are necessary to explain these features. We estimate that the typical values of bulk viscosity and bulk yield strength are less than 10 Pa s and less than 10 Pa, respectively. The fluid rheology can be explained by a water‐related flow with a solid content less than about 20%. An alternative explanation is a dry grain flow with extremely low cohesion and friction angle supported by dispersive pressure or a lubricant, such as electric conditions of particles. The continuous nature of anastomosing slope streaks that originate from point sources is best explained by continuous discharges of material or lubricant. In this case, the estimated flow rate is less than several cubic meters per second, and the flow duration is estimated to be less than a day.

Thickness changes on Whillans Ice Stream and Ice Stream C, West Antarctica, derived from laser altimeter measurements

AbstractRepeat airborne laser altimeter measurements are used to derive surface elevation changes on parts of Whillans Ice Stream and Ice Stream C, West Antarctica. Elevation changes are converted to estimates of ice equivalent thickness change using local accumulation rates, surface snow densities and vertical bedrock motions. The surveyed portions of two major tributaries of Whillans Ice Stream are found to be thinning almost uniformly at an average rate of ∼1 m a−1. Ice Stream C has a complicated elevation-change pattern, but is generally thickening. These results are used to estimate the contribution of each surveyed region to the current rate of global sea-level rise.

Effective aerodynamic roughness estimated from airborne laser altimeter measurements of surface features

Aerodynamic roughness length (z 0) and displacement height (d 0) are important surface parameters for estimating surface fluxes in numerical models. These parameters are generally determined from wind flow characteristics using logarithmic wind profiles measured at a meteorological tower or by balloon release. It would be an advantage to use measurements of land surface characteristics instead of wind flow characteristics to estimate the z 0 and d 0 for large areas. Important land surface characteristics are the size and distribution of roughness elements (obstacles). This research evaluates the use of high resolution laser altimeter data to obtain these land surface characteristics. Data were collected at the US Department of Agriculture, Agricultural Research Service (USDA‐ARS), Jornada Experimental Range in southern New Mexico, USA over a coppice dune dominated area. These dunes are covered by honey mesquite (Prosopis glandulosa Torr.) with flat and mostly bare interdunal areas. For this analysis, three 450 m laser transects with a 2 cm measurement interval were used. The distribution and size of dunes were calculated from these laser transects and used to compute z 0. Analysis gave an average z 0 = 4.3 cm and d 0 = 70 cm for the three laser transects, which compares to z 0 = 7 ± 4 cm and d 0 = 98 ± 48 cm calculated from wind profile data measured at a 10 m tower near the laser transects. These results show that the estimation of z 0 and d 0 for a complex terrain is possible using simple land surface features computed from high resolution laser altimeter data.

Coherent radar ice thickness measurements over the Greenland ice sheet

We developed two 150‐MHz coherent radar depth sounders for ice thickness measurements over the Greenland ice sheet. We developed one of these using connectorized components and the other using radio frequency integrated circuits (RFICs). Both systems are designed to use pulse compression techniques and coherent integration to obtain the high sensitivity required to measure the thickness of more than 4 km of cold ice. We used these systems to collect radar data over the interior and margins of the ice sheet and several outlet glaciers. We operated both radar systems on the NASA P‐3B aircraft equipped with GPS receivers. Radar data are tagged with GPS‐derived location information and are collected in conjunction with laser altimeter measurements. We have reduced all data collected since 1993 and derived ice thickness along all flight lines flown in support of Program for Regional Climate Assessment (PARCA) investigations and the North Greenland Ice Core Project. Radar echograms and derived ice thickness data are placed on a server at the University of Kansas (http://tornado.rsl.ukans.edu/Greenlanddata.htm) for easy access by the scientific community. We obtained good ice thickness information with an accuracy of ±10 m over 90% of the flight lines flown as a part of the PARCA initiative. In this paper we provide a brief description of the system along with samples of data over the interior, along the 2000‐m contour line in the south and from a few selected outlet glaciers.

Multifractal analysis of canopy height measures in a longleaf pine savanna

Spatial patterns of forest canopies are fractal as they exhibit variation over a continuum of scales. A measure of fractal dimension of a forested landscape represents the spatial summation of physiologic (leaf-level), demographic (population-level), and abiotic (e.g., edaphic) processes, as well as exogenous disturbances (e.g., fire and hurricane) and thus provides a basis to classify or monitor such systems. However, forests typically exhibit a spectrum of fractal parameters which yields further insight to the geometric structure of the system and potentially the underlying processes. We calculated multifractal properties of longleaf pine flatwoods, the predominant ecosystem of central Florida, from canopy profile data derived from an airborne laser altimeter and ground-based measurements in The Nature Conservancy’s Disney Wilderness Preserve located near Kissimmee, Florida. These metrics were compared for six ≈500 m transects to determine the level of consistency between remotely sensed and field measures and within a forest community. Multifractal techniques uncovered subtle differences between transects that could correspond to unique, underlying abiotic and biotic processes. These techniques should be considered a valuable tool for ecological analysis.

Control of the radiative behavior of the Martian polar caps by surface CO2 ice: Evidence from Mars Global Surveyor measurements

The seasonal polar caps of Mars are composed primarily of solid CO2, and their growth and decay play a large part in the planet's atmospheric CO2 cycle. The thermodynamic temperature of the caps is ∼145 K, in equilibrium with the ∼6 mbar atmosphere. The optical properties of CO2 ice require that pure snow with a sufficiently small particle size has a high visible/near‐infrared albedo and a low infrared emissivity in parts of the thermal infrared, particularly in the region between 20 and 50 μm. Dust mixed into or on top of the CO2 ice will lower the visible albedo and bring the thermal infrared emissivity closer to 1. Water ice mixed with the CO2 will have little effect in the visible and near‐infrared to 1.4 μm but can raise the 20–50 μm emissivity if the particle size is >50 μm. Observations of both seasonal polar caps during two of the assessment orbits of the Mars Global Surveyor spacecraft are used to show that there is a strong correlation between visible brightness and low 20–50 μm emissivity. The infrared spectra from the Thermal Emission Spectrometer from regions with low 20–50 μm brightness temperature are consistent with surface deposits of CO2 with millimeter‐sized grains and containing varying small amounts of dust, and they are not consistent with the expected signature of water ice or clouds. Large regions of low emissivity in the spring seasonal caps have not been observed previously. They are correlated with visible bright regions which are known to become brighter as the spring progresses. The visible brightness of the south polar deposits examined is inferred from historical observations, while the brightness of the north polar region is determined from Mars Orbiter Laser Altimeter measurements at 1.06 μm. The model of seasonal CO2 caps with bright, low‐emissivity regions agrees with previous visible observations of bright crater rims, streaks, and other bright areas within the polar caps, some of which may evolve in time from dark, high‐emissivity sheet ice to brighter, fractured, lower‐emissivity ice layers. It is also in agreement with models of the CO2 cycle, which require average polar cap emissivities <0.9.

Georeferencing of airborne laser altimeter measurements

Abstract The primary purpose of airborne laser altimetry is to determine the ellipsoidal or geoidal coordinates of a series of points on the surface of the Earth. An aircraft that is instrumented with a laser altimeter, an inertial navigation system, and a Global Positioning System (GPS) receiver provides the following data: (1) laser range to the Earth's surface, (2) measurement platform spatial location and orientation, and (3) aircraft kinematic trajectory in ellipsoidal coordinates. These data are sufficient to determine (georeference) the three dimensional coordinates of the points where the beam from a pulsed laser intersects the Earth. We develop the exact equations necessary to georeference the laser points. We also discuss calibrating the laser pulse timing, laser positioning and alignment relative to the local-level reference frame, correcting atmospheric refraction effects on the laser pulse, and time synchronizing the various data streams. We use a laser altimeter mission flown over Lake Crowl...

Airborne laser altimeter measurements of landscape topography

Measurements of topography can provide a wealth of information on landscape properties for managing hydrologic and geologic systems and conserving natural and agricultural resources. This article discusses the application of an airborne laser altimeter to measure topography and other landscape surface properties. The airborne laser altimeter makes 4000 measurements per second with a vertical recording resolution of 5 cm. Data are collected digitally with a personal computer. A video camera, borehole sighted with the laser, records an image for locating flight lines. GPS data are used to locate flight line positions on the landscape. Laser data were used to measure vegetation canopy topography, height, cover, and distribution and to measure microtopography of the land surface and gullies with depths of 15–20 cm. Macrotopography of landscape profiles for segments up to 4 km were in agreement with available topographic maps but provided more detail. Larger gullies with and without vegetation, and stream channel cross sections and their associated floodplains have also been measured and reported in other publications. Landscape segments for any length could be measured for either micro- or macrotopography. Airborne laser altimeter measurements of landscape profiles can provide detailed information on landscape properties or specific needs that will allow better decisions on the design and location of structures (i.e., roads, pipe, and power lines) and for improving the management and conservation of natural and agricultural landscapes.

Comparison of retracking algorithms using airborne radar and laser altimeter measurements of the Greenland ice sheet

Compares four continental ice sheet radar altimeter retracking algorithms using airborne radar and laser altimeter data taken over the Greenland ice sheet in 1991. The refurbished Advanced Application Flight Experiment (AAFE) airborne radar altimeter has a large range window and stores the entire return waveform during flight. Once the return waveforms are retracked, or post-processed to obtain the most accurate altitude measurement possible, they are compared with the high-precision Airborne Oceanographic Lidar (AOL) altimeter measurements. The AAFE waveforms show evidence of varying degrees of both surface and volume scattering from different regions of the Greenland ice sheet. The AOL laser altimeter, however, obtains a return only from the surface of the ice sheet. Retracking altimeter waveforms with a surface scattering model results in a good correlation with the laser measurements in the wet and dry-snow zones, but in the percolation region of the ice sheet, the deviation between the two data sets is large due to the effects of subsurface and volume scattering. The Martin et al. model results in a lower bias than the surface scattering model, but still shows an increase in the noise level in the percolation zone. Using an offset center of gravity algorithm to retrack altimeter waveforms results in measurements that are only slightly affected by subsurface and volume scattering and, despite a higher bias, this algorithm works well in all regions of the ice sheet. A cubic spline provides retracked altitudes that agree with AOL measurements over all regions of Greenland. This method is not sensitive to changes in the scattering mechanisms of the ice sheet and it has the lowest noise level and bias of all the retracking methods presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Estimation of effective aerodynamic roughness of Walnut Gulch watershed with laser altimeter measurements

A new method to estimate the effective aerodynamic roughness of a complex landscape is presented. High‐resolution elevation profiles measured with an airborne laser altimeter have been used to compute geometrical parameters of three landscape elements of the Walnut Gulch experimental watershed, Arizona. Mean crop height, its standard deviation, and the frequency distribution (as a function of penetration depth) of plant elements hit by the laser beam have been calculated for short segments (1 m) measured over intervening grass between shrubs. Longer segments have been applied to obtain the mean height and spacing of taller shrubs and trees. Finally, laser profiles covering the entire watershed gave the mean amplitude and wavelength of hillocks and ridges. The aerodynamic roughness length of the intervening grass is estimated using the ratio of standard deviation to vegetation height, corrected for instrument noise, times mean height. An effective roughness length which parameterizes the total stress due to grass and taller shrubs and trees is calculated first. Finally, a watershed‐scale effective roughness length is calculated combining the amplitude and wavelength of hillocks and ridges with the roughness length obtained in the previous step.

Two-color short-pulse laser altimeter measurements of ocean surface backscatter

The timing and correlation properties of pulsed laser backscatter from the ocean surface have been measured with a two-color short-pulse laser altimeter. The Nd: YAG laser transmitted 70-and 35-ps wide pulses simultaneously at 532 and 355 nm at nadir, and the time-resolved returns were recorded by a receiver with 800-ps response time. The time-resolved backscatter measured at both 330- and 1291-m altitudes showed little pulse broadening due to the submeter laser spot size. The differential delay of the 355- and 532-nm backscattered waveforms were measured with a rms error of ~75 ps. The change in aircraft altitudes also permitted the change in atmospheric pressure to be estimated by using the two-color technique.