Laser Altimeter Measurements Research Articles

Extracting iceberg freeboard using shadow length in high-resolution optical images

ABSTRACT Icebergs are big chunks of ice floating on the ocean surface, and melting of icebergs contributes for the major part of freshwater flux into ocean. Dynamic monitoring of the icebergs in Antarctica and accurate estimation of their volume are important for predicting the trend of freshwater budget of the Southern Ocean. The iceberg freeboard is a key parameter for measuring the thickness and volume of an iceberg and is defined as the difference between the elevation of iceberg surface and sea level. So far, freeboards of icebergs have been successfully extracted using InSAR DEM, and the laser and radar altimeter. However, uncertainties exist in these results mainly caused by missed detection of small icebergs due to the spatially sparse and temporally incomplete data coverage. In addition to the above techniques, optical images can also be used to extract the iceberg freeboard based on its geometric relationship with shadow length, which can effectively compensate for the above shortcomings. Although the feasibility has been preliminarily presented, the precision and extensive application of shadow-height method deserves further research, such as estimating the basal melting of icebergs. In this work, we tested an optical image-based freeboard extraction method over icebergs in Prydz Bay, Antarctica. A normalized shadow pixel index (NSPI) is designed to identify iceberg shadows with different shapes in HY-1C/D CZI and Sentinel-2 MSI optical images. The iceberg freeboard can be determined with an acceptable precision (2 m) in optical images compared with laser altimeter (i.e. ICESat-2) measurements. Moreover, basal melting of icebergs has been assessed according to the variation of freeboard using repeated optical observations. The results indicate that icebergs in the study area were with a mean freeboard of about 56 m in early December 2022, and experienced a decrease in freeboard of 1.9 m within two months, in correspondence with the Antarctic seasonal trend. The methodological framework, therefore, turns out to be a reliable complementary approach to studying the iceberg freeboard in polar regions.

Automated astronaut traverses with minimum metabolic workload: Accessing permanently shadowed regions near the lunar south pole

The Artemis exploration zone is a topographically complex impact-cratered terrain. Steep undulating slopes pose a challenge for walking extravehicular activities (EVAs) anticipated for the Artemis III and subsequent missions. Using 5 m/pixel Lunar Orbiter Laser Altimeter (LOLA) measurements of the surface, an automated Python pipeline was developed to calculate traverse paths that minimize metabolic workload. The tool combines a Monte Carlo method with a minimum-cost path algorithm that assesses cumulative slope over distances between a lander and stations, as well as between stations. To illustrate the functionality of the tool, optimized paths to permanently shadowed regions (PSRs) are calculated around potential landing sites 001, nearby location 001(6), and 004, all within the Artemis III ‘Connecting Ridge’ candidate landing region. We identified 521 PSRs and computed (1) traverse paths to accessible PSRs within 2 km of the landing sites, and (2) optimized descents from host crater rims into each PSR. Slopes are limited to 15° and previously identified boulders are avoided. Surface temperature, astronaut body illumination, regolith bearing capacity, and astronaut-to-lander direct view are simultaneously evaluated. Travel times are estimated using Apollo 12 and 14 walking EVA data. A total of 20 and 19 PSRs are accessible from sites 001 and 001(6), respectively, four of which maintain slopes <10°. Site 004 provides access to 11 PSRs, albeit with higher EVA workloads. From the crater rims, 94 % of PSRs can be accessed. All round-trip traverses from potential landing sites can be performed in under 2 h with a constant walk. Traverses and descents to PSRs are compiled in an atlas to support Artemis mission planning.

Statistical and Dynamical Characteristics of Extreme Wave Crests Assessed with Field Measurements from the North Sea

Abstract Wave crests of unexpected height and steepness pose a danger to activities at sea, and long-term field measurements provide important clues for understanding the environmental conditions that are conducive to their generation and behavior. We present a novel dataset of high-frequency laser altimeter measurements of the sea surface elevation gathered over a period of 18 years from 2003 to 2020 on an offshore platform in the central North Sea. Our analysis of crest height distributions in the dataset shows that mature, high sea states with high spectral steepness and narrow directional spreading exhibit crest height statistics that significantly deviate from standard second-order models. Conversely, crest heights in developing sea states with similarly high steepness but wide directional spread correspond well to second-order theory adjusted for broad frequency bandwidth. The long-term point time series measurements are complemented with space–time stereo video observations from the same location, collected during five separate storm events during the 2019/20 winter season. An examination of the crest dynamics of the space–time extreme wave crests in the stereo video dataset reveals that the crest speeds exhibit a slowdown localized around the moment of maximum crest elevation, in line with prevailing theory on nonlinear wave group dynamics. Extending on previously published observations focused on breaking crests, our results are consistent for both breaking and nonbreaking extreme crests. We show that wave crest steepness estimated from time series using the linear dispersion relation may overestimate the geometrically measured crest steepness by up to 25% if the crest speed slowdown is not taken into account. Significance Statement Better understanding of the statistics and dynamical behavior of extreme ocean surface wave crests is crucial for improving the safety of various operations at sea. Our study provides new, long-term field evidence of the combined effects of wave field steepness and directionality on the statistical distributions of crest heights in storm conditions. Moreover, we show that the dynamical characteristics of extreme wave crests are well described by recently identified nonlinear wave group dynamics. This finding has implications, for example, for wave force calculations and the treatment of wave breaking in numerical wave models.

Точність вимірювання висоти дерев різними способами

In this paper, the possibilities of using stereophotogrammetry methods for measurements using unmanned aerial vehicles (UAVs) for the conditions of a mature pine stand with uneven density are examined. Here, we carried out a comparison of measurements using altimeters and remote sensing data collected with a UAV optical camera. In particular, the height of growing trees was estimated by three different field-based devices and applying the three methods of data collection and processing with UAVs. Specifically, one method implied the direct measurements using on-board UAV equipment. The following two methods are based on the data provided by the stereophotogrammetrical approach, while the aerial images for that were collected using a UAV optical camera. In particular, there was a modeling of the points cloud from one-sided vertical shooting of sample trees and determination of height of trees from digital canopy height model (CHM) from data of aerial photography of horizontal spans over a stand. Our investigation confirmed the highest accuracy of laser measuring tools among the ground measuring devices used in research. Respective value of the average random measurement error was less than 3 % (0.88 m). Among the results obtained from the analysis of the original data collected by UAVs, the best method was to utilize the CHM, namely, the average random error was less than 2% (0.64 m). This exceeds the accuracy of laser altimeter measurements 33 %. Thus, this method of measuring height in pine stands meets the standards of accuracy in determining the height for production assessment, according to the “Inventory guidelines for the forest fund of Ukraine”, and can be used for survey, inventory, forest management and other works related to forestry and monitoring the changes in forest ecosystems.

Monthly Surface Elevation Changes of the Greenland Ice Sheet From ICESat-1, CryoSat-2, and ICESat-2 Altimetry Missions

The Greenland Ice Sheet (GrIS) mass balance shows significant variabilities over a range of time scales. As geodetic records lengthen over time, it becomes insufficient to characterize the temporal evolution of the ice sheet by using a best-fit linear trend over a given observation period. This study investigates the joint analysis of laser and radar satellite altimeter measurements for estimating GrIS surface elevation changes (SECs) with a 30-day resolution. We first apply a crossover analysis to assess the precisions of the surface elevations measured by ICESat-1/2 laser altimeters and CryoSat-2 radar altimeter over the GrIS, which are needed for assigning weights for each data set in the joint analysis. Then, based on a modified repeat-track approach, we analyze the surface elevation measurements of ICESat-1/2 and CryoSat-2 to produce monthly SEC estimates for the past two decades, together with their associated uncertainties. The multimission SEC estimates are further assessed by using IceBridge airborne laser measurements, showing differences with a median value of −12 cm ± 60 cm. The monthly SEC time series reveal important variations over a range of time scales across different parts of the GrIS and would facilitate the investigation of complex spatiotemporal patterns of GrIS changes.

Improvement of orbit determination using laser altimeter crossovers: JUICE mission case study

This work evaluates the added benefit of using laser altimeter measurements for orbit reconstruction. As a spacecraft orbits a celestial body, its altimetry swaths progressively cross previous swaths. These locations, known as crossover points, yield valuable information about the orbited body and the spacecraft trajectory. The mathematical expressions for the inclusion of crossover measurements into orbit determination algorithms are presented and evaluated. It is shown that a first-order approximation of these expressions is insufficient and a more detailed expression is developed. To evaluate the impact of altimetry crossover measurements on orbit determination, the planetary mission Jupiter Icy moons Explorer (JUICE) by the European Space Agency (ESA) is used as a case study by means of a covariance analysis. In this initial analysis, the assumption is made that all altimetry measurements are obtained with nadir pointing which limits the direct applicability of our method until pointing is accounted for.

Integration of allometric equations in the water cloud model towards an improved retrieval of forest stem volume with L-band SAR data in Sweden

Much attention is paid to the estimation of forest biomass-related variables (stem volume and above-ground biomass) with synthetic aperture radar (SAR) backscatter images because of the increasing number of sensors in space providing global and repeated coverage and the sensitivity of the backscattered intensity to forest properties. One of the most popular models used to estimate a biomass-related variable from SAR backscatter observations is the Water Cloud Model (WCM) because of its simplicity allowing for a straightforward retrieval. Nonetheless, a common feature of these estimates is the tendency to over- or underestimate specific ranges due to simplifying assumptions in the model. In this study, the WCM has been revisited by exploring pathways for a physically-based, Light Detection and Ranging (LiDAR)-aided, model parameterization at larger scale with the overall aim to reduce systematic retrieval errors associated with empirical assumptions in the model. The study was undertaken in Sweden where repeated observations of backscatter by the Advanced Land Observing Satellite (ALOS) Phased Array-type L-band Synthetic Aperture Radar (PALSAR) were available. The integration was prototyped in Sweden thanks to detailed allometries relating forest variables in the WCM. These were derived from spatially dense estimates of canopy density and vegetation height from observations by the Ice, Cloud and land Elevation Satellite (ICESat) Geoscience Laser Altimeter System (GLAS) and measurements of height and stem volume from the Swedish National Forest Inventory (NFI). The SAR backscatter predicted by the revisited WCM was in strong agreement with the observations. When evaluated against stem volumes estimated from the NFI data, the SAR-based stem volumes presented strong dispersion at the pixel level. Average stem volume at the level of five or more pixels, i.e., for an area larger than 0.3 ha, were instead unbiased and similar to the average values obtained from the NFI data (relative root mean square error: 21.4%, estimation bias: 0.9 m3/ha and coefficient of determination: 0.67). This study demonstrates that the integration of allometries in the WCM effectively reduces estimation errors. The method here prototyped in Sweden qualifies to provide large-scale estimates of biomass-related variables using multiple observations of L-band backscatter with potential application worldwide.

Assessing Scale Dependence on Local Sea Level Retrievals from Laser Altimetry Data over Sea Ice

The measurement of sea ice elevation above sea level or the “freeboard” depends upon an accurate retrieval of the local sea level. The local sea level has been previously retrieved from altimetry data alone by the lowest elevation method, where the percentage of the lowest elevations over a particular segment length scale was used. Here, we provide an evaluation of the scale dependence on these local sea level retrievals using data from NASA Operation IceBridge (OIB) which took place in the Ross Sea in 2013. This is a unique dataset of laser altimeter measurements over five tracks from the Airborne Topographic Mapper (ATM), with coincidently high-spatial resolution images from the Digital Mapping System (DMS), that allows for an independent sea level validation. The local sea level is first calculated by using the mean elevation of ATM L1B data over leads identified by using the corresponding DMS imagery. The resulting local sea level reference is then used as ground truth to validate the local sea levels retrieved from ATM L2 by using nine different percentages of the lowest elevation (0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, and 4%) at seven different segment length scales (1, 5, 10, 15, 20, 25, and 50 km) for each of the five ATM tracks. The closeness to the 1:1 line, R2, and root mean square error (RMSE) is used to quantify the accuracy of the retrievals. It is found that all linear least square fits are statistically significant (p &lt; 0.05) using an F test at every scale for all tested data. In general, the sea level retrievals are farther away from the 1:1 line when the segment length scale increases from 1 or 5 to 50 km. We find that the retrieval accuracy is affected more by the segment length scale than the percentage scale. Based on our results, most retrievals underestimate the local sea level; the longer the segment length (from 1 to 50 km) used, especially at small percentage scales, the larger the error tends to be. The best local sea level based on a higher R2 and smaller RMSE for all the tracks combined is retrieved by using 0.1–2% of the lowest elevations at the 1–5 km segment lengths.

Estimating Penetration-Related X-Band InSAR Elevation Bias: A Study over the Greenland Ice Sheet

Accelerating melt on the Greenland ice sheet leads to dramatic changes at a global scale. Especially in the last decades, not only the monitoring, but also the quantification of these changes has gained considerably in importance. In this context, Interferometric Synthetic Aperture Radar (InSAR) systems complement existing data sources by their capability to acquire 3D information at high spatial resolution over large areas independent of weather conditions and illumination. However, penetration of the SAR signals into the snow and ice surface leads to a bias in measured height, which has to be corrected to obtain accurate elevation data. Therefore, this study purposes an easy transferable pixel-based approach for X-band penetration-related elevation bias estimation based on single-pass interferometric coherence and backscatter intensity which was performed at two test sites on the Northern Greenland ice sheet. In particular, the penetration bias was estimated using a multiple linear regression model based on TanDEM-X InSAR data and IceBridge laser-altimeter measurements to correct TanDEM-X Digital Elevation Model (DEM) scenes. Validation efforts yielded good agreement between observations and estimations with a coefficient of determination of R2 = 68% and an RMSE of 0.68 m. Furthermore, the study demonstrates the benefits of X-band penetration bias estimation within the application context of ice sheet elevation change detection.

Using TanDEM-X Pursuit Monostatic Observations with a Large Perpendicular Baseline to Extract Glacial Topography

Space-based Interferometric Synthetic Aperture Radar (InSAR) applications have been widely used to monitor the cryosphere over past decades. Owing to temporal decorrelation, interferometric coherence often severely degrades on fast moving glaciers. TanDEM-X observations can overcome the temporal decorrelation because of their simultaneous measurements by two satellite constellations. In this study, we used the TanDEM-X pursuit monostatic mode with large baseline formation following a scientific phase timeline to develop highly precise topographic elevation models of the Petermann Glacier of Northwest Greenland. The large baseline provided the advantage of extracting topographic information over low relief areas, such as the surface of a glacier. As expected, coherent interferometric phases (&gt;0.8) were well maintained over the glaciers, despite their fast movement, due to the nearly simultaneous TanDEM-X measurements. The height ambiguity, which was defined as the altitude difference corresponding to a 2π phase change of the flattened interferogram, of the dataset was 10.63 m, which was favorable for extracting topography in a low relief region. We validated the TanDEM-X derived glacial topography by comparing it to the SAR/Interferometric radar altimeter observations acquired by CryoSat-2 and the IceBridge Airborne Topographic Mapper laser altimeter measurements. Both observations showed very good correlation within a few meters of the offsets (−12.5~−3.1 m), with respect to the derived glacial topography. Routine TanDEM-X observations will be very useful to better understand the dynamics of glacial movements and topographic change.

In-flight characterization of the lunar orbiter laser altimeter instrument pointing and far-field pattern.

The Lunar Orbiter Laser Altimeter (LOLA) aboard the Lunar Reconnaissance Orbiter (LRO) has collected nearly seven billion measurements of surface height on the Moon with an absolute accuracy of ∼1 m and a precision of ∼10 cm. Converting time-of-flight laser altimeter measurements to topographic elevations requires accurate knowledge of the laser pointing with respect to the spacecraft body-fixed coordinate system. To that end, we have utilized altimetric crossovers from LOLA, as well as bidirectional observations of the LOLA laser and receiver boresight via an Earth-based laser tracking ground station. Based on a sample of ∼780,000 globally distributed crossovers from the circular-orbit phase of LRO's mission (∼27 months), we derive corrections to the LOLA laser boresight. These corrections improve the cross-track and along-track agreement of the crossovers by 24% and 33%, respectively, yielding RMS residuals of ∼10 m. Since early in the LRO mission, the bidirectional laser tracking experiments have confirmed a pointing anomaly when the LOLA instrument is facing toward deep space or the night side of the Moon and have allowed the reconstruction of the laser far-field pattern and receiver telescope pointing. By conducting such experiments shortly after launch and nearly eightyears later, we have directly measured changes in the laser characteristics and obtained critical data to understand the laser behavior and refine the instrument pointing model. The methods and results presented here are also relevant to the design, fabrication, and operation of future planetary laser altimeters and their long-term behavior in the space environment.

Shipboard Wave Measurements in the Southern Ocean

AbstractSurface wave measurements from ships pose difficulties because of motion contamination. Cifuentes-Lorenzen et al. analyzed laser altimeter and marine X-band radar (MR) wave measurements from the Southern Ocean Gas Exchange Experiment (SOGasEx). They found that wave measurements from both sensors deteriorate precipitously at ship speeds3 m s−1. This study demonstrates that MR can yield accurate wave frequency–direction spectra independent of ship motion. It is based on the same shipborne SOGasEx wave data but uses the MR wave retrieval method proposed by Lund et al. and a novel empirical transfer function (ETF). The ETF eliminates biases in the MR wave spectra by redistributing energy from low to high frequencies. The resulting MR wave frequency–direction spectra are shown to agree well with laser altimeter wave frequency spectra from times when the ship was near stationary and with WAVEWATCH III (WW3) model wave parameters over the full study period.

Evidence for surface water ice in the lunar polar regions using reflectance measurements from the Lunar Orbiter Laser Altimeter and temperature measurements from the Diviner Lunar Radiometer Experiment

We find that the reflectance of the lunar surface within 5° of latitude of the South Pole increases rapidly with decreasing temperature, near ∼110K, behavior consistent with the presence of surface water ice. The North polar region does not show this behavior, nor do South polar surfaces at latitudes more than 5° from the pole. This South pole reflectance anomaly persists when analysis is limited to surfaces with slopes less than 10° to eliminate false detection due to the brightening effect of mass wasting, and also when the very bright south polar crater Shackleton is excluded from the analysis. We also find that south polar regions of permanent shadow that have been reported to be generally brighter at 1064nm do not show anomalous reflectance when their annual maximum surface temperatures are too high to preserve water ice. This distinction is not observed at the North Pole. The reflectance excursion on surfaces with maximum temperatures below 110K is superimposed on a general trend of increasing reflectance with decreasing maximum temperature that is present throughout the polar regions in the north and south; we attribute this trend to a temperature or illumination-dependent space weathering effect (e.g. Hemingway et al., 2015). We also find a sudden increase in reflectance with decreasing temperature superimposed on the general trend at 200K and possibly at 300K. This may indicate the presence of other volatiles such as sulfur or organics. We identified and mapped surfaces with reflectances so high as to be unlikely to be part of an ice-free population. In this south we find a similar distribution found by Hayne et al. (2015) based on UV properties. In the north a cluster of pixels near that pole may represent a limited frost exposure.

On retrieving sea ice freeboard from ICESat laser altimeter

Abstract. Sea ice freeboard derived from satellite altimetry is the basis for the estimation of sea ice thickness using the assumption of hydrostatic equilibrium. High accuracy of altimeter measurements and freeboard retrieval procedure are, therefore, required. As of today, two approaches for estimating the freeboard using laser altimeter measurements from Ice, Cloud, and land Elevation Satellite (ICESat), referred to as tie points (TP) and lowest-level elevation (LLE) methods, have been developed and applied in different studies. We reproduced these methods for the ICESat observation periods (2003–2008) in order to assess and analyse the sources of differences found in the retrieved freeboard and corresponding thickness estimates of the Arctic sea ice as produced by the Jet Propulsion Laboratory (JPL) and Goddard Space Flight Center (GSFC). Three main factors are found to affect the freeboard differences when applying these methods: (a) the approach used for calculation of the local sea surface references in leads (TP or LLE methods), (b) the along-track averaging scales used for this calculation, and (c) the corrections for lead width relative to the ICESat footprint and for snow depth accumulated in refrozen leads. The LLE method with 100 km averaging scale, as used to produce the GSFC data set, and the LLE method with a shorter averaging scale of 25 km both give larger freeboard estimates comparing to those derived by applying the TP method with 25 km averaging scale as used for the JPL product. Two factors, (a) and (b), contribute to the freeboard differences in approximately equal proportions, and their combined effect is, on average, about 6–7 cm. The effect of using different methods varies spatially: the LLE method tends to give lower freeboards (by up to 15 cm) over the thick multiyear ice and higher freeboards (by up to 10 cm) over first-year ice and the thin part of multiyear ice; the higher freeboards dominate. We show that the freeboard underestimation over most of these thinner parts of sea ice can be reduced to less than 2 cm when using the improved TP method proposed in this paper. The corrections for snow depth in leads and lead width, (c), are applied only for the JPL product and increase the freeboard estimates by about 7 cm on average. Thus, different approaches to calculating sea surface references and different along-track averaging scales from one side and the freeboard corrections as applied when producing the JPL data set from the other side roughly compensate each other with respect to freeboard estimation. Therefore, one may conclude that the difference in the mean sea ice thickness between the JPL and GSFC data sets reported in previous studies should be attributed mostly to different parameters used in the freeboard-to-thickness conversion.

The comparison of topographic long profiles of gullies on Earth to gullies on Mars: A signal of water on Mars

The topographic signature of a landform can give important clues as to its formation process. Here, we have used topographic long profiles to study the process of gully formation on Mars. We studied topographic long profiles of gullies on Earth to (1) confirm that previously published generalisations of how long profile shape varies with process also applies at the kilometre-scale of martian gullies, and (2) use as a direct comparison with the martian data. We have compared 24 fluvial and 22 debris flow long profiles of terrestrial gullies derived from laser altimeter and GPS measurements, to 78 long profiles of a range of gullies on Mars derived from a stereo-photogrammetry point-matching technique. We have confirmed that this manual point-matching technique is reliable for the martian data by comparison with full digital elevation models. We used nine different characteristics of the long profiles, including slope and curvature parameters, to perform a canonical discriminant analysis, which allowed us to identify the variables most important for differentiating between fluvial and debris flow gullies on Earth. In agreement with published literature for larger-scale features, we found that terrestrial debris flow gullies tend to be steeper and less concave than fluvial gullies. We have found that gully long profiles on Mars can resemble long profiles of terrestrial gullies formed by either fluvial or debris flow processes, with slightly more affinity to fluvial systems. Gullies on Mars can only be weakly separated from those on Earth: they can be separated from terrestrial fluvial gullies on curvature parameters and from terrestrial debris flow gullies by slope parameters. In addition, we have found that different alcove types identified from planview morphology are also distinctive in terms of their long profile morphology: gullies which incise back into the bedrock are more similar to terrestrial debris flows whereas polar-pit gullies are most similar to terrestrial fluvial gullies. Our findings suggest that the presence of a bedrock alcove promotes debris flow behaviour in gullies on Mars.

Recovery of elevation from estimated gradient fields constrained by digital elevation maps of lower lateral resolution

Depth measurement techniques like stereo analysis and laser range scanning often yield a lateral resolution below image resolution. In contrast, shading-based methods estimate the elevation model at image resolution. We present a computationally efficient approach to recover a surface of high vertical and high lateral resolution from a noisy gradient field and independently measured elevation data of lower lateral resolution, relying on a minimization of the mean squared difference between the low-pass component of the surface obtained based on shading information and that of the independently measured elevation data. The presented method is compared to a reference approach that minimizes a weighted sum comprised of the mean squared difference between the low-pass components of the estimated gradient field and the optimized model, respectively. The presented algorithm is applied using lunar orbital image data and stereo elevation data and is evaluated regarding orbital laser altimeter measurements of high vertical accuracy.

A new 1 km digital elevation model of the Antarctic derived from combined satellite radar and laser data – Part 1: Data and methods

Abstract. Digital elevation models (DEMs) of the whole of Antarctica have been derived, previously, from satellite radar altimetry (SRA) and limited terrestrial data. Near the ice sheet margins and in other areas of steep relief the SRA data tend to have relatively poor coverage and accuracy. To remedy this and to extend the coverage beyond the latitudinal limit of the SRA missions (81.5° S) we have combined laser altimeter measurements from the Geosciences Laser Altimeter System onboard ICESat with SRA data from the geodetic phase of the ERS-1 satellite mission. The former provide decimetre vertical accuracy but with poor spatial coverage. The latter have excellent spatial coverage but a poorer vertical accuracy. By combining the radar and laser data using an optimal approach we have maximised the vertical accuracy and spatial resolution of the DEM and minimised the number of grid cells with an interpolated elevation estimate. We assessed the optimum resolution for producing a DEM based on a trade-off between resolution and interpolated cells, which was found to be 1 km. This resulted in just under 32% of grid cells having an interpolated value. The accuracy of the final DEM was assessed using a suite of independent airborne altimeter data and used to produce an error map. The RMS error in the new DEM was found to be roughly half that of the best previous 5 km resolution, SRA-derived DEM, with marked improvements in the steeper marginal and mountainous areas and between 81.5 and 86° S. The DEM contains a wealth of information related to ice flow. This is particularly apparent for the two largest ice shelves – the Filchner-Ronne and Ross – where the surface expression of flow of ice streams and outlet glaciers can be traced from the grounding line to the calving front. The surface expression of subglacial lakes and other basal features are also illustrated. We also use the DEM to derive new estimates of balance velocities and ice divide locations.

Comparison of ICESat Data With Airborne Laser Altimeter Measurements Over Arctic Sea Ice

Surface elevation and roughness measurements from NASA's Ice, Cloud, and land Elevation Satellite (ICESat) are compared with high-resolution airborne laser altimeter measurements over the Arctic sea ice north of Alaska, which were taken during the March 2006 EOS Aqua Advanced Microwave Scanning Radiometer sea ice validation campaign. The comparison of the elevation measurements shows that they agree quite well with correlations of around 0.9 for individual shots and a bias of less than 2 cm. The differences are found to decrease quite rapidly when applying running means. The comparison of the roughness measurements show that there are significant differences between the two data sets, with ICESat generally having higher values. The roughness values are only moderately correlated on an individual-shot basis, but applying running means to the data significantly improves the correlations to as high as 0.9. For the conversion of the elevation measurements into snow-ice freeboard, ocean surface elevation estimates are made with the high-resolution laser altimeter data, as well as several methods using lower resolution ICESat data. Under optimum conditions, i.e., when leads that are larger than the ICESat footprint are present, the ICESat- and Airborne Topographic Mapper-derived freeboards are found to agree to within 2 cm. For other areas, ICESat tends to underestimate the freeboard by up to 9 cm.

ICESat measurements of sea ice freeboard and estimates of sea ice thickness in the Weddell Sea

Sea ice freeboard heights in the Weddell Sea of Antarctica are derived from the Ice, Cloud, and Land Elevation Satellite (ICESat) laser altimeter measurements, which have a unique range precision to flat surfaces of 2 cm within 70 m footprints spaced at 172 m along track. Although elevations of flat surfaces can be obtained to an accuracy of ∼10 cm (1σ) per footprint, direct determination of freeboard heights is precluded by errors in knowledge of the geoid and temporal variability of the ocean surface. Therefore freeboards are determined relative to an ocean reference level detected over areas of open water and very thin ice within the sea ice pack using an along‐track filtering method. The open water/thin ice selections show good agreement in the combined analysis of ICESat segments and Envisat Synthetic Aperture Radar (SAR) imagery. The average residual between the ICESat‐measured ocean level and the EGM96 geoid is 1.4 m. Estimates of snow depth on the sea ice from AMSR‐E passive microwave data along with nominal densities of snow, water, and sea ice are used to estimate sea ice thickness. Four periods of ICESat data in May–June (fall) and October–November (late winter) of 2004 and 2005 between longitudes 298°E and 360°E are analyzed. In the fall the mean freeboards are 0.28 m in 2004 and 0.29 m in 2005, and the mean thicknesses are 1.33 m in 2004 and 1.52 m in 2005. In late winter the freeboards grew to 0.37 m in 2004 and 0.35 in 2005, and the thicknesses grew to 2.23 m in 2004 and 2.31 m in 2005. The interannual differences in freeboard are small, and the larger interannual change in estimated thickness mainly represents differences in the snow depth estimates. Seasonal changes in the spatial patterns of freeboard and thickness over the 4 months correlate with the expected circulation of sea ice in the Weddell Sea, as indicated by sea ice velocity fields.

Combined airborne laser and radar altimeter measurements over the Fram Strait in May 2002

Knowledge of sea ice thickness is critical for the prediction of future climate, and for assessing the significance of changes in thickness. Sea ice thickness can be calculated from radar or laser satellite altimetry measurements of freeboard. However, a lack of knowledge of snow depth introduces significant uncertainties into these calculations. This paper compares the first coincident airborne laser and radar altimetry data over sea ice, collected during the Laser Radar Altimetry (LaRA) field campaign. LaRA was a flight of opportunity that provided valuable data to explore techniques to validate satellite measurements of ice freeboard, and the possibility of combining laser and radar measurements over snow covered sea ice to calculate the snow depth. Two new methods were created to analyse these data sets: a new radar retracker and a radar power simulator, which models radar returns from the laser data. We present the first quantitative analysis of data from the LaRA laser and radar altimeters, and demonstrate the potential of combining laser and radar altimetry to estimate snow depth. LaRA elevation estimates compare well with elevations from the radar altimeter onboard ERS-2 at the sub-meter level and the study provides lessons for future validation of satellite altimetry data over sea ice. Laser elevations are consistently higher than the radar elevations over snow covered sea ice. As LaRA was a flight of opportunity, no coincident in-situ measurements were available. Nevertheless, the difference between the reflecting surface of the laser and radar is consistent with snow depth from climatology and the analysis techniques developed in this paper will be useful for future radar and laser altimetry comparisons.