A surface and volume scattering retracking algorithm for ice sheetsatellite altimetry

An algorithm based on a combined surface and volume scattering model that is used to retrack individual altimeter waveforms from the ice sheets is developed. Because the combined model is nonlinear, an iterative least-squares procedure is used to fit the combined model to the return waveforms. This retracking algorithm can be used to assess the accuracy of elevations produced by current retracking algorithms, which do not account for subsurface volume scattering. This is extremely important if repeated altimeter elevation measurements are to be used to accurately detect changes in the mass balance of the ice sheets. In addition, by analyzing the distribution of the model parameters over large portions of the ice sheet, quantitative estimates of regional and seasonal variations in the near-surface properties of the ice sheets can be obtained     

Comparison of ice-sheet satellite altimeter retracking algorithms

The NASA and ESA retracking algorithms are compared with an algorithm based upon a combined surface and volume (S/V) scattering model. First, the S/V, NASA, and ESA algorithms were used to retrack over 1.3 million altimeter return waveforms from the Greenland and Antarctic ice sheets. The surface elevations from the S/V algorithm were compared with the elevations produced by the NASA and ESA algorithms to determine the relative accuracy of these algorithms when subsurface volume scattering occurs. The results show that the ESA_25% algorithm produced slightly higher surface elevations than the S/V algorithm. The NASA retracking algorithm produced lower surface elevations than the SN retracking algorithm, with average differences ranging from -0.3 to -0.9 m. The lower NASA elevations can only account for a portion of previously reported differences between altimeter and geoceiver surface elevations, suggesting that the remainder is probably due to orbital differences. Next, by analyzing several thousand satellite crossover points from the Greenland and Antarctic ice sheets, the author estimated the repeatability of the surface elevations derived from the different retracking algorithms. The elevations derived from the ESA_25% and S/V algorithm had the smallest standard deviations for the crossover differences for a time period where no significant change in surface elevation should occur. The NASA standard deviations were approximately 0.2 m larger than those from the ESA_25% and S/V algorithm, which represents an average increase in error of approximately 0.5 m in the datasets. Since previous ice-sheet growth estimates have been based upon the elevations produced by the NASA retracking algorithm, further work needs to be conducted to determine if the ESA_25% or S/V retracking algorithms produce growth estimates that are significantly different from the previous estimates     

Growth of the Greenland ice sheet: a performance assessment ofaltimeter retracking algorithms

The authors compare the performance of different altimeter retracking algorithms for measuring ice sheet elevations and growth rates. The results show that the threshold, ESA, and S/V retracking algorithms produce growth rates that are 30-55% smaller than those produced by the NASA algorithm. Based upon a comparison of crossover-point standard deviations, the analysis indicates that the surface elevation estimates produced by these algorithms are more repeatable than the NASA surface elevations. An analysis of the NASA algorithm shows that a mixing of its 5 and 9 parameter functional fits on the crossover-point altimeter waveforms occurs in over 70% of the crossover data. The mixing of the functional fits is shown to reduce the repeatability of the NASA elevations and this may be responsible for the larger estimates of ice sheet growth produced by the NASA retracking algorithm. The extremely close agreement between the standard deviations and the growth-rate estimates from the threshold, ESA, and S/V retracking algorithms: lead the authors to conclude that 0.10 m/yr is a more accurate estimate of the growth of the Greenland ice sheet from 1978-1987 (south of 72°N)     

Improved elevation-change measurement of the southern Greenland icesheet from satellite radar altimetry

A new analysis of Seasat and Geosat satellite radar altimeter measurements over the Greenland ice sheet was performed to determine surface elevation change. The new analysis includes twice as many measurements and has 50% greater spatial coverage than the authors' previous study. In addition, a precise global ocean reference network created from four years of Topex/Poseidon altimeter data is used to obtain improved estimates of altimeter orbit errors and measurement system biases. The results show that the average elevation change of the southern Greenland ice sheet above 2000 m from 1978 to 1988 is not significantly different than zero. This contradicts earlier and even more recent studies that reported positive ice sheet growth rates and suggested increased precipitation due to a warmer polar climate     

A waveform model for surface and volume scattering from ice andsnow

A method for combining surface and volume scattering effects to model the radar altimeter power waveforms recorded over continental ice and snow, such as that present on Greenland and Antarctica, is discussed. The surface model that is used is a variant of the well-known impulse response based method that is capable of modeling the returns for an altimeter having a general configuration of altitude, antenna beamwidth, transmitted pulsewidth, and pointing angle. The newly formulated volume model is also an impulse response based method that differs from previously introduced versions in that it can also be applied to a general altimeter configuration. The two models are added together assuming completely incoherent scattering. An example of fitting this combined model to altimeter data is given to illustrate the importance of accounting for the volume scattering present in ice/snow data. An optimization method can then be used to achieve a least-squares fit to actual altimeter data, providing an estimate of the parameters for that area on the ice shelf     

Penetration depths inferred from interferometric volume decorrelation observed over the Greenland Ice Sheet

Radar interferometry provides a novel way to study the subsurface of glaciers: interferometric correlation. Since the two complex images that comprise the interferogram have slightly different incidence angles at each point on the ground, a decorrelating phase noise, with statistics related to the scattering medium, is present in the interferogram. The amount of "surface" decorrelation is increased by volume scatter. The larger the vertical extent of the scatterers contributing to the radar echo, the greater the decorrelation will be. By modeling this effect, the authors can estimate radio wave penetration depths within the upper layers of the glacier or ice sheet. Observations of the Greenland Ice Sheet using ERS data yield penetration depths (one-way, 1/e point for power) that range from 12 to 35 m. Due to the contribution of volume scatter, the critical interferometer baseline is decreased, and the authors find for the Greenland data, the baseline must be restricted to be less than 300 m. The authors also compare penetration depths measured within the dry snow zone with those found in the percolation zone and coastal areas. They find that as expected, the rocky coastal areas evidence minimal penetration. Interestingly, the penetration depths that the authors measure in the percolation zone, /spl sim/23 m, indicate a large degree of volume scatter, which is contrary to earlier results that found the scattering in the percolation zone dominated by structures in the first few meters. This discrepancy may be due to unmodeled scattering behavior, or the radar return may indeed include significant contributions from scatterers far beneath the surface.     

Surface roughness characterizations of sea ice and ice sheets: case studies with MISR data

This work is an examination of potential uses of multiangular remote sensing imagery for mapping and characterizing sea ice and ice sheet surfaces based on surface roughness properties. We use data from the Multi-angle Imaging SpectroRadiometer (MISR) to demonstrate that ice sheet and sea ice surfaces have characteristic angular signatures and that these angular signatures may be used in much the same way as spectral signatures are used in multispectral classification. Three case studies are examined: sea ice in the Beaufort Sea off the north coast of Alaska, the Jakobshavn Glacier on the western edge of the Greenland ice sheet, and a region in Antarctica south of McMurdo station containing glaciers and blue-ice areas. The MISR sea ice image appears to delineate different first-year ice types and, to some extent, the transition from first-year to multiyear ice. The MISR image shows good agreement with sea ice types that are evident in concurrent synthetic aperture radar (SAR) imagery and ice analysis charts from the National Ice Center. Over the Jakobshavn Glacier, surface roughness data from airborne laser altimeter transects correlate well with MISR-derived estimates of surface roughness. In Antarctica, ablation-related blue-ice areas, which are difficult to distinguish from bare ice exposed by crevasses, are easily detected using multiangular data.     

Temporal change in the extinction coefficient of snow on theGreenland ice sheet from an analysis of Seasat and Geosat altimeter data

The extinction coefficient of snow k_e along the central portion of the Greenland ice sheet is mapped using data from the Seasat (1978) and Geosat (1985-1989) altimeters. The extinction coefficient is obtained by fitting altimeter waveforms with a surface/volume scattering model. The authors find that in the lower latitudes the Seasat and Geosat extinction coefficients are very nearly the same, while in a specific higher latitude region of the ice sheet the Seasat k_e values exceed the Geosat values by over 100%. By analyzing 18 months of the Geosat data, the author quantified the variability inherent in the extinction coefficient measurements. The results show that the observed temporal variation in the extinction coefficient from 1978 to 1985 is three times larger than the measured variability. This indicates that the average grain size of the near surface snow in this region may have decreased during the time span between the two altimeter datasets. The temporal change in extinction coefficient found in this study demonstrates the important contributions that time-series analysis of satellite datasets can make to the study of the polar ice sheets. In addition, these results have important implications for the study of long-term elevation change over the ice sheets using altimeter data. The author's study demonstrates that significant biases could be introduced into ice-sheet elevation change estimates because of temporal variations in the surface conditions of the ice sheet. Future investigations of ice-sheet mass balance using altimetry data should be aware of this possibility     

A robust threshold retracking algorithm for measuring ice-sheetsurface elevation change from satellite radar altimeters

A threshold retracking algorithm for processing ice-sheet altimeter data is presented. The primary purpose for developing this algorithm is detection of ice-sheet elevation change, where it is critical that a retracking algorithm produce repeatable elevations. The more consistent an algorithm is in selecting the retracking point the less likely that errors and/or biases will be introduced by the retracking scheme in the elevation-change measurement. The author performed extensive comparisons between the threshold algorithm and two widely used ice-sheet retracking algorithms on Geosat datasets comprised of over 60000 crossover points. The results show that the threshold retracking algorithm, with a 10% threshold level, produces ice-sheet surface elevations that are more repeatable than the elevations derived from the other retracking algorithms. For this reason, the threshold retracking algorithm has been adopted by NASA/GSFC as an alternative to their existing algorithm for production of ice sheet altimeter datasets under the NASA Pathfinder Program. The threshold algorithm will be used to re-process existing ice-sheet altimeter datasets and to process the datasets from future altimeter missions     

Focused synthetic aperture radar processing of ice-sounder datacollected over the Greenland ice sheet

The authors developed a synthetic aperture radar (SAR) processing algorithm for airborne/spaceborne ice-sounding radar systems and applied it to data collected in Greenland. By using focused SAR (phase-corrected coherent averaging), they improved along-track resolution by a factor of four and provided a 6-dB processing gain over unfocused SAR (coherent averaging without phase correction) based on a point-target analysis for a Greenland ice-sounding data set. Also, They demonstrated that the focused-SAR processing reduced clutter and enabled them to identify bedrock-interface returns buried in clutter. Using focused-SAR technique, they processed data collected over a key 360-km-long portion of the 2000-m contour line of southwest Greenland. To the best of their knowledge, these are the first high-quality radar ice thickness measurements over this key location. Moreover, these ice-thickness measurements have been used for improving mass-balance estimates of the Greenland ice sheet     

Precision and Accuracy of Satellite Radar and Laser Altimeter Data Over the Continental Ice Sheets

The unprecedented accuracy of elevations retrieved from the Ice Cloud and Land Elevation Satellite (ICESat) laser altimeter is investigated and used to characterize the range errors in the Environmental Satellite (Envisat) and European Remote Sensing 2 Satellite (ERS-2) radar altimeters over the continental ice sheets. Cross-mission crossover analysis between time-coincident ERS-2-, Envisat-, and ICESat-retrieved elevations and comparisons to an ICESat-derived digital elevation map are used to quantify the radar elevation error budget as a function of surface slope and to investigate the effectiveness of a method to account for the radar altimeter slope-induced error. The precision and accuracy of the elevations retrieved from the ICESat Geoscience Laser Altimeter System and the European Space Agency radar altimeters on ERS-2 and Envisat are calculated over the Greenland and Antarctic ice sheets using a crossover analysis. As a result of this work, the laser precision is found to vary as a function of surface slope from 14 to 59 cm, and the radar precision varies from 59 cm to 3.7 m for ERS-2 and from 28 cm to 2.06 m for Envisat. Envisat elevation retrievals when compared with ICESat results over regions with less than 0.1deg surface slopes show a mean difference of 9plusmn5 cm for Greenland and -40plusmn98 cm over Antarctica. ERS-2 elevation retrievals over these same low surface slope regions differ from ICESat results by -56plusmn72 cm over Greenland and 1.12plusmn1.16 m over Antarctica. At higher surface slopes of 0.7deg to 0.8deg, the Envisat/ICESat differences increase to -2.27plusmn23 m over Greenland and to 0.05plusmn26 m over Antarctica     

Use of a new high-speed digital data acquisition system in airborneice-sounding

A high-speed digital data acquisition and signal averaging system for borehole, surface, and airborne radio-frequency geophysical measurements was designed and built by the US Geological Survey. The system permits signal averaging at rates high enough to achieve significant signal-to-noise- enhancement in profiling, even in airborne applications. The first field use of the system took place in Greenland in 1987 for recording data on a 150 by 150-km grid centered on the summit of the Greenland ice sheet. About 6000-line km were flown and recorded using the new system. The data can be used to site a proposed scientific corehole through the ice sheet     

利用GLAS激光测高仪计算格陵兰冰盖高程变化

星载激光测高系统亚毫弧量级的发散角和冰层表面几乎没有穿透效应的优势使其非常适于监测南北极冰盖变化。利用GLAS激光测高卫星的高程数据,通过交叉和重复点方法分析2003~2009年3月格陵兰2 000 m以上区域冰盖高程变化,并改进了交叉点计算方法,使其适合纬度跨度较大的格陵兰地区。经过粗差剔除和时序解算,研究结果表明,该区域7年间冰盖高程年均变化+3.80 cm/年,中误差0.91 cm,呈缓慢增长趋势;交叉点和重复点方法所得结果趋势一致,重复点数量为交叉点数量的4~15倍,但位置分布不均匀,使用星载激光测高数据分析极地冰盖变化时,较大区域适合使用交叉点方法,较小区域适合使用重复点方法。     

基于半解析模型的激光测高回波海水海冰波形分类方法

现有利用激光测高波形的地物分类方法绝大多数基于机器学习的分类原理,是一种基于经验的分类方法。从激光回波的理论模型出发,通过推导纯海水表面回波和含有海冰的表面回波的解析模型,对纯海水回波和含有海冰回波逐个采样点按时域距离加权计算总振幅差异值,以该差异值作为依据建立一种半解析型的海水、海冰分类方法;通过机载LiDAR将在格陵兰北部海冰区的实测点云数据判断GLAS激光脚点对应的地面类型,对GLAS在该区域实测波形进行基于论文方法的分类准确性验证;结果显示,在剔除饱和波形影响后,分类总体精度OA大于95%,Kappa系数接近0.89,具有非常好的分类效果。论文将使得激光测高仪地物类型分类方法由基于机器学习为依据向半理论解析模型为依据的分类方向延伸,为后续基于激光回波数据的地物分类方法提供重要的参考思路。     

基于小光斑复合激光足印的植被属性研究

用机载激光测高仪对单木树冠进行自动提取和轮廓描绘,是获取森林信息的一种快速有效的方法。机载激光测高仪可通过记录回波波形来描绘单树植被冠盖等细节信息,但由于激光足印较小,很难给出树木种群的细节尺度参数,如树的冠盖大小,树径等。提出把若干小激光足印复合成大激光足印的方法,并通过高斯分解算法导出植被冠层高度,植被冠层反射率和中值能量高度等大尺度参数。实验结果表明,通过把固定数量的小光斑激光足印复合成一个大光斑激光足印的方法,能反演出更多的单树外围尺度参数,为植被垂直结构分析提供更多的可靠保证。     

一种利用点目标响应和加权窗 进行海洋雷达高度计重跟踪的新方法

重跟踪是海洋雷达高度计1B级数据处理中的重要内容,依靠重跟踪可进一步精确提取测量信息,在重跟踪中利用雷达系统的点目标响应和加权窗进行数据校正十分重要.本文提出了一种利用点目标响应和加权窗进行雷达高度计重跟踪的新方法,在该方法中利用点目标响应和星上加权窗重新构造了重跟踪时使用的信号模型,从而有效地提高了重跟踪的精度.本文利用该方法对SZ-4高度计的数据进行了处理,并将处理结果分别与同期的ERS-2高度计测量数据和海面浮标数据进行了对比,对比结果显示该方法对参数提取效果有显著改善.     

Satellite Laser Altimeter for Measurement of Ice Sheet Topography

We describe the design and expected performance of a spaceborne laser altimeter based on a nadir-oriented laser transmitter, a 0.5-m-diam receiver telescope, and a high-accuracy satellite attitude measurement system. This instrument is designed for high-resolution mapping of polar ice sheet topography where conventional radar altimeters have difficulty. This instrument could also provide useful data on cloud-top heights and the ocean surface.     

Estimating Snow Accumulation From InSAR Correlation Observations

Snow accumulation in remote regions, such as Greenland and Antarctica, is a key factor for estimating the Earth's ice mass balance. In situ data are sparse; hence, they are useful to derive snow accumulation from remote sensing observations, such as microwave thermal emission and radar brightness. These data are usually interpreted using electromagnetic models in which volume scattering is the dominant mechanism. The main limitation of this approach is that microwave brightness is not well related to backscatter if the ice sheet is layered. Because larger grain size and thicker annual layers both increase radar image brightness, with the first corresponding to lower accumulation rate and the second to higher accumulation rate, models of radar brightness alone cannot accurately reflect accumulation. Consideration of correlation measurements can also resolve this ambiguity. We introduce an interferometric ice scattering model that relates the interferometric synthetic aperture radar correlation and radar brightness to both ice grain size and hoar layer spacing in the dry-snow zone of Greenland. We use this model and the European Remote Sensing satellite radar observations to derive several parameters related to snow accumulation rates in a small area in the dry-snow zone. These parameters show agreement with four in situ core accumulation rate measurements in this area, whereas models using only radar brightness data do not match the observed variation in accumulation rates     

A wideband radar for high-resolution mapping of near-surface internal layers in glacial ice

Snow accumulation rate is an important parameter in determining the mass balance of polar ice sheets. Accumulation rate is currently determined by analyzing ice cores and snow pits. Inadequate sampling of the spatial variations in the ice sheet accumulation has resulted in accumulation rate uncertainties as large as 24%. We designed and developed a 600-900-MHz airborne radar system for high-resolution mapping of the near-surface internal layers for estimating the accumulation rate of polar ice sheets. Our radar system can provide improved spatial and temporal coverage by mapping a continuous profile of the isochronous layers in the ice sheet. During the 2002 field season in Greenland, we successfully mapped the near-surface layers to a depth of 200 m in the dry-snow zone, 120 m in the percolation zone, and 20 m in the melt zone. We determined the water equivalent accumulation rate at the NASA-U/spl I.bar/1 site to be 34.9/spl plusmn/5.1 cm/year from 1964 to 1992. This is in close agreement with the ice-core derived accumulation rate of 34.6 cm/year for the same period.     

机载正下视SAR探测极地冰川的回波数值模拟

建立了大尺度分层粗糙面散射物理模型, 基于Kirchhoff近似、几何光学和射线追踪方法, 推导给出粗糙面散射场的计算公式, 提出了一种快速模拟机载正下视合成孔径雷达(Synthetic Aperture Radar, SAR)探测冰川回波的数值仿真方法, 以帮助分析实测数据.对SAR系统设计、雷达回波信号处理算法研究都有很大帮助, 可以应用到星体地下结构探测回波的模拟.理论公式及数值仿真结果验证了此数值仿真方法的正确性.在不同的仿真场景和雷达系统参数下, 数值仿真模拟了冰川散射回波, 定量分析了冰层表面粗糙度、次表面粗糙度、山体倾斜度、雷达系统参数等对天底区域和非天底区域散射回波的影响.本文的仿真方法可以快速计算任意大尺度特定地形所对应的雷达探测仪回波数据.