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Reduction of ICESat systematic geolocation errors and the impact on ice sheet elevation change detection
Geophysical Research Letters
Abstract
The Ice Cloud and land Elevation Satellite (ICESat) has been acquiring unprecedented observations of our planet's varied surfaces with impressive precision. However, systematic errors remain in the data including orbital variation and long-term bias trend pointing errors. These errors are particularly troublesome because they cannot be separated from true surface elevation change and confound instrument range bias determination. We present a method to correct these systematic pointing errors to the sub-arcsecond level. We demonstrate the impact of these errors and their corrections on Greenland and Antarctica ice sheet surface elevation change observations and sea surface anomaly observations, as well as instrument range bias determination. While further improvements in the ICESat data will certainly be made, the analysis presented here demonstrates a major error source can be significantly reduced, resulting in improved surface elevation accuracies for ice sheet elevation change detection.
... TOOs can be implemented via uploaded instrument command files if not in violation of the operational limit of 5º. These off-nadir instances are important for validation studies as they often magnify errors or biases in the position or pointing determination solutions not normally observed during typical near-nadir pointing [18][19][20]. The success of a TOO is indicative of the observatory pointing control capability and confirms that the instrument command process, and on-board attitude control systems are working appropriately. ...
... TOOs can be implemented via uploaded instrument command files if not in violation of the operational limit of 5 • . These off-nadir instances are important for validation studies as they often magnify errors or biases in the position or pointing determination solutions not normally observed during typical near-nadir pointing [18][19][20]. The success of a TOO is indicative of the observatory pointing control capability and confirms that the instrument command process, and on-board attitude control systems are working appropriately. ...
... The precision orbit determination (total position) for the mission to date is 4.9 cm, also well within the (20 cm) requirement. With time (and orbit cycles) the pointing calibration can be resolved through dedicated on-orbit maneuvers that isolate the range residuals to determine corrections across the full orbital cycle [19]. Table 2 provides the uncertainties associated with each of the aforementioned processes that contribute to the overall ICESat-2 geolocation. ...
The Ice, Cloud and Land Elevation Satellite-2 (ICESat-2), an Earth-observing laser altimetry mission, is currently providing global elevation measurements. Geolocation validation confirms the altimeter’s ability to accurately position the measurement on the surface of the Earth and provides insight into the fidelity of the geolocation determination process. Surfaces well characterized by independent methods are well suited to provide a measure of the ICESat-2 geolocation accuracy through statistical comparison. This study compares airborne lidar data with the ICESat-2 along-track geolocated photon data product to determine the horizontal geolocation accuracy by minimizing the vertical residuals between datasets. At the same location arrays of corner cube retro-reflectors (CCRs) provide unique signal signatures back to the satellite from their known positions to give a deterministic solution of the laser footprint diameter and the geolocation accuracy for those cases where two or more CCRs were illuminated within one ICESat-2 transect. This passive method for diameter recovery and geolocation accuracy assessment is implemented at two locations: White Sands Missile Range (WSMR) in New Mexico and along the 88°S latitude line in Antarctica. This early on-orbit study provides results as a proof of concept for this passive validation technique. For the cases studied the diameter value ranged from 10.6 to 12 m. The variability is attributed to the statistical nature of photon-counting lidar technology and potentially, variations in the atmospheric conditions that impact signal transmission. The geolocation accuracy results from the CCR technique and airborne lidar comparisons are within the mission requirement of 6.5 m.
... This method is simple, and laser-pointing calibration can be achieved only by comparing the laser-derived elevation with the DSM elevation. In addition, satellite attitude maneuvering can be used for calibration [13], [14]. This technique, which is easy to implement and has high reliability, involves rotating the satellite laser conically over the sea surface to perform a Bayesian least square estimation of the sea level ranging residual to calibrate the light detection and ranging (LiDAR) pointing angle. ...
... The GF-7 satellite laser surface footprints on sloping terrain are selected to repeat Steps 1 and 2. The parameters of the fit plane corresponding to each laser footprint, the corrected laser ranging value, satellite orbit, attitude, and other data are incorporated into (11) to establish a system of equations. The GF-7 satellite laser system error θ k is then solved according to (13). ...
The GaoFen-7 (GF-7) satellite, which is used for Earth observations, is equipped with China’s first civil full-waveform laser altimeter. The geometric calibration of the GF-7 satellite laser is vital for ensuring its long-term stability and high precise global measurements. In this study, a geometric of calibrating satellite laser is proposed. Notably, this method does not require outdoor calibration field but instead relies on a mathematical surface model. This calibration method converts that to calibrate laser pointing angle error in traditional calibration method to calibrate the three-axis rotation angle error between laser frame and the satellite body fixed-frame. Assuming that the laser footprints always fall on a fitted spatial plane, a new satellite laser geometry calibration model is established. In addition, the GF-7 satellite laser is calibrated by using airborne LiDAR point clouds collected from the sloped terrain of Xinjiang. Taking the laser footprints that are captured by the laser ground detectors as the true values, the positioning error of GF-7 beam 1 is improved from 430 m before calibration to 1.8 m after calibration. And the beam 2 positioning error is improved from 1025 m to 6.4 m. Finally, according to the surface elevation of the laser footprints that were measured by a real time kinematic, the elevation error of the GF-7 laser on flat terrain was verified to be 0.14 m after calibration. In summary, the calibration method proposed in this study is effective and feasible and provides a new method for satellite laser calibration.
... The scanning maneuver method uses the ranging residuals (the difference between the measured data and the data calculated 2 of 21 based on the ranging model) in spaceborne laser scanning data to estimate the pointing and ranging system bias [13,14]. The ICESat and the ICESat-2 official mission teams have used scanning maneuvers on the ocean surface to achieve sub-arcsecond calibration of system pointing angle errors [15,16]. The ground detector method is a relatively direct calibration method. ...
... In recent years, many researchers improved the DEM's accuracy on the lunar surface using trajectory crossovers between single or multiple laser altimeter systems [25][26][27][28][29]. Many scholars have calibrated and corrected the long-term bias trend and orbit variation in the ICESat system pointing error in polar regions using the crossover adjustment method [15,30]. Restricted by factors such as laser altimeter performance and the detection mode, there is usually no direct measurement value at the traditional crossover location, and it is necessary to interpolate based on several adjacent points to calculate the measurement value at the crossover, which usually introduces interpolation errors [24,26,28,29]. ...
The new-generation photon-counting laser altimeter aboard the Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) has acquired unprecedented high-density laser data on the global surface. The continuous analysis and calibration of potential systematic biases in laser data are important for generating highly accurate data products. Current studies mainly calibrate the absolute systematic bias of laser altimeters based on external reference data. There are few studies that focus on the analysis and calibration of relative systematic biases in long-term laser data. This paper explores a method for systematic biases analysis and calibration of ICESat-2 laser data based on track crossovers for the first time. In the experiment, the simulated data and ICESat-2 data were used to verify the algorithm. The results show that, during the three-year period in orbit, the standard deviation (STD) and bias of the crossover differences of the ICESat-2 terrain data were 0.82 m and −0.03 m, respectively. The simulation validation well demonstrate that the crossover adjustment can calibrate the relative bias between different beams. For ICESat-2 data, the STD of the estimated systematic bias after crossover adjustment was 0.09 m, and the mean absolute error (MAE) was 0.07 m. Compared with airborne lidar data, the bias and root mean square error (RMSE) of the ICESat-2 data remained basically unchanged after adjustment, i.e., −0.04 m and 0.38 m, respectively. This shows that the current ICESat-2 data products possess excellent internal and external accuracy. This study shows the potential of crossover for evaluating and calibrating the accuracy of spaceborne photon-counting laser altimeter data products, in terms of providing a technical approach to generate global/regional high-accuracy point cloud data with consistent accuracy.
... Achieving these objectives requires high-precision measurement of the surface elevation at each laser footprint. GLAS combines its own geocentric position vector-a range vector formed by a 1,064-nm laser, inferred from the round-trip travel time of the laser pulse-and a laser pointing angle determination system, ascertained by Precision attitude determination, to obtain the precision location of each footprint [4,5]. ...
... The correlation of fit served as a good indicator and the fitting accuracy was up to 0.12 pixel (approximately 0.37 arcsec), which was computed from the RMSE of the x-axis and y-axis. Moreover, according to Kepler's third law, the satellite cycle (T) can be defined by Eq. (9). 3 2 T a G M π = (9) where a is the semi-major axis, and the geocentric gravitational constant GM is equal to 5 3 2 ...
Satellite laser altimeter data are used for polar ice sheet elevation mapping, vegetation mapping, etc. Data quality mainly depends on complex relationships among several factors in the path of laser transmission and on illuminated surfaces, including clouds, atmospheric aerosol, satellite pointing, laser energy, topography, footprint size, shape and orientation. The precise pointing of the transmitted laser pulse is critical for improving the horizontal accuracy of the footprint on the ground. Thus, we extracted the centroid of the laser profile array (LPA) image of ICESat/GLAS by 1/e² maximum energy distribution method. The results show that the accuracy of extraction of the LPA’s centroid improved by 0.3 pixel, and the relative positioning accuracy improved by 0.11 pixel. The fast Fourier transform and Fourier series fitting of the LPA centroid has been implemented to detect the periodic change and analyze the model regularity. The results show that the centroid of the LPA undergoes four periodic changes: 1.83 × 10⁻⁴, 3.36 × 10⁻⁴, 5.19 × 10⁻⁴, and 6.71 × 10⁻⁴Hz. The correlation of fit is a good indicator (R²=0.86) and accurate up to 0.4 arcsec (approximately 0.13 pixel). Finally, we extract and estimate the LPA characteristic parameters (eccentricity, orientation, total intensity, and major axis) in different campaigns. We observe that the results obtained by the 1/e²maximum energy distribution are only approximate.
... Data from the Geoscience Laser Altimeter System (GLAS) aboard NASA's ICESat Mission (2003)(2004)(2005)(2006)(2007)(2008)(2009)) brought an improvement in spatial resolution to 173-m along-track spacing of points, determined from a 65-m footprint, with large gaps between tracks (of up to 40 km) [36], [50]. The ICESat Mission facilitated significant cryospheric research [5], [8], [13], [14], [18], [23], [25]- [28], [37], [38], [40], [49], [51], [52]; however, the GLAS data still lack information on 0196-2892 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. ...
... This yields a series of bins I g,t ⊂ R as I g,t = ((t − 1)g, tg] for t = 1, 2, . . . , t n (28) with the first bin I g,1 = (0, g]. ...
Glacial acceleration is a main source of uncertainty in sea-level-change assessment. Measurement of ice-surface heights with a spatial and temporal resolution that not only allows elevation-change calculation, but also captures ice-surface morphology and its changes is required to aid in investigations of the geophysical processes associated with glacial acceleration. The Advanced Topographic Laser Altimeter System aboard NASA's future ICESat-2 Mission (launch 2017) will implement multibeam micropulse photon-counting lidar altimetry aimed at measuring ice-surface heights at 0.7-m along-track spacing. The instrument is designed to resolve spatial and temporal variability of rapidly changing glaciers and ice sheets and the Arctic sea ice. The new technology requires the development of a new mathematical algorithm for the retrieval of height information. We introduce the density-dimension algorithm (DDA) that utilizes the radial basis function to calculate a weighted density as a form of data aggregation in the photon cloud and considers density an additional dimension as an aid in autoadaptive threshold determination. The autoadaptive capability of the algorithm is necessary to separate returns from noise and signal photons under changing environmental conditions. The algorithm is evaluated using data collected with an ICESat-2 simulator instrument, the Slope Imaging Multi-polarization Photon-counting Lidar, over the heavily crevassed Giesecke Br\ae r in Northwestern Greenland in summer 2015. Results demonstrate that ICESat-2 may be expected to provide ice-surface height measurements over crevassed glaciers and other complex ice surfaces. The DDA is generally applicable for the analysis of airborne and spaceborne micropulse photon-counting| lidar data over complex and simple surfaces.
... On-orbit alignment errors and time-varying pointing biases in the ATLAS laser instrument can degrade the horizontal positioning accuracy of ICESat-2 photon returns (Agca et al., 2024;Brunt et al., 2019;Magruder et al., 2021a,b;Wang et al., 2019). While the mission specification allows for up to 6.5 m of horizontal error (Luthcke et al., 2005;Neumann et al., 2019), actual offsets can vary from pass to pass depending on orbit ephemeris solutions, attitude control, calibration drifts, and data product updates. Such horizontal displacement ℎ can induce a vertical elevation error on sloped terrain, approximated by: ...
Monocular height estimation (MHE) from very-high-resolution (VHR) remote sensing imagery via deep learning is notoriously challenging due to the lack of sufficient structural information. Conventional digital elevation models (DEMs), typically derived from airborne LiDAR or multi-view stereo, remain costly and geographically limited. Recently, models trained on synthetic data and refined through domain adaptation have shown remarkable performance in MHE, yet it remains unclear how these models make predictions or how reliable they truly are. In this paper, we investigate a state-of-the-art MHE model trained purely on synthetic data to explore where the model looks when making height predictions. Through systematic analyses, we find that the model relies heavily on shadow cues, a factor that can lead to overestimation or underestimation of heights when shadows deviate from expected norms. Furthermore, the inherent difficulty of evaluating regression tasks with the human eye underscores additional limitations of purely synthetic training. To address these issues, we propose a novel correction pipeline that integrates sparse, imperfect global LiDAR measurements (ICESat-2) with deep-learning outputs to improve local accuracy and achieve spatially consistent corrections. Our method comprises two stages: pre-processing raw ICESat-2 data, followed by a random forest-based approach to densely refine height estimates. Experiments in three representative urban regions -- Saint-Omer, Tokyo, and Sao Paulo -- reveal substantial error reductions, with mean absolute error (MAE) decreased by 22.8\%, 6.9\%, and 4.9\%, respectively. These findings highlight the critical role of shadow awareness in synthetic data-driven models and demonstrate how fusing imperfect real-world LiDAR data can bolster the robustness of MHE, paving the way for more reliable and scalable 3D mapping solutions.
... This method requires the spaceborne laser to perform conical motions by periodically adjusting the satellite's roll and pitch angles over the open ocean. Geometric calibration is then achieved by applying range residual minimization to the periodic ranging data [26][27][28][29]. To meet the ATLAS plane accuracy requirement of 6.5 m, attitude maneuver calibrations were conducted twice weekly for extended periods and twice daily for short durations [13,30]. ...
Geometric calibration, as a crucial method for ensuring the precision of spaceborne single-photon laser point cloud data, has garnered significant attention. Nonetheless, prevailing geometric calibration methods are generally limited by inadequate precision or are unable to accommodate spaceborne lasers equipped with multiple payloads on a single platform. To overcome these limitations, a novel geometric calibration method for spaceborne single-photon lasers that integrates laser detectors with corner cube retroreflectors (CCRs) is introduced in this study. The core concept of this method involves the use of triggered detectors to identify the laser footprint centerline (LFC). The geometric relationships between the triggered CCRs and the LFC are subsequently analyzed, and CCR data are incorporated to determine the coordinates of the nearest laser footprint centroids. These laser footprint centroids are then utilized as ground control points to perform the geometric calibration of the spaceborne single-photon laser. Finally, ATLAS observational data are used to simulate the geometric calibration process with detectors and CCRs, followed by conducting geometric calibration experiments with the gt2l and gt2r beams. The results demonstrate that the accuracy of the calibrated laser pointing angle is approximately 1 arcsec, and the ranging precision is better than 2.1 cm, which verifies the superiority and reliability of the proposed method. Furthermore, deployment strategies for detectors and CCRs are explored to provide feasible implementation plans for practical calibration. Notably, as this method only requires the positioning of laser footprint centroids using ground equipment for calibration, it provides exceptional calibration accuracy and is applicable to single-photon lasers across various satellite platforms.
... These changes directly affect the geometric positioning accuracy of the lasers. Even with precision orbit determination [10] and precision pointing determination [11], alignment errors and time-varying pointing biases have significant impact [12,13]. Laser pulses emitted by satellites introduce ranging errors, including atmospheric delays, as they traverse the atmosphere. ...
The horizontal positioning error in spaceborne photon point clouds seriously constrains their elevation accuracy. To improve data quality for enhanced performance in scientific applications, this study proposes a photon correction method based on terrain feature identification, specifically for the photoncounting spaceborne lidar. Unlike the conventional terrain matching method, this approach accurately determines the horizontal positions of photons within a small-range area by establishing a matching relationship between the laser elevation turning points and the surface boundary lines. The feasibility of this method was verified using the satellite laser altimetry simulation platform, and the horizontal correction accuracy can reach within 0.6 m. Subsequently, the experiments were conducted to verify the geometric positioning accuracy of ICESat-2 across different areas, leveraging high-precision digital surface models. The results indicate that the average horizontal accuracy of ICESat-2 was 3.81 m, and the elevation accuracy was better than 0.5 m
... Additionally, the methods in another category take full advantage of natural terrain features to perform on-orbit geometric calibration of satellite-borne laser altimeters. One of them involves satellite attitude maneuver calibration based on various flat terrains such as sea or flat land to achieve angular calibration (Rowlands, Pavlis et al. 1999, Luthcke, Rowlands et al. 2000, Luthcke, Carabajal and Rowlands 2002, Luthcke, Rowlands et al. 2005. This method relies on satellite attitude maneuvers to measure satellite laser altimeter range by means of ocean scans (OS) or attitude maneuvers across entire orbits ("Round"-The-World Scans, RTWS). ...
Spaceborne laser altimetry represents a novel active remote sensing technology applicable to earth observation, which together with imaging spectroscopy and synthetic aperture radar as a core technology for data acquisition in the earth observation systems. However, the accuracy of horizontal positioning for laser footprints from spaceborne laser altimeters declines due to various factors such as the changes in the orbital environment and the deterioration of performance. Moreover, the limited frequency of in-orbit calibration of the spaceborne laser altimeters and the non-disclosure of calibration parameters mean that users are heavily reliant on positioning accuracy of the altimetry data provided. To address this issue, a new algorithm is proposed in this study for enhancing the accuracy of horizontal positioning for laser footprints in the absence of satellite altimeter pointing and ranging parameters. In this algorithm, high-resolution DSM is taken as the reference terrain data to take advantage of the higher precision in elevation over horizontal positioning of the laser footprints. By adjusting the horizontal position of the laser footprint within a small area, the algorithm achieves the optimal alignment of laser elevation data with the reference terrain. Then, the resulting shift in the horizontal position of the laser footprints is referenced to correct their horizontal positioning during that period. Based on the high-accuracy DSM data collected from the Xinjiang autonomous region in China and the data collected by the GF-7 satellite, simulation experiments are performed in this study to analyze and validate the proposed algorithm. According to the experimental results, the horizontal accuracy of the laser footprints improves significantly from 12.56 m to 3.11 m after optimization by the proposed method. With the elimination of 9.45 m horizontal error, accuracy is improved by 75.23%. This method is demonstrated as effective in further optimizing the horizontal position of laser altimetry data products in the absence of altimeter parameters and original data, which promotes the application of spaceborne laser data.
... As depicted in Tables 1 and 2, artificial ground marker calibration method comprise the Laser Ground Detector (LGD) method (Magruder et al. 2001;Tang et al. 2021), airborne infrared camera imaging calibration method (Magruder et al. 2010), Corner Cube Retroreflector (CCR) calibration method (Magruder et al. 2007). The natural surface calibration method includes satellite attitude maneuver calibration method (Luthcke et al. 2005;Luthcke et al. 2002;Luthcke et al. 2000;Rowlands et al. 1999), slope terrain calibration method , as well as terrain matching calibration method (Tang et al. 2019). The advantages and disadvantages of various calibration methods are described in detail in Tables 1 and 2. Terrain matching calibration method is one of the effective means to achieve highfrequency on-orbit geometry calibration of satellite lasers. ...
The satellite laser geometry calibration method based on terrain matching (terrain matching calibration) has been extensively employed in satellite laser geometry calibration for its simplicity and lack of need for ground probes. In this study, the key factors for the accuracy of the above-mentioned calibration method, i.e. namely the terrain slope and the number of laser points, are examined with the GaoFen-7 (GF-7) satellite as an example. Terrain is classified into six levels in according with the slope classification standards, i.e. Flat (<2°), Micro-slope (2°–5°), Gentle-slope (5°–15°), Moderate-slope (15°–25°), Slope (25°–35°) and Steep-slope (35°–55°). Moreover, a different number of laser points are randomly selected from each the respective terrain slope for calibration. The accuracy of calibration is verified using the true laser pointing obtained based on the ground detector calibration method. As indicated by the experimental results, the terrain matching calibration achieves the optimal experimental conditions when there are over 50 laser points with a terrain slope greater than 15°, or there exist over 20 laser points with a terrain slope greater than 25°. In both cases, the laser pointing accuracy after calibration can exceed 3 arc seconds. This study can provide technical guidance for high-precision terrain matching calibration.
... This technique offers the highest accuracy for laser pointing and ranging calibration, and its results are commonly used as a benchmark to validate alternative calibration methods. Luthcke and associates employed a satellite attitude excitation method for calibrating laser altimeters [13][14][15], which produced commendable results. However, this method is not suitable for satellites equipped with multiple instruments, such as the GaoFen-7 (GF-7), which includes a stereo mapping camera. ...
Satellite laser altimetry technology, a novel space remote sensing technique, actively acquires high-precision elevation information about the Earth’s surface. However, the accuracy of laser altimetry can be compromised by alterations in the satellite-ground environment, thermal dynamics, and cosmic radiation. These factors may induce subtle variations in the installation and internal structure of the spaceborne laser altimeter on the satellite platform, diminishing measurement precision. In-orbit calibration is thus essential to enhancing the precision of laser altimetry. Through collaborative calculations between satellite and ground stations, we can derive correction parameters for laser pointing and ranging, substantially improving the accuracy of satellite laser altimetry. This paper introduces a sophisticated calibration method for laser altimeter pointing and ranging that utilizes dense control points. The approach interpolates discrete ground control point data into continuous simulated terrain using empirical Bayesian kriging, subsequently categorizing the data for either pointing or ranging calibration according to their respective functions. Following this, a series of calibration experiments are conducted, prioritizing “pointing” followed by “ranging” and continuing until the variation in the ranging calibration results falls below a predefined threshold. We employed experimental data from ground control points (GCPs) in Xinjiang and Inner Mongolia, China, to calibrate the GaoFen-7 (GF-7) satellite Beam 2 laser altimeter as per the outlined method. The calibration outcomes were then benchmarked against those gleaned from infrared laser detector calibration, revealing disparities of 1.12 s in the pointing angle and 2 cm in the ranging correction value. Post validation with ground control points, the measurement accuracy was refined to 0.15 m. The experiments confirm that the proposed calibration method offers accuracy comparable to that of infrared laser detector calibration and can facilitate the updating of 1:10,000 topographic maps utilizing stereo optical imagery. Furthermore, this method is more cost-effective and demands fewer personnel for ground control point collection, enhancing resource efficiency compared to traditional infrared laser detector calibration. The proposed approach surpasses terrain-matching limitations when calibrating laser ranging parameters and presents a viable solution for achieving frequent and high-precision in-orbit calibration of laser altimetry satellites.
... This calibration method currently has the highest accuracy. Luthcke et al. achieved geometric calibration of the GLAS profile lasers (Luthcke et al. 2005) and ATLAS single-photon lasers (Luthcke et al. 2020) using a satellite attitude maneuver method. Magruder et al. successfully tagged ATLAS return photon clouds using corner-cube retroreflectors (CCRs) and verified a mean ICESat-2 geolocation measurement accuracy of 3.5 m ± 2.1 m (Magruder et al. 2021). ...
After being launched into orbit, the geometric calibration of a satellite laser altimeter will reduce errors in laser pointing and ranging caused by satellite vibrations during launch, environmental changes, and thermal effects during long-term operation, which guarantees the accuracy of measurement data. In this study, a satellite laser geometric calibration method combining infrared detectors and corner-cube retroreflectors (CCRs) is proposed. First, a CCR-based laser ranging error calibration method was established, and then a laser pointing error calibration model was derived based on a single infrared detector array. Taking GaoFen-7 (GF-7) satellite laser beam 2 as the experimental object, laser geometric calibration was realized using an infrared detector and CCR-measured data. Then, the accuracy of the proposed method was compared with that of other calibration methods, the CMLID and the CMSPR. The results show that the accuracy of the proposed calibration method is equivalent to that of the CMLID and higher than that of the CMSPR. Among them, the accuracy of the laser pointing after calibration using the proposed method is better than 0.8 arcsec, and the elevation accuracy of the laser on flat, sloping, and mountainous terrains is better than 0.11 m, 0.30 m, and 1.80 m, respectively.
... In recent years, polar ice sheet mass balance studies have revealed that the Antarctic and Greenland ice sheets as a whole are in a state of accelerated melting, which has important implications for both global sea level rise and climate change [85,86]. Satellite laser altimetry, represented by ICESat-2, is an important tool for mapping changes in polar ice sheet elevation and thus analyzing the ice sheet material balance [87][88][89][90]. ...
The objective of spatial analysis techniques is to describe the patterns existing in spatial data and to establish, preferably quantitatively, the relationships between different geographic variables. The notion of spatial analysis in a Geographic Information Systems (GIS) environment encompasses the idea of integrating spatial data and alphanumeric attributes and translating it into a series of functions related to selection and data search, on the one hand, and with modeling, on the other. There have been substantial advances in spatial analysis techniques in GIS, mainly in the form of more faithfully apprehending the relationships inherent to the geographic phenomenon, something that was proven impossible to do with non-spatial techniques. Nowadays, spatial analysis involves a set of techniques used to analyze and model variables with distribution in space and/or time. The new era of spatial analysis must also consider the possibilities of integrating artificial intelligence in simulation (geosimulation) processes in computerized environments (geocomputation) in close relationship with models developed in real situations. GIS have emerged as useful tools in geographic modeling processes, helping to answer questions about the time variability of the landscape structure, study the behavior of fire, predict areas of urban expansion, analyze propagation phenomena, model animal movement and behavior, and determine periods and areas of high risk of flooding, among other phenomena. GIS and Spatial Analysis is a critical book that provides different methodologies that combine the potential data (including Big Data) analysis with GIS applications. It gives readers a comprehensive overview of the current state-of-the-art methods of spatial analysis, focusing both on the new philosophical and theoretical foundations for spatial analysis and on a flexible framework for analysis in the real world, for problems such as complexity and uncertainty.
... At present, there are two primary high-precision calibration methods for existing profile satellite lasers: (1) The satellite attitude maneuver calibration method [3][4][5][6], which causes the satellite laser to rotate within a cone over the middle of the ocean. Then, according to the principle of minimum residual error of satellite laser ranging, the geometric calibration of the satellite laser is completed. ...
Satellite laser altimeters have been widely used in the surveying, mapping, forestry, and polar regions and by other industries due to their excellent elevation measurement accuracy. Satellite laser on-orbit geometry calibration is a necessary means to ensure elevation accuracy. This study proposes an iterative geometry calibration method for satellite laser altimeter pointing and ranging separation that does not require the use of field detectors. The DSM data were first used to complete the laser pointing calibration, and then the laser footprint elevation was measured accurately to complete the laser ranging calibration. The iterative calibration experiment was repeated until the convergence condition (i.e., the laser point difference was less than 1× 10-5 degrees and the laser ranging difference was less than 0.01 m) was met, with the calibrated laser pointing angle and ranging separation used as the input parameters. In this work, the GaoFen-7 (GF-7) satellite laser was used as the test object and the actual laser pointing and ranging values derived from ground detector calibrations. The results verified that the pointing accuracy of the GF-7 beam 1 was 2 arcsec and that the ranging accuracy was 2 cm after applying the calibration method presented in this paper. The pointing accuracy of the GF-7 beam 2 was 2.2 arcsec, and the ranging accuracy was approximately 1 cm. This analysis demonstrated that the GF-7 laser mission exceeded its pointing angle requirement of 3 arcsec after laser pointing and ranging separation iterative calibrations were applied. Finally, ground control points were used to verify the calibrated elevation accuracy of the GF-7 satellite laser, and its accuracy on flat terrain was 0.18 m. In summary, it was proven that the satellite laser geometry calibration method proposed in the article is effective.
... Laser altimetry data have long been used for geodetic analyses for Earth (e.g., Luthcke et al. 2000Luthcke et al. , 2002Luthcke et al. , 2003Luthcke et al. , 2005 and planetary applications (e.g., Rowlands et al. 1999;Lemoine et al. 2001;Neumann et al. 2001). In particular, altimetric crossovers are based on the assumption that two spacecraft groundtracks that intersect should essentially measure the same topography, in the absence of changes on the surface. ...
The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission collected a sample from the rubble-pile asteroid (101955) Bennu for return to Earth. For the successful Touch And Go sample acquisition maneuver, the shape and mass of the asteroid needed to be known precisely. Here we use a combination of radiometric, image landmark, and laser altimetry data to determine Bennu’s mass, shape, and orientation simultaneously and to verify existing models thereof. Our shape determination consists of estimating a scale factor and three frame rotation angles that apply to both the global digital terrain model (GDTM) and the landmark coordinates. We use a data type called image constraints, where we take the difference of the observation of the same landmark in images taken at two different times. We analyze data from two phases of the OSIRIS-REx mission, Orbital B and Recon B, and show that interphase image constraints greatly reduce interdependencies between estimated parameters for mass, GDTM scale, and biases on the altimetry data. This results in an improved solution for the mass and shape relative to considering a single mission phase. We find Bennu’s gravitational parameter GM to be 4.89256 ± 0.00035 m ³ s ⁻² , and we find a scale factor of 1.000896 ± 0.00036 for the altimetry-based GDTM. Using the scaled volume, this results in a bulk density of 1191.57 ± 1.74 kg m ⁻³ , which is within the uncertainties of previous analyses but more precise.
... The PPD quality is dependent on the ability to resolve the influence of thermal variations and spacecraft orientation on the pointing efficacy (Bae et al., 2021;Luthcke et al., 2021;Magruder et al., 2021). On-orbit biases (static and dynamic) for instrument pointing, ranging, and timing are determined using a regular sequence of maneuvers over the ocean ("ocean scans") and within a full orbit ("around the world scans") to recover the range residuals that inform the geolocation corrections (Luthcke, et al., 2005;; Thomas et al., 2021). Although the methodology of assessing the on-orbit errors with dedicated satellite maneuvers is well established, independent assessments of geolocation at specific locations validate the results and provide the opportunity to evaluate the data quality at varying length scales. ...
Plain Language Summary
NASA launched its second Earth observing laser altimeter in 2018 with mission objectives of studying the changes in our climate by monitoring global elevations, particularly in the polar regions. Since the mission is focused on generating accurate elevations and elevation change assessments, the geolocation accuracy of the measurements is of upmost importance to each of the scientific disciplines supported by these observations. Geolocation validation is required to ensure that the mission is meeting its science objectives. One validation technique uses small optical reflectors that provide a unique signal back to the satellite. These signal locations within the ICESat‐2 data can be compared to the surveyed positions of the optics to determine the data quality. Results from this technique indicate that the measurements are accurate to within 3.5 m, with a standard deviation of 2.1 m. In addition, the optics can be used to determine the effective laser footprint diameter on the surface when multiple optics are illuminated during a single satellite overpass. For those qualifying overpasses, the diameter is estimated to have an average value of 10.9 m with a standard deviation of 1.2 m. The variation in diameter is thought to be a result of environmental influences on the laser‐energy‐level at the surface, most likely linked to atmospheric conditions.
... CAMS schedules ICAL activities across ideal locations over water and ice and at ideal times for capturing orbital and solar-dependent variations. These scans are critical for calibrating pointing to improve pointing control and geolocation knowledge (Luthcke et al., 2000(Luthcke et al., , 2005. Additional geographic and temporal constraints are autonomously enforced on science and instrument activities. ...
The Ice, Cloud and land Elevation Satellite‐2 was launched on 15 September 2018 with mission goals of collecting surface elevation data that can precisely measure ice sheet topography, cloud and aerosol heights, land topography, as well as vegetation height for ecosystems studies. The Constraint Analysis and Monitoring System (CAMS) was implemented as an ICESat‐2 ground system element to perform Mission Planning and Spacecraft Safety Monitoring. Mission Planning requires CAMS to serve as an interface between the Project Science Office, the Instrument Support Facility and the Mission Operation Control Center. By ingesting inputs received from all three groups, the CAMS builds an optimized and deconflicted timeline of science and instrument activities. Based upon the timeline of activities, the CAMS utilizes a sophisticated set of algorithms to model and predict the location and pointing of the spacecraft instrument relative to the Earth and Sun. From the predicted position and pointing, the CAMS provides precise monitoring of instrument health, and performs space asset laser conjunction detection. Furthermore, the CAMS determines alternative plans to prevent detected health constraint violations or mitigate potential laser conjunctions. This paper provides an overview of the CAMS and presents the operational performance of planning science and instrument activities, the monitoring of instrument constraints for health and safety, and the space asset laser conjunction screening and mitigation process.
... The full angle impact on the attitude determination retrievals for pointing control is not known until calibrations can be estimated throughout a complete solar cycle. Static and time varying biases for instrument pointing, ranging and timing are determined through observatory maneuvers and dynamic crossovers (Luthcke et al., 2005). These maneuvers happen regularly and are performed primarily over the ocean over a partial orbit (ocean scans: OS) or during a complete orbit on a weekly cadence (round-the-world scans: RTW). ...
Plain Language Summary
Space‐based remote sensing provides an unequaled point of view for observing changes on Earth's surface. Collecting precise elevation data from this perspective with modern measurement technology holds promise for a wide range of science disciplines given the coverage over all surface types (e.g., land ice, sea ice, inland water, ocean, and vegetation). Over time this high quality data can not only reveal global elevations but elevation change in those regions with repeat measurements. ICESat‐2 carries a state‐of‐the‐art laser ranging system for accurate elevation measurements and utilizes onboard instrumentation and databases to help control the laser pointing enabling repeat measurements and individual geodetic (latitude and longitude) points of interest. The repeat measurements are critical for looking at elevation change over time, while the collection of elevations over specific locations on the Earth enable studies for science and validation efforts. ICESat‐2 has been on‐orbit for over two years and has collected nearly a trillion measurements. Evaluation of the repeat measurements over time indicate the satellite can point to within 10 m while the ability to collect a measurement of a single position on the surface is within the mission specification of ±45 m.
... The LRS is the custom-built instrument that measures stars and each of the six ATLAS laser beams in a single, co-axial apparatus constructed by rigidly attaching two optical devices with focal planes back-to-back as shown in Figure 1 (Correll, 2016). The concept of the LRS design originated from the PPD experience of its predecessor, ICESat, in which the correction of a star tracker movement against a laser measurement device served as a major calibration task throughout the mission (Bae et al., 2010;Luthcke et al., 2005;Sirota et al, 2005). The design of merging two instruments into one system with a stellar side and laser side was planned to significantly restrict relative motion between the two observations, and meet the stability requirement associated with the uncompensated IAS, in Table 1. ...
Global elevations are critical to understanding the Earth's dynamic processes and changing climate. These measurements are best acquired from a space‐based vantage point and are most accurate using laser altimetry technology. However, the accuracy associated with the elevation retrievals from laser altimetry relies heavily on the ability to precisely determine the pointing angle of the laser beams from the satellite to the illuminated spot on the surface. The Ice, Cloud, and land Elevation Satellite 2 (ICESat‐2) has a system consisting of instruments that support the determination of the laser pointing direction through a process called Precision Pointing Determination (PPD). In this paper, we describe the PPD conceptual implementation, instrument details, data processing approach, calibration/validation techniques, and performance assessment. We show that the PPD has successfully achieved the allocated accuracy goal essential to meeting the ICESat‐2 geolocation mission requirement.
... The nominal accuracy of horizontal co-registration for GEDI observations is between 8 and 10 m underestimation of upper RH metrics at low canopy cover (see Fig. 7d in Hancock et al. 2019). The integration of GEDI data will benefit from improved horizontal positioning accuracy and locally calibrated algorithms (Luthcke et al. 2000(Luthcke et al. , 2005. The processing of ALS data is always oriented to rescale the elevation of many laser echoes to ground level. ...
Background
The NASA’s Global Ecosystem Dynamics Investigation (GEDI) satellite mission aims at scanning forest ecosystems on a multi-temporal short-rotation basis. The GEDI data can validate and update statistics from nationwide airborne laser scanning (ALS). We present a case in the Northwest of Spain using GEDI statistics and nationwide ALS surveys to estimate forest dynamics in three fast-growing forest ecosystems comprising 211,346 ha. The objectives were: i) to analyze the potential of GEDI to detect disturbances, ii) to investigate uncertainty source regarding non-positive height increments from the 2015–2017 ALS data to the 2019 GEDI laser shots and iii) to estimate height growth using polygons from the Forest Map of Spain (FMS). A set of 258 National Forest Inventory plots were used to validate the observed height dynamics.
Results
The spatio-temporal assessment from ALS surveying to GEDI scanning allowed the large-scale detection of harvests. The mean annual height growths were 0.79 (SD = 0.63), 0.60 (SD = 0.42) and 0.94 (SD = 0.75) m for Pinus pinaster, Pinus radiata and Eucalyptus spp., respectively. The median annual values from the ALS-GEDI positive increments were close to NFI-based growth values computed for Pinus pinaster and Pinus radiata, respectively. The effect of edge border, spatial co-registration of GEDI shots and the influence of forest cover in the observed dynamics were important factors to considering when processing ALS data and GEDI shots.
Discussion
The use of GEDI laser data provides valuable insights for forest industry operations especially when accounting for fast changes. However, errors derived from positioning, ground finder and canopy structure can introduce uncertainty to understand the detected growth patterns as documented in this study. The analysis of forest growth using ALS and GEDI would benefit from the generalization of common rules and data processing schemes as the GEDI mission is increasingly being utilized in the forest remote sensing community
... It is worth noting that none of 116 the geolocation models presented here have dealt with the 117 inherent orbit, pointing and timing errors which could sig-118 nificantly degrade the geolocation process. Thus, additional 119 calibrations to eliminate these error types are needed to fur-120 ther improve the geolocation accuracy, e.g., terrain matching, 121 ground-based laser detector approach, cross-over analysis, 122 satellite commanded maneuver method and so on (Row-123 lands et al. 1999;Neumann et al. 2001;Luthcke et al. 2005;124 Magruder et al. 2005). In particular, the accuracy of plan-125 etary laser altimetry is often limited by significantly larger 126 unmodeled orbit, attitude errors and less precise transforma-127 tion from inertial to planet body-fixed coordinates than in the 128 terrestrial case. ...
Laser altimeters are commonly used in planetary research for their high geodetic accuracy. A key procedure in processing of laser altimeter data is the geolocation. In this process, the time-of-flight measurements are converted to coordinates of laser pulse footprints on the surface of the target body. Here, we present a consistent and systematic formulation of three commonly used geolocation models with increasing complexity: static model, spacecraft motion model, pointing aberration model and special relativity model. We show that for small velocities of the spacecraft relative to the target the special relativity model can be reduced to the pointing aberration model without significant loss in the geolocation accuracy. We then discuss the respective accuracies of the proposed models and apply them to time-of-flight measurements from the Mars Orbiter Laser Altimeter (MOLA) onboard the Mars Global Surveyor (MGS) spacecraft and the Mercury Laser Altimeter (MLA) onboard the MErcury Surface, Space ENvironment, GEochemistry and Ranging spacecraft (MESSENGER). While, the archived datasets had not considered the effect of pointing aberration, we demonstrate that a correction due to pointing aberration makes insignificant improvements of 4–5 m laterally and up to ± 3 cm radially for MOLA profiles, these figures enormously increase to up to about 150 m laterally and ± 25 m radially when applied to the MLA orbital profiles.
... All other photons in the same segment are geolocated using the reference photon's geolocation. For details on geolocation, the reader is referred to (Luthcke et al., 2000(Luthcke et al., , 2002(Luthcke et al., , 2005 and the ATBD for Receive-Photon Geolocation ATL03g (Luthcke et al., 2019). ...
NASA’s Ice, Cloud and land Elevation Satellite ICESat-2, launched 15 September 2018, carries the first space-borne multi-beam micro-pulse photon-counting laser altimeter system, the Advanced Topographic Laser Altimeter System (ATLAS). Observations from ATLAS are acquired in three pairs of weak and strong beams with 0.7m nominal along-track spacing (under clear-sky conditions). The recording of the observations as a photon point cloud, which includes signal and background/noise events, requires a dedicated algorithm for identification of signal photons and determination of surface heights.
The objectives of this paper are to demonstrate that measurements from ICESat-2 allow determination of heights over heavily crevassed ice surfaces and yield elevation profiles that present morphological characteristics that are typical of fast-moving and accelerating glaciers. Surface-height determination from the photon point cloud is facilitated by the density-dimension algorithm for ice surfaces, the DDA-ice. The DDA-ice returns surface heights at the 0.7 m sensor resolution for strong and weak beams, it utilizes a radial basis function for data aggregation and automatically adapts to changing environmental conditions and background characteristics, including time of day and apparent surface reflectance. In contrast, the official Land-Ice Along-Track Height Product, ATL06, provides surface heights at 40 m resolution with 20 m postings. The DDA-ice signal classification consistently identifies photons from complex reflectors in both the strong and weak ATLAS beams and hence constitutes a significant advance over the signal classification on the ATL03 Global Geolocated Photons Product.
Results are evaluated using (1) airborne laser altimeter data collected during our ICESat-2 validation campaign over Negribreen, Svalbard, during surge, and (2) high-resolution (0.72m or 0.86m) satellite image data from Planet SkySat acquired over Ilulissat Ice Stream (Jakobshavn Isbræ), Greenland. Using DDA-ice analysis, ICESat-2 data allow discrimination of ice-surface types from surging glaciers (Negribreen) and continuously fast-moving and accelerating glaciers (Jakobshavn Isbræ) based on morphological characteristics.
... The CCR calibration method mainly relies on the waveform to extract the echo energy, and the atmosphere and reflectivity of ground objects will introduce errors (Ma et al., 2018;Magruder et al., 2006). The attitude-maneuver calibration method requires high-agility satellite platform, and will introduce additional high-frequency attitude noise (Luthcke et al., 2000;Luthcke et al., 2005). Terrain matching calibration method relies on high-precision reference DSM, so it is necessary to select special terrain for test (F. ...
The GaoFen-7 (GF-7) satellite is successfully launched on November 3, 2019, and its laser altimeter system is officially and firstly employed as the main payload for earth observations in China, which includes two sets of laser altimeters and laser footprint cameras. The Laser Footprint Image (LFI) is used to capture laser spots on the ground. In order to make up for the shortcomings of high cost field work for the traditional laser altimeter ground detector-based calibration method, this paper proposes a novel laser altimeter calibration method based on LFI. Firstly, the spaceborne laser calibration model and the Laser Footprint Camera (LFC) geolocation model are established. Secondly, the image coordinates of laser spot centroid are extracted from LFI, and the ground location of is obtained by ray intersecting with the reference Digital Surface Model (DSM). Finally, the centroid of laser spot is considered as Ground Control Point (GCP), and the pointing bias of GF-7 laser altimeter is calibrated by the Least Squares Estimation (LSE). The ALOS Global Digital Surface Model “ALOS World 3D-30m” (AW3D30) is used to evaluate the elevation accuracy of GF-7 laser altimeter before and after the calibration. The results indicate that elevation accuracy of the GF-7 laser altimeter is improved significantly after calibration. The proposed method can be effectively applied for high-frequency geometric calibration of GF-7 laser altimeter.
... The post-launch calibration consists of an integrated residual analysis of the returned waveform ranging observations. For GEDI, altimeter range observations from ocean scans and "round"-the-world scans along with dynamic crossovers are used to calibrate and correct the systematic pointing and ranging errors in the form of biases, trends and orbital variation parameters (Luthcke et al., 2000(Luthcke et al., , 2005. The calibration process is expected to reduce the geolocation error down to 8 m horizontal and 10 cm vertical. ...
Obtaining accurate and widespread measurements of the vertical structure of the Earth’s forests has been a long-sought goal for the ecological community. Such observations are critical for accurately assessing the existing biomass of forests, and how changes in this biomass caused by human activities or variations in climate may impact atmospheric CO2 concentrations. Additionally, the three-dimensional structure of forests is a key component of habitat quality and biodiversity at local to regional scales. The Global Ecosystem Dynamics Investigation (GEDI) was launched to the International Space Station in late 2018 to provide high-quality measurements of forest vertical structure in temperate and tropical forests between 51.6° N & S latitude. The GEDI instrument is a geodetic-class laser altimeter/waveform lidar comprised of 3 lasers that produce 8 transects of structural information. Over its two-year nominal lifetime GEDI is anticipated to provide over 10 billion waveforms at a footprint resolution of 25 m. These data will be used to derive a variety of footprint and gridded products, including canopy height, canopy foliar profiles, Leaf Area Index (LAI), sub-canopy topography and biomass. Additionally, data from GEDI are used to demonstrate the efficacy of its measurements for prognostic ecosystem modeling, habit and biodiversity studies, and for fusion using radar and other remote sensing instruments. GEDI science and technology are unique: no other space-based mission has been created that is specifically optimized for retrieving vegetation vertical structure. As such, GEDI promises to advance our understanding of the importance of canopy vertical variations within an ecological paradigm based on structure, composition and function.
... Several factors affect the geolocation of ICESat ground footprints at repeat-track and crossover locations. These include orbital vibrations, orbital drift, random errors in ICESat's laser orientation determination system, and declines in laser transmit power through time that modify the illuminated footprint diameter [164,167,170]. These factors produce both real and apparent deviations between illuminated ground tracks and reference tracks up to a few hundred meters [163,171]. ...
The Greenland Ice Sheet is now the largest land ice contributor to global sea level rise, largely driven by increased surface meltwater runoff from the ablation zone, i.e., areas of the ice sheet where annual mass losses exceed gains. This small but critically important area of the ice sheet has expanded in size by ~50% since the early 1960s, and satellite remote sensing is a powerful tool for monitoring the physical processes that influence its surface mass balance. This review synthesizes key remote sensing methods and scientific findings from satellite remote sensing of the Greenland Ice Sheet ablation zone, covering progress in (1) radar altimetry, (2) laser (lidar) altimetry, (3) gravimetry, (4) multispectral optical imagery, and (5) microwave and thermal imagery. Physical characteristics and quantities examined include surface elevation change, gravimetric mass balance, reflectance, albedo, and mapping of surface melt extent and glaciological facies and zones. The review concludes that future progress will benefit most from methods that combine multi-sensor, multi-wavelength, and cross-platform datasets designed to discriminate the widely varying surface processes in the ablation zone. Specific examples include fusing laser altimetry, radar altimetry, and optical stereophotogrammetry to enhance spatial measurement density, cross-validate surface elevation change, and diagnose radar elevation bias; employing dual-frequency radar, microwave scatterometry, or combining radar and laser altimetry to map seasonal snow depth; fusing optical imagery, radar imagery, and microwave scatterometry to discriminate between snow, liquid water, refrozen meltwater, and bare ice near the equilibrium line altitude; combining optical reflectance with laser altimetry to map supraglacial lake, stream, and crevasse bathymetry; and monitoring the inland migration of snowlines, surface melt extent, and supraglacial hydrologic features.
... Several factors affect geolocation of ICESat ground footprints at repeat-track and crossover locations. These include orbital vibrations, orbital drift, random errors in ICESat's laser orientation determination system, and declines in laser transmit power through time that modify the illuminated footprint diameter [156,159,162]. These factors produce both real and apparent deviations between illuminated ground tracks and reference tracks up to a few hundred meters [155,163]. ...
The Greenland Ice Sheet is now the largest land ice contributor to global sea level rise, largely driven by increased surface meltwater runoff from the ablation zone, i.e. areas of the ice sheet where annual mass losses exceed gains. This small but critically important area of the ice sheet has expanded in size by ~50% since the early 1960s, and satellite remote sensing is a powerful tool for monitoring the physical processes that influence its surface mass balance. This review synthesizes key remote sensing methods and scientific findings from satellite remote sensing of the Greenland Ice Sheet ablation zone, covering progress in 1) radar altimetry, 2) laser (lidar) altimetry, 3) gravimetry, 4) multispectral optical imagery and, 5) microwave and thermal imagery. Physical characteristics and quantities examined include surface elevation change, gravimetric mass balance, reflectance, albedo, and mapping of surface melt extent and glaciological facies and zones. The review concludes that future progress will benefit most from methods that combine multi-sensor, multi-wavelength, and cross-platform datasets designed to discriminate the widely varying surface processes in the ablation zone. Specific examples include fusing laser altimetry, radar altimetry, and optical stereophotogrammetry to enhance spatial measurement density, cross-validate surface elevation change, and diagnose radar elevation bias; fusing optical imagery, radar imagery, and microwave scatterometry to discriminate between snow, liquid water, refrozen meltwater, and bare ice near the equilibrium line altitude; combining optical reflectance with laser altimetry to map supraglacial lake, stream, and crevasse bathymetry; and monitoring the inland migration of snowlines, surface melt extent, and supraglacial hydrologic features.
... Altimeter measurements must meet high standards for accuracy and precision, since even small height changes over the large expanse of the Greenland and Antarctic ice sheets would imply significant changes in global sea level. To meet these standards, altimeters undergo extensive prelaunch and postlaunch efforts to identify (validate) and correct (calibrate) measurement error from various sources [1], [2], [12], [27], [44], [46], [47], [54]. This process is commonly referred to as calibration and validation, or "Cal/Val." ...
The primary goal of NASA's Ice, Cloud, and land Elevation Satellite (ICESat) mission was to detect centimeter-level changes in global ice sheet elevations at the spatial scale of individual ice streams. Confidence in detecting these small signals requires careful validation over time to characterize the uncertainty and stability of measured elevations. A common validation approach compares altimeter elevations to an independently characterized and stable reference surface. Using a digital elevation model (DEM) from geodetic surveys of one such surface, the salar de Uyuni in Bolivia, we show that ICESat elevations at this location have a 0.0-cm bias relative to the WGS84 ellipsoid, 4.0-cm (1-sigma) uncertainty overall, and 1.8-cm uncertainty under ideal conditions over short (50 km) profiles. We observe no elevation bias between ascending and descending orbits, but we do find that elevations measured immediately after transitions from low to high surface albedo may be negatively biased. Previous studies have reported intercampaign biases (ICBs) between various ICESat observation campaigns, but we find no statistically significant ICBs or ICB trends in our data. We do find a previously unreported 3.1-cm bias between ICESat's Laser 2 and Laser 3, and we find even larger interlaser biases in reanalyzed data from other studies. For an altimeter with an exact repeat orbit like ICESat, we also demonstrate that validation results with respect to averaged elevation profiles along a single ground track are comparable to results obtained using reference elevations from an in situ survey.
... There are many calibration methods that were studied in depth for spaceborne laser altimeter, including the ground-based detector method [7], [9], [10], the airborne-based infrared camera method [11], the terrain-matching method [12], [13], and the satellite maneuver method [14]. The ground-based detector method includes laser footprint location prediction, laser triggered detectors laying, footprint centroiding, and parameter estimation, and the calibration accuracy would be high based on the precise footprint center location. ...
The pointing bias of the laser altimeter would change because of the launch vibration, variations in the space environment or other factors, which is one of the most important factors affecting the geometric accuracy of a laser altimeter. To calibrate pointing bias and improve the measuring accuracy of spaceborne laser altimeter, this paper proposed an in-orbit calibration method based on terrain matching with pyramid-search for this spaceborne laser altimeter. First, we used published digital terrain data as a reference to match with stripes of a point cloud obtained by the geolocation model of the laser altimeter, and then the optimal matching terrain could be determined by a pyramid search. Finally, the pointing bias of the laser altimeter was calibrated. The proposed method was applied in calibration experiment of ZY3-02 laser altimeter. Several tracks of ZY3-02 laser data and advanced Land Observation Satellite Digital Surface Model (AW3D30) with a 30-m grid size were deployed for calibration and validation. After calibration, the systematic error was effectively eliminated. The elevation accuracy of the laser altimeter was improved from more than 100 to 3 m approximately, and the algorithm efficiency with pyramid search was improved by 10 times at least. The experimental result demonstrates that the proposed method is an effective means to calibrate the current spaceborne laser altimeters.
... As there are some systematic errors in the GLAS, many researchers have studied calibration methods for it. For example, Luthcke et al. (2000Luthcke et al. ( , 2005 adopted sea level as a reference surface, and obtained GLAS ranging measurements by satellite attitude manoeuvres to correct the pointing error. Magruder et al. (2003Magruder et al. ( , 2010 used a unique signal that was caused by a reflected waveform from corner-cube retroreflectors to calibrate a time synchronisation error. ...
ZY 3‐02 is the first Chinese earth‐observation satellite equipped with a laser altimeter, designed to assist stereo‐image mapping elevations. Launch vibrations, the space environment and other factors, both expected and unexpected, cause bias in the designed ranging and pointing of the laser altimeter, influencing its height measurement accuracy. This paper proposes an in‐orbit geometric calibration model which considers this bias as unknown systematic errors. The unknown parameters were calibrated by the minimum ranging error principle using laser spot geolocation data captured by a ground‐based laser detector array. Based on precise orbit and attitude data, the precise geolocation information of the laser footprints could be obtained. Validation experiments show that the elevation precision of the laser altimeter can reach 1 m.
... Some of the techniques will provide independent assessments of specific data parameters such as range or geolocation. Similar to calibration efforts of ICESat, pointing and range biases will be determined using periodic maneuvers executed by the spacecraft over ocean surfaces [27]. Other techniques will support a direct validation of the ICESat-2 higher level geophysical products through comparison with established ground truth. ...
Two airborne photon-counting laser altimeters have been deployed in direct support of National Aeronautics and Space Adminsitration (NASA's) upcoming Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) mission. Multiple Altimeter Beam Experimental Lidar (MABEL) was developed specifically for ICESat-2 testing and development. MABEL data are used to simulate key aspects of the ICESat-2 measurement strategy and are critical to the development of the algorithms for geophysical data-product generation. Slope Imaging Multi-polarization Photon-counting Lidar (SIMPL) is a NASA Goddard Space Flight Center instrument that has also been deployed in support of ICESat-2 performance discovery. Both instruments are photon-counting, small footprint laser altimeters that sample in both the 532- and 1064-nm wavelengths. And both instruments serve as a proxy for ICESat-2 operational performance and error assessment and a basis for the development of potential validation strategies. This paper provides an overview of how data from MABEL and SIMPL overflights have specifically provided the foundation for understanding the quality of ICESat-2 data and how we can plan to evaluate ICESat-2 products through comparison with other modalities of lidar data and/or ground-truth locations using ground fiducials.
... Up to the writing, the first and the only satellite laser altimeter explicitly targeted at tracking surface elevation dynamics is the Ice, Cloud, and Land Elevation Satellite (ICESat) [14] , carrying the Geoscience Laser Altimeter System (GLAS) onboard. It was launched on January 12, 2003, and was decommissioned in 2009 due to instrument failures [20] . A major goal of ICESat is to measure changes in elevation over glaciers and lands for estimating ice sheet mass balance or predicting canopy change and carbon dynamics [21,22] . ...
Elevation measurements from the Ice, Cloud and Land Elevation Satellite (ICESat) have been applied to monitor dynamics of lakes and other surface water bodies. Despite such potential, the true utility of ICEsat--more generally, satellite laser altimetry--for continuously tracking surface water dynamics over time has not been adequately assessed, especially in the continental or global contexts. This study analyzed elevation derived from ICESat data for the conterminous United States and examined the potential and limitations of satellite laser altimetry in monitoring the water level dynamics. Owing to a lack of spatially-explicit ground-based water-level data, the high-fidelity land elevation data acquired by airborne lidar were firstly resorted to quantify ICESat’s ranging accuracy. Trend and frequency analyses were then performed to evaluate how reliably ICESat could capture water-level dynamics over a range of temporal scales, as compared to in-situ gauge measurements. The analytical results showed that ICESat had a vertical ranging error of 0.16 m at the footprint level—an lower limit on the detectable range of water-level dynamics. The sparsity of data over time was identified as a major factor limiting the use of ICESat for water dynamics studies. Of all the US lakes, only 361 had reliable ICESat measurements for more than two flight passes. Even for those lakes with sufficient temporal coverage, ICESat failed to capture the true interannual water-level dynamics in 32% of the cases. Our frequency analysis suggested that even with a repeat cycle of two months, ICESat could capture only 60% of the variations in water-level dynamics for at most 34% of the US lakes. To capture 60% of the water-level variation for most of the US lakes, a weekly repeat cycle (e.g., less than 5 d) is needed - a requirement difficult to meet in current designs of spaceborne laser altimetry. Overall, the results highlight that current or near-future satellite laser missions, though with high ranging accuracies, are unlikely to fulfill the general needs in remotely monitoring water surface dynamics for lakes or reservoirs. © 2017, Chinese Society of Agricultural Engineering. All rights reserved.
... To eliminate the influence of system errors on the accuracy of the laser altimeter, it is necessary to carry out an on-orbit geometric calibration test. Several methods have been used to perform this test in past space-borne laser altimeter calibrations, and most can acquire reliable results, but on-orbit geometric calibration based on electro-optical ground based detector arrays is presently the best calibration method among them due to its reliability, operability, accuracy, etc., and this geometric calibration method is also one of the most important ways to calibrate the laser altimeter of an Earth observation satellite [10][11][12][13][14][15]. The electro-optical ground based detector array must be laid before the satellite passes the calibration area. ...
Successfully launched on 30 May 2016, ZY3-02 is the first Chinese surveying and mapping satellite equipped with a lightweight laser altimeter. Calibration is necessary before the laser altimeter becomes operational. Laser footprint location prediction is the first step in calibration that is based on ground infrared detectors, and it is difficult because the sample frequency of the ZY3-02 laser altimeter is 2 Hz, and the distance between two adjacent laser footprints is about 3.5 km. In this paper, we build an on-orbit rigorous geometric prediction model referenced to the rigorous geometric model of optical remote sensing satellites. The model includes three kinds of data that must be predicted: pointing angle, orbit parameters, and attitude angles. The proposed method is verified by a ZY3-02 laser altimeter on-orbit geometric calibration test. Five laser footprint prediction experiments are conducted based on the model, and the laser footprint prediction accuracy is better than 150 m on the ground. The effectiveness and accuracy of the on-orbit rigorous geometric prediction model are confirmed by the test results. The geolocation is predicted precisely by the proposed method, and this will give a reference to the geolocation prediction of future land laser detectors in other laser altimeter calibration test.
... Analysis of altimetry data during ocean scan maneuvers will be used to calibrate pointing and separate these errors from ranging errors (Luthcke et al., 2000;Luthcke et al., 2005). Ocean scans are routine calibration activities where the instrument will be pointed off-nadir by ≤5°a nd perform conical scans. ...
The Ice, Cloud, and land Elevation Satellite (ICESat) mission used laser altimetry measurements to determine changes in elevations of glaciers and ice sheets, as well as sea ice thickness distribution. These measurements have provided important information on the response of the cryopshere (Earth's frozen surfaces) to changes in atmosphere and ocean condition. ICESat operated from 2003 to 2009 and provided repeat altimetry measurements not only to the cryosphere scientific community but also to the ocean, terrestrial and atmospheric scientific communities. The conclusive assessment of significant ongoing rapid changes in the Earth's ice cover, in part supported by ICESat observations, has strengthened the need for sustained, high accuracy, repeat observations similar to what was provided by the ICESat mission. Following recommendations from the National Research Council for an ICESat follow-on mission, the ICESat-2 mission is now under development for planned launch in 2018. The primary scientific aims of the ICESat-2 mission are to continue measurements of sea ice freeboard and ice sheet elevation to determine their changes at scales from outlet glaciers to the entire ice sheet, and from 10s of meters to the entire polar oceans for sea ice freeboard. ICESat carried a single beam profiling laser altimeter that produced ~70m diameter footprints on the surface of the Earth at ~150m along-track intervals. In contrast, ICESat-2 will operate with three pairs of beams, each pair separated by about 3km cross-track with a pair spacing of 90m. Each of the beams will have a nominal 17m diameter footprint with an along-track sampling interval of 0.7m. The differences in the ICESat-2 measurement concept are a result of overcoming some limitations associated with the approach used in the ICESat mission. The beam pair configuration of ICESat-2 allows for the determination of local cross-track slope, a significant factor in measuring elevation change for the outlet glaciers surrounding the Greenland and Antarctica coasts. The multiple beam pairs also provide improved spatial coverage. The dense spatial sampling eliminates along-track measurement gaps, and the small footprint diameter is especially useful for sea surface height measurements in the often narrow leads needed for sea ice freeboard and ice thickness retrievals. The ICESat-2 instrumentation concept uses a low energy 532nm (green) laser in conjunction with single-photon sensitive detectors to measure range. Combining ICESat-2 data with altimetry data collected since the start of the ICESat mission in 2003, such as Operation IceBridge and ESA's CryoSat-2, will yield a 15+ year record of changes in ice sheet elevation and sea ice thickness. ICESat-2 will also provide information of mountain glacier and ice cap elevations changes, land and vegetation heights, inland water elevations, sea surface heights, and cloud layering and optical thickness.
... Nonetheless, also those measurements are not free of systematic errors. Pointing errors and orbital variations (Luthcke et al., 2005) or saturation effects (Scambos and Shuman, 2016) may cause laser campaign biases which induce spurious trends of up to 2 cm/yr (Hofton et al., 2013;Gunter et al., 2014). ...
Ice-surface elevation profiles of more than 30 000 km in total length are derived from kinematic GNSS (GPS and the Russian GLONASS) observations on sledge convoy vehicles along traverses between Vostok Station and the East Antarctic coast. These profiles have accuracies between 4 and 9 cm. They are used to validate elevation data sets from both radar and laser satellite altimetry as well as four digital elevation models. A crossover analysis with three different processing versions of Envisat radar altimetry elevation profiles yields a clear preference for the relocation method over the direct method of slope correction and for threshold retrackers over functional fit algorithms. The validation of CryoSat-2 low-resolution mode and SARIn mode data sets documents the progress made from baseline B to C elevation products. ICESat laser altimetry data are demonstrated to be accurate to a few decimetres over a wide range of surface slopes. A crossover adjustment in the region of subglacial Lake Vostok combining ICESat elevation data with our GNSS profiles yields a new set of ICESat laser campaign biases and provides new, independent evidence for the stability of the ice-surface elevation above the lake. The evaluation of the digital elevation models reveals the benefits of combining laser and radar altimetry.
The Ice, Cloud and Land Elevation Satellite (ICESat-2) offers an unprecedented high density of laser data on the Earth’s surface. Evaluation and calibration of geolocation errors are important for the scientific application of laser data. In this study, a crossover method is proposed to evaluate and calibrate errors. This method mainly relies on small-scale reference data and consistency conditions at the trajectory crossovers. By matching the profile measurements of the ascending and descending altimeter tracks to find the location where the crossover discrepancy is minimized, the geolocation errors of the wide-area laser data are estimated. Three years of ICESat-2 measurements over the Ridgecrest (CA19) in the southern United States and McMurdo Dry Valleys (MDV) in eastern Antarctica were used to test the performance of the method. It was shown that the method can be used to estimate the geolocation error of the ICESat-2 laser data, which ranged from 4.65 m to 5.62 m for both sites. After calibration, the mean absolute error of the crossover discrepancies was reduced by approximately 50%. In addition, with the reference digital elevation model (DEM) as the true value, the elevation accuracy (RMSE) of the laser data at the CA19 site improved from 0.38 m before calibration to 0.34 m after calibration, and the improvement at the MDV site was even more significant, about 0.22m. In conclusion, the proposed method is effective and feasible and can improve the consistency of long-period laser data quality in large areas.
The Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2), launched in September 2018, has been widely used in forestry and surveying. A high-accuracy digital elevation model (DEM)/digital surface model (DSM) for terrain matching can effectively evaluate the ICESat-2 design requirements and provide essential data support for further study. The conventional terrain-matching methods regard the laser ground track as a whole, ignoring the individual differences caused by the interaction of photons during flight. Therefore, a novel terrain-matching method using a two-dimensional affine transformation model was proposed to describe the deformation of laser tracks. The least-square optimizes the model parameters with the high-accuracy terrain data to obtain the best matching result. The results in McMurdo Dry Valley (MDV), Antarctica, and Zhengzhou (ZZ), China, demonstrate that the proposed method can verify geolocation accuracy and indicate that the average horizontal accuracy of ICESat-2 V5 data is about 3.86 m in MDV and 4.67 m in ZZ. It shows that ICESat-2 has good positioning accuracy, even in mountainous areas with complex terrain. Additionally, the random forest (RF) model was calculated to analyze the influence of four factors on geographic location accuracy. The slope and signal-to-noise ratio (SNR) are considered the crucial factors affecting the accuracy of ICESat-2 data.
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.
Abstract ICESat‐2 science requirements are dependent on the accurate real‐time pointing control (i.e., geolocation control) and postprocessed geolocation knowledge of the laser altimeter surface returns. Prelaunch pointing alignment errors and postlaunch pointing alignment variation result in large geolocation errors that must be calibrated on orbit. In addition, the changing sun‐orbit geometry causes thermal‐mechanical forced laser frame alignment variations at the orbit period and trends from days, weeks, and months. Early mission analysis computed precise postlaunch laser beam alignment calibration. The alignment calibration was uploaded to the spacecraft and enabled the pointing control performance to achieve 4.4 ± 6.0 m, a significant improvement over the 45 m (1 σ) mission requirement. Laser frame alignment calibrations are used to reduce the alignment bias and time variation, as well as the orbital variation contributions to geolocation knowledge error from 6 to 1.7 m (1 σ). Relative beam alignment of the six beams is calibrated and shown to contribute between 0.5 ± 0.1 m and 2.4 ± 0.2 m of remaining geolocation knowledge error. Independent geolocation assessment based on comparison to high‐resolution digital elevation models agrees well with the calibration geolocation error estimates. The analysis demonstrates the ICESat‐2 mission is performing far better than its geolocation knowledge requirement of 6.5 m (1 σ) after the laser frame alignment bias variation and orbital variation calibrations have been applied. Remaining geolocation error is beam dependent and ranges from 2.5 m for beam 6 to 4.4 m for beam 2 (mean + 1 σ).
The Ice, Cloud, and land Elevation Satellite - 2 (ICESat-2) observatory was launched on 15 September 2018 to measure ice sheet and glacier elevation change, sea ice freeboard, and enable the determination of the heights of Earth's forests. ICESat-2's laser altimeter, the Advanced Topographic Laser Altimeter System (ATLAS) uses green (532 nm) laser light and single-photon sensitive detection to measure time of flight and subsequently surface height along each of its six beams. In this paper, we describe the major components of ATLAS, including the transmitter, the receiver and the components of the timing system. We present the major components of the ICESat-2 observatory, including the Global Positioning System, star trackers and inertial measurement unit. The ICESat-2 Level 1B data product (ATL02) provides the precise photon round-trip time of flight, among other data. The ICESat-2 Level 2A data product (ATL03) combines the photon times of flight with the observatory position and attitude to determine the geodetic location (i.e. the latitude, longitude and height) of the ground bounce point of photons detected by ATLAS. The ATL03 data product is used by higher-level (Level 3A) surface-specific data products to determine glacier and ice sheet height, sea ice freeboard, vegetation canopy height, ocean surface topography, and inland water body height.
The Japan Aerospace Exploration Agency’s (JAXA) Kaguya spacecraft carried a suite of instruments to map the Moon and its environment globally. During its extended mission, the average altitude was 50 km or lower, and Kaguya science products using these data hence have an increased spatial resolution. However, the geodetic position quality of these products is much worse than that of those acquired during the primary mission (at an altitude of 100 km) because of reduced radiometric tracking and frequent thrusting to maintain spacecraft attitude after the loss of momentum wheels. We have analyzed the Kaguya tracking data using gravity models based on the Gravity Recovery and Interior Laboratory (GRAIL) mission, and by making use of a new data type based on laser altimeter data collected by Kaguya: we adjust the spacecraft orbit such that the altimetry tracks fit a precise topographic basemap based on the Lunar Reconnaissance Orbiter’s (LRO) Lunar Orbiter Laser Altimeter (LOLA) data. This results in geodetically accurate orbits tied to the precise LOLA/LRO frame. Whereas previously archived orbits show errors at the level of several kilometers, the inclusion of altimetry greatly improves the orbit precision, to a level of several tens of meters. When altimetry data are not available, the combination of GRAIL gravity and radio tracking results in an orbit precision of around several hundreds of meters for the low-altitude phase of the extended mission. Our greatly improved orbits result in better geolocation of the Kaguya extended mission data set.
Experiments conducted onboard the International Space Station (ISS) that rely on GPS for positioning and timing must take into account potential performance degradation caused by the complex multipath environment around the station. The Advanced GNSS Multipath Model (AGMM) has been developed to simulate the station structure, dynamics, and receiver characteristics necessary for accurate multipath analysis. Results from case studies at the planned location of the Global Ecosystem Dynamics Investigation Lidar (GEDI) mission show code and carrier multipath error magnitudes of at least 1.5 m and 1.4 cm. Model validation was attempted by comparison to a short segment of on‐orbit data gathered from the SIGI receiver. The simulation produced realistic code and carrier multipath for some satellite passes. However, low elevation passes along the truss line of sight were not well matched due to structures on the ISS that were not included in the AGMM ISS CAD model.
Light Detection and Ranging, or lidars, are similar to radars but at optical wavelengths. They have the unique advantages of high temporal and spatial resolution and high measurement efficiency because of the much shorter wavelengths. Lidars have been used to map Mars, Mercury, and the Moon, with unprecedented precision and accuracy. There have been several Earth-orbiting lidar missions which measured the surface elevation, atmosphere backscattering profiles, and vegetation coverage. Several Earth remote-sensing lidars are being developed at present. Here we give a brief description of various types of lidar sensors for remote sensing from space and a summary of the space lidar missions by various space agencies over the years.
The different mapping systems is analyzed in this paper, and as a way of acquiring elevation with high accuracy and effectiveness, laser altimeter can improve the capability of 3- dimensional earth observation of satellite optical remote sensing imagery. And using high accuracy elevation observation of space-borne laser altimeter as control points accords with the trend of satellite photogrammetric without ground control points. In this paper, the problems of satellite photogrammetric are firstly introduced, and then the method of space-borne laser altimeter supported aero triangulation is described, which can solve these problems due to the characteristics of laser altimeter. The bundle adjustment error equations are established according to the earth observation of satellite image and space-borne laser altimeter, as well as the relationship of their exterior orientation elements. Elevation observations supplied by laser altimeter can be used as elevation control points to improve the accuracy of aero triangulation and the positioning accuracy. Finally, the improvement of elevation accuracy is shown by simulation using aero triangulation experiment of satellite image and space-borne laser altimeter, which can be improved from 9m to 1.6m.
Satellite altimetry has become an important tool for studying the Earth's oceans. Here, we summarize the basic concepts of satellite radar altimeters, starting from how range is measured, how the precise orbit height is calculated, and how these are combined to determine sea surface height (SSH). Corrections needed to account for path delays of the radar pulse in the atmosphere and biases at the surface are also discussed. These include the ionosphere, wet troposphere, and dry troposphere atmospheric corrections and the sea state bias and inverted barometer surface corrections. The calibration and verification of the SSH measurement are explained, using specific examples from TOPEX/Poseidon, Jason-1, and Jason-2/Ocean Surface Topography Mission. We also comment on some geophysical applications of the altimeter measurement, including measuring the ocean geoid, bathymetry, and global mean sea level. In closing, we briefly discuss other types of satellite altimeters, including laser, delay-Doppler, and wide swath altimeters.
A precise estimate of the present-day ice mass balance of the Greenland ice shield requires spatio-temporally resolved knowledge on the dynamic regime of its outlet glaciers. We demonstrate for the Jakobshavn Isbra? glacier, how a combination of different geodetic and photogrammetrie techniques can be used to investigate specific phenomena of outlet glaciers like changes of the glacier front position, of the flow velocity and the ice surface height as well as the interactions of the ice with the ocean and the solid Earth. Providing this data at a high precision and reliability, geodesy and photogrammetry deliver a very valuable contribution to Earth system research.
Mass balance of Antarctic ice sheet remains the largest source of uncertainty in estimation of global sea level change. Depending on our developed procedure on the basis of crossover analysis method, the current short-term rates of elevation change in Lambert-Amery system have been evaluated using ICESat/GLAS altimetry data over the period 2003 ∼ 2007 comprising 10 ICESat observation periods. The interannual variability of elevation has been obtained in sub-regions of Lambert-Amery System (LAS), which are defined and calibrated on principle of glacier dynamics, using regression analysis method. We get the new conclusions shown as follows: (1) the elevations are almost increasing in all sub-regions except for AIS sub-region; (2) It is very obvious that there are largely regional variations of elevation in the sub- regions, the largest interannual variability of elevation in AIS and MGLup are -6. 1 cm/a and 6. 6 cm/a respectively; (3)Interannual variation of elevation in western LAS is 2∼3 times that of the eastern part. This is the first time to get the series of elevation change in LAS with high- density and high-precision and its interannual variability in the sub-regions are fully analyzed.
The range between satellite and surface target was acquired by processing the weak received waveform which was transmitted from the space-borne laser altimeter and reflected by earth surface. Combined with the precise orbit and attitude data, the accurate location and elevation of laser footprint were calculated. As for the altimeter with elevation accuracy of 10 cm magnitude, the systematic error on attitude angles influencing the accuracy severely should be calibrated effectively. The analytic model of attitude angle error associated with priori knowledge of earth surface was deduced, and the calibration method used to eliminate the attitude error was designed, which utilized the ocean surface as calibration field, was by way of satellite attitude maneuver and based on least squares estimation algorithm. The results of simulation show that the designed method can estimate the systematic error precisely and effectively, even if the mass observed data were lost, the estimated bias is less than 5%. This on-orbit calibration method is beneficial to the systematic error correction for the space-borne laser altimeter, and is of reference significance.
Altimetry from the Mars Observer Laser Altimeter (MOLA), an instrument on board the Mars Global Surveyor (MGS) spacecraft, has been analyzed for the period of the MGS Science Phasing Orbit-1 (SPO-1) mission phase. Altimeter ranges have been used to improve significantly the orbit and attitude knowledge of the spacecraft by the use of crossover constraint equations derived from short passes of the MOLA data. These constraint equations differ from traditional crossover constraints and exploit the small footprint associated with laser altimetry. The rationale for using this technique with laser altimetry over sloping terrain is laid out and evidence of the resulting benefit is presented.
A mean sea surface, GSFC00, and its derivatives, the altimetry gravity
anomaly and vertical gravity gradient fields, were computed using sea
surface heights of TOPEX, ERS, and Geosat satellite altimeter missions
provided by the NASA Ocean Altimeter Pathfinder Project [Koblinsky et
al., 1998]. A new estimation method, distinct from the GSFC98 mean sea
surface computation [Wang, 2000a], was explored and used to reduce ocean
variability in the mean sea surface heights. The gravity anomaly and
vertical gravity gradient fields were then computed from the mean sea
surface height implied geoid undulations. Validation of the GSFC00 mean
sea surface and gravity anomaly was also performed.
For many science applications of laser altimetry, the precise location of the point on the Earth's surface from which the laser energy reflects is required. The geolocation accuracy of this illuminated laser bounce point is primarily dependent on the accuracy of the laser-pointing knowledge. Laser and spacecraft body systematic misalignments are significant contributors to the pointing knowledge error. We present a method fur accounting for these systematic misalignments through a Bayesian least-squares-estimation process using laser-altimeter range residuals. This range-residual calibration method uses commanded spacecraft attitude maneuvers and ocean range residuals for the recovery of the pointing and range biases. The performance of this method is evaluated through both consider-covariance and simulation studies applied to the upcoming Vegetation Canopy Lidar and the Ice and Cloud Elevation Satellite missions. The results of these analyses suggest are-second level pointing bias knowledge can be achieved. A calibration maneuver is developed for both the Vegetation Canopy Lidar and the Ice and Cloud Elevation Satellite mission. The laser altimeter-range measurement model algorithm is presented.
The accurate geolocation of a laser altimeter’s surface return, the spot from which the laser energy reflects on the Earth’s surface, is a critical issue in the scientific application of these data. Pointing, ranging, timing and orbit errors must be compensated to accurately geolocate these data. Detailed laser altimeter measurement models have been developed and implemented within precision orbit determination software providing the capability to simultaneously estimate the orbit and geolocation parameters from a combined reduction of altimeter range and spacecraft tracking data. In preparation for NASA’s future dedicated Earth observing spaceborne laser altimeter missions, the Vegetation Canopy Lidar (VCL) and the Ice, Cloud and land Elevation Satellite (ICESat), data from two Shuttle Laser Altimeter (SLA) missions have been reprocessed to test and refine these algorithms and to develop the analysis methodologies for the production and verification of enhanced geolocation products. Both direct altimetry and dynamic crossover data have been reduced in combination with navigation tracking data to obtain significant improvement in SLA geolocation accuracy. Residual and overlap precision tests indicate a factor of two improvement over the previously released SLA Standard Data Products, showing 40-m RMS horizontal and 26-cm RMS elevation geolocation precision for the long SLA-01 arcs. Accuracy estimates by comparing SLA profiles to Digital Elevation Models show horizontal positioning accuracy at the 60-m (1σ) level. Vertical accuracies, on the order of 1 m (1σ) for low slope surfaces are now dominated by the ±75-cm one-way range resolution of the instrument. Comparable relative improvements are also observed in the analysis of the SLA-02 data. The analyses show that complex temporal variations in parameters (i.e., pointing) can be recovered and not just simple biases. The methodology and results obtained from the detailed analysis are discussed in this paper, along with their applicability to VCL and ICESat.
Goddard Ocean Tide model GOT99.2 is a new solution for the amplitudes and phases of the global oceanic tides, based on over six years of sea-surface height measurements by the TOPEX/POSEIDON satellite altimeter. Comparison with deep-ocean tide-gauge measurements show that this new tidal solution is an improvement over previous global models, with accuracies for the main semidiurnal lunar constituent M2 now below 1.5 cm (deep water only). The new solution benefits from use of prior hydrodynamic models, several in shallow and inland seas as well as the global finite-element model FES94.1. This report describes some of the data processing details involved in handling the altimetry, and it provides a comprehensive set of global cotidal charts of the resulting solutions. Various derived tidal charts are also provided, including tidal loading deformation charts, tidal gravimetric charts, and tidal current velocity (or transport) charts. Finally, low-degree spherical harmonic coefficients are computed by numerical quadrature and are tabulated for the major short-period tides; these are useful for a variety of geodetic and geophysical purposes, especially in combination with similar estimates from satellite laser ranging.
Science Division, SGT Inc., 7701 Greenbelt Road
- T A Williams
T. A. Williams, Science Division, SGT Inc., 7701 Greenbelt Road, Suite 400, Greenbelt, MD 20770, USA.
Sigma Space Corp, 4801 Forbes Blvd
- M Sirota
M. Sirota, Sigma Space Corp, 4801 Forbes Blvd., Lanham, MD 20706,
USA.
T. A. Williams, Science Division, SGT Inc., 7701 Greenbelt Road, Suite
400, Greenbelt, MD 20770, USA.