5,509 research outputs found
Roadmap to a mutually consistent set of offshore vertical reference frames
No full textThis thesis presents a combined approach for the realization of the (quasi-)geoid as a height reference surface and the vertical reference surface at sea (chart datum). This approach, specifically designed for shallow seas and coastal waters, provides the relation between the two vertical reference surfaces without gaps down to the coast. It uses a shallow water hydrodynamic model which provides water levels relative to a given (quasi-)geoid. The latter requires that the hydrodynamic model is vertically referenced to the same (quasi-)geoid. Vice versa, the hydrodynamic model is also used to realize a (quasi-)geoid by providing corrections to the dynamic sea surface topography, which are used to reduce radar altimeter-derived sea surface heights to the (quasi-)geoid. The coupled problem of vertically referencing the hydrodynamic model and computing the (quasi-)geoid is solved iteratively. After convergence of the iteration process, the vertically referenced hydrodynamic model is used to realize the chart datum. In this way, consistency between the chart datum and (quasi-)geoid is ensured. The feasibility and performance of this approach is demonstrated for the Dutch North Sea and mainland. It is shown that the differences between modeled and observed instantaneous and mean dynamic sea surface topography is 8-10 cm and 5.8 cm, respectively, for the Dutch North Sea. On land, it is shown that the methodology provides a (quasi-)geoid which has a lower standard deviation than the European Gravimetric Geoid 2008 (EGG08) and the official Netherlands (quasi-)geoid NLGEO2004-grav when compared to GPS-levelling data. The standard deviation at 81 GPS-levelling points is below 1 cm; no correction surface is needed. Finally, it is shown that the chart datum (lowest astronomical tide, LAT) agrees with the observed chart datum at 92 onshore tide gauges to within 21.5 cm standard deviation. This study also examines the impact of instantaneous dynamic sea surface topography (DT) corrections to be applied to altimeter-derived sea surface slopes on the quasi-geoid in the shallow and coastal waters of the North Sea. It is found that the steric and surge parts of the DT mainly contribute to improvements in the signal-to-noise ration at longer wavelengths down to 100-200 km and that the improvements increase towards the southern North Sea. It is also found that the shallow water hydrodynamic model provides better tidal corrections compared to a global ocean tide model, which are most pronounced in the southern North Sea and affect almost the entire spectrum. In terms of quasi-geoid heights, the differences are very small differences (mostly below ±2 cm). This is explained by the fact that altimeter-derived (quasi-)geoid slopes hardly contribute to the estimated quasi-geoid if shipboard gravity data are included. The last question treated in this thesis is whether a spherical Slepian basis representation enables to obtain spectral consistency between a high- and low-resolution data set (following recent studies, this question is treated in the context of mean dynamic topography (MDT) estimation by computing the difference between a high-resolution mean sea level (MSL) model obtained from satellite altimetry and a low-resolution gravimetric geoid). The answer is no; a Slepian representation of the low-resolution MSL signal suffers from broadband leakage. Furthermore, it is shown that a meaningful definition of a low-resolution MSL over incomplete spherical domains involves orthogonal basis functions with additional properties that Slepian functions do not possess. One of these sets of orthogonal basis functions are computed using the Gram-Schmidt orthogonalization for spherical harmonics. For the oceans, an orthogonal basis could be constructed only for resolutions equivalent to a spherical harmonic degree 36. The computation of a basis with a higher resolution failed due to inherent instabilities. More research is needed to solve the instability problem.Geoscience and Remote SensingCivil Engineering and Geoscience
Multiple scatterer retracking and interferometric swath processing of CryoSat-2 data for ice sheet elevation changes
No full textIn the processing of data acquired by conventional radar altimeters, algorithms designed to compute elevation of signal backscatterers assume that the point closest to the satellite antenna is the nadir point. While this is a valid assumption over the oceans, it is often not valid in non-oceanic surfaces. Radar altimetry data acquired over ice sheets, for instance, are dominated by multiple scatterers off-nadir to the satellite, which implies that more than one scattering surfaces in the radar footprint contribute to the received signal. A SAR/Interferometric Radar Altimeter, which is the primary payload of CryoSat, allows the geolocation of an off-nadir scatterer by recording the receiving signal in two distinct receiving antennae separated by a baseline of 1.2 m. In recent studies on ice sheet elevation changes based on CryoSat data, however, only one scatterer is ‘tracked’. Hence, the full potential of the backscattered signal is not exploited. In this study, we design an algorithm that enables elevation estimation and geolocation of multiple off-nadir scatterers within the radar footprint from CryoSat Level 1b waveforms. With the results obtained by our CryoSat level 2 data processor, we obtain elevation change measurements over a part of the Jakobshavn Isbræ (or the Jakobshavn drainage basin) at West-Greenland. Our algorithm provides about twice as much retracked elevations compared to the number of elevations from the CryoSat ESA level 2 products, and hence a better spatial sampling of the elevation change signal.Physical and Space GeodesyGeoscience & Remote SensingCivil Engineering and Geoscience
Towards a combined estimation of Greenland’s ice sheet mass balance using GRACE and ICESat data
No full textThe Greenland ice sheet is sensitive to climate change. Global heating is expected to result in ice mass losses that will contribute to global sea level rise. For this reason monitoring Greenland’s ice mass balance is of utmost importance. Data of both the Ice, Cloud, and land Elevation Satellite (ICESat) laser altimetry mission and the Gravity Recovery and Climate Experiment (GRACE) gravity mission are used to create two independent estimates of Greenland’s ice sheet mass balance over the full measurement period of about 2003 until 2007. For ICESat data, a processing strategy is developed that uses the elevation differences of geometrically overlapping footprints of both crossing and repeated tracks. The dataset is cleaned using quality flags defined by the Geoscience Laser Altimeter System (GLAS) science team. (The GLAS is the sole scientific instrument on ICESat). The cleaned dataset reveals some strong, spatially correlated signals that are shown to be related to physical phenomena like melting glaciers. On the other hand, strong correlation is also visible between the observed elevation differences and the combined effect of roughness and surface slope. Different processing strategies applied to different sets of laser campaigns are used to convert the observed temporal elevation differences to mass changes for 6 different drainage systems, further divided into a region above and below 2000 meter elevation. Here all available laser campaigns are used and outliers are removed using N-sigma thresholding. Both a uniform and non-uniform weighting scheme,used to estimate the elevation changes with respect to a reference epoch, is evaluated. The non-uniform weighting scheme is developed to account for the influence of roughness and surface slope, but it turns out that also signals of interest are sometimes suppressed. In order to obtain our final estimates based on ICESat data, the uniform weighting scheme is applied. For the whole of Greenland the estimated mass change rate is equal to -142.6 Gton/year. This value can be mainly attributed to strong mass losses in the region below 2000 meter elevation. On the other hand we show that for different processing strategies this value ranges between approximately 0 and -200 Gton/year. In general, the obtained results confirm trends discovered by other authors who use altimetry. Differences can be explained by different time spans of the used datasets, but mainly by differences in sampling of the data in the region below 2000 meter. Furthermore, GRACE based monthly variations of the Earth’s gravity field as processed by CNES, CSR, DEOS and GFZ are used to estimate the mass change rate for North and South Greenland. Here, both a Gaussian filter, for half-widths of 300, 500 and 800 km, and a Wiener filter is used. It turns out that the Gaussian filter with a half-width of 500 km has the best performance. The final estimates obtained after application of this filter for the different GRACE solutions range between -60.9 and -93.1 Gton/year. The differences in estimates among different GRACE solutions can be mainly explained by differences in processing strategies used by the processing centers to obtain the monthly gravity fields. Only for the DEOS solutions, these differences can also be attributed to the different time span of this dataset. In any case the estimates are low compared with recently published GRACE estimates, which can be explained by an unaccounted leakage effect in our estimates. The unaccounted leakage effect also mainly explains the differences between estimates based on ICESat and GRACE data. Due to their global coverage and high temporal resolution both the ICESat and GRACE mission have improved the estimations of Greenland’s ice sheet mass balance. Further improvements are possible when both datasets are combined. Hence an attempt is made for a joint inversion of both datasets. Depending on the used GRACE solution the estimated combined mass change rates range between -114.3 and -124.7 Gton/year.GeomaticsDepartment of Earth Observation and Space Systems (DEOS)Aerospace Engineerin
Rites of Spring concert flier, Food For Thought, Washington, D.C. - December 15, 1984
No full textPhotocopy of an advertising flier promoting a concert by the Washington, D.C. punk band, Rites Of Spring. The concert occurred on December 15, 1984 at Food For Thought, a restaurant/concert venue in Washington, D.C. The other bands on the bill were the Washington, D.C. punk bands Gray Matter and Grand Mal. The photocopy was made by D.C. artist, author, and musician Sharon Cheslow as part of the research for "Banned in D.C.," a book she co-authored with Cynthia Connolly and Leslie Clague
Variance–covariance analysis of two high-resolution regional least-squares quasi-geoid models
No full textThis paper investigates the full variance–covariance (VC) matrix of two high-resolution regional quasi-geoid models, utilizing a spherical radial basis function parameterization. Model parameters were estimated using weighted least-squares techniques and variance component estimation (VCE) for data weighting. The first model, known as the “RCR model,” is computed through the remove–compute–restore method, incorporating various local gravity and radar altimeter datasets. The second model, the “combined model,” includes the GOCO05s satellite-only global geopotential model as an additional dataset with a full-noise VC matrix. Validation of the noise VC matrix scaling for each quasi-geoid model is achieved by comparing observed and formal noise standard deviations of differences between geometric and gravimetric height anomalies at GPS height markers in the Netherlands. Analysis of the noise VC matrix of height anomalies at grid nodes reveals significantly smaller formal noise standard deviations for the RCR model compared to the combined model. This difference is attributed to VCE assigning larger weights to the GOCO05s dataset, which exhibits greater noise standard deviations for the specific spatial scales used. Additionally, the formal noise standard deviations of height anomaly differences, relevant for GNSS-heighting, favor the RCR model. However, the disparity between the two models is smaller than implied by the height anomaly noise standard deviations. This is due to the combined model’s noise autocorrelation function displaying a longer correlation length (67 km) in contrast to the RCR model’s (17 km). Consequently, the combined model exhibits a greater reduction in noise variance for height anomaly differences relative to white noise compared to the RCR model.Physical and Space Geodes
Estimating the rates of mass change, ice volume change and snow volume change in Greenland from ICESat and GRACE data
No full textThe focus of this paper is on the quantification of ongoing mass and volume changes over the Greenland ice sheet. For that purpose, we used elevation changes derived from the Ice, Cloud, and land Elevation Satellite (ICESat) laser altimetry mission and monthly variations of the Earth’s gravity field as observed by the Gravity Recovery and Climate Experiment (GRACE) mission. Based on a stand alone processing scheme of ICESat data, the most probable estimate of the mass change rate from 2003 February to 2007 April equals ?139 ± 68 Gton yr?1. Here, we used a density of 600±300 kgm?3 to convert the estimated elevation change rate in the region above 2000m into a mass change rate. For the region below 2000 m, we used a density of 900±300 kgm?3. Based on GRACE gravity models from half 2002 to half 2007 as processed by CNES, CSR, DEOS and GFZ, the estimated mass change rate for the whole of Greenland ranges between ?128 and ?218 Gton yr?1. Most GRACE solutions show much stronger mass losses as obtained with ICESat, which might be related to a local undersampling of the mass loss by ICESat and uncertainties in the used snow/ice densities. To solve the problem of uncertainties in the snow and ice densities, two independent joint inversion concepts are proposed to profit from both GRACE and ICESat observations simultaneously. The first concept, developed to reduce the uncertainty of the mass change rate, estimates this rate in combination with an effective snow/ice density. However, it turns out that the uncertainties are not reduced, which is probably caused by the unrealistic assumption that the effective density is constant in space and time. The second concept is designed to convert GRACE and ICESat data into two totally new products: variations of ice volume and variations of snow volume separately. Such an approach is expected to lead to new insights in ongoing mass change processes over the Greenland ice sheet. Our results show for different GRACE solutions a snow volume change of ?11 to 155 km3 yr?1 and an ice loss with a rate of ?136 to ?292 km3 yr?1.Geoscience & Remote SensingCivil Engineering and Geoscience
Impact of systematic errors in gravity and heights on a quasi-geoid model for the Netherlands and Belgium
No full textIn this study, we quantified systematic errors in surface gravity anomalies, which were caused by systematic errors in gravity and heights of the gravity stations, and computed their impact on the quasi-geoid model of the Netherlands and Belgium. We found that 70% of the gravity datasets have statistically significant biases ranging from −2 mGal to 1.5 mGal. The primary impact of the biases are long-wavelength systematic distortions in the quasi-geoid model with a peak-to-peak amplitude of 8 cm. We also found systematic errors in the height networks of the Netherlands and Belgium, which cause corresponding errors in the heights of the gravity stations. They range from −3.0 cm to 1.7 cm and −12.0 cm to 5.0 cm, respectively. They also introduce errors in the transformation parameters to EVRF2007 of several centimetres. However, the impact of the height errors on the quasi-geoid model is negligible with a peak-to-peak amplitude of less than 0.1 cm.Physical and Space Geodes
How to deal with the high condition number of the noise covariance matrix of gravity field functionals synthesised from a satellite-only global gravity field model?
No full textThe posed question arises for instance in regional gravity field modelling using weighted least-squares techniques if the gravity field functionals are synthesised from the spherical harmonic coefficients of a satellite-only global gravity model (GGM), and are used as one of the noisy datasets. The associated noise covariance matrix, appeared to be extremely ill-conditioned with a singular value spectrum that decayed gradually to zero without any noticeable gap. We analysed three methods to deal with the ill-conditioned noise covariance matrix: Tihonov regularisation of the noise covariance matrix in combination with the standard formula for the weighted least-squares estimator, a formula of the weighted least-squares estimator, which does not involve the inverse noise covariance matrix, and an estimator based on Rao’s unified theory of least-squares. Our analysis was based on a numerical experiment involving a set of height anomalies synthesised from the GGM GOCO05s, which is provided with a full noise covariance matrix. We showed that the three estimators perform similar, provided that the two regularisation parameters each method knows were chosen properly. As standard regularisation parameter choice rules do not apply here, we suggested a new parameter choice rule, and demonstrated its performance. Using this rule, we found that the differences between the three least-squares estimates were within noise. For the standard formulation of the weighted least-squares estimator with regularised noise covariance matrix, this required an exceptionally strong regularisation, much larger than one expected from the condition number of the noise covariance matrix. The preferred method is the inversion-free formulation of the weighted least-squares estimator, because of its simplicity with respect to the choice of the two regularisation parameters.Physical and Space GeodesyNovel Aerospace Material
The RTM harmonic correction revisited
No full textIn this paper, we derive improved expressions for the harmonic correction to gravity and, for the first time, expressions for the harmonic correction to potential and height anomaly. They need to be applied at stations buried inside the masses to transform internal values into harmonically downward continued values, which are then input to local quasi-geoid modelling using least-squares collocation or least-squares techniques in combination with the remove-compute-restore approach. Harmonic corrections to potential and height anomaly were assumed to be negligible so far resulting in yet unknown quasi-geoid model errors. The improved expressions for the harmonic correction to gravity, and the new expressions for the harmonic correction to potential and height anomaly are used to quantify the approximation errors of the commonly used harmonic correction to gravity and to quantify the magnitude of the harmonic correction to potential and height anomaly. This is done for two test areas with different topographic regimes. One comprises parts of Norway and the North Atlantic where the presence of deep, long, and narrow fjords suggest extreme values for the harmonic correction to potential and height anomaly and corresponding large errors of the commonly used approximation of the harmonic correction to gravity. The other one is located in the Auvergne test area with a moderate topography comprising both flat and hilly areas and therefore may be representative for many areas around the world. For both test areas, two RTM surfaces with different smoothness are computed simulating the use of a medium-resolution and an ultra-high-resolution reference gravity field, respectively. We show that the errors of the commonly used harmonic correction to gravity may be as large as the harmonic correction itself and attain peak values in areas of strong topographic variations of about 100 mGal. Moreover, we show that this correction may introduce long-wavelength biases in the computed quasi-geoid model. Furthermore, we show that the harmonic correction to height anomaly can attain values on the order of a decimetre at some points. Overall, however, the harmonic correction to height anomaly needs to be applied only in areas of strong topographic variations. In flat or hilly areas, it is mostly smaller than one centimetre. Finally, we show that the harmonic corrections increase with increasing smoothness of the RTM surface, which suggests to use a RTM surface with a spatial resolution comparable to the finest scales which can be resolved by the data rather than depending on the resolution of the global geopotential model used to reduce the data.Physical and Space Geodes
Water Level Monitoring in the Karnali River, Nepal: Evaluating Satellite SAR Altimetry Techniques through Field Observations
No full textRivers play a crucial role in shaping landscapes and supporting ecosystems. This is demonstrated by the tiger habitats in and around Bardia National Park in West-Nepal, which rely on the Karnali River. This study contributes to a larger effort aimed at sustainably managing these tiger habitats. Monitoring the rivers in this remote area is challenging, suggesting a role for remote sensing. An exploration is presented regarding the potential of satellite synthetic aperture radar altimetry (sat-SARA) for monitoring rivers situated in diverse topographic landscapes. Focusing on the Bheri, Karnali, and Geruwa Rivers, the applicability of sat-SARA techniques for water level monitoring, multiple channel identification, and channel activation detection was evaluated. For deriving water surface heights from sat-SARA data, an empirical Gaussian retracker was used. The findings are promising. While resulting water level variations align with field observations, complementary in-situ measurements are imperative for a comprehensive evaluation. Additionally, the study reveals the potential for identifying multiple channels from sat-SARA return signals, extending to channel classification and detecting channel activation. Leveraging the labour-intensive nature of sat-SARA data processing, the technique holds great promise for monitoring rivers in remote and difficult-to-access landscapes. Therewith, this study contributes to advancing the understanding of the hydrodynamics of the Lower Karnali River and opens doors for sat-SARA applications for river monitoring in challenging terrains.Save the Tigers, Save the Grasslands, Save the Water!Water Managemen
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