Showing papers on "Geodetic datum published in 1990"

GPS satellite surveying

Abstract: Elements of Satellite Surveying The Global Positioning System Adjustment Computations Least-Squares Adjustment Examples Links to Physical Observations The Three-Dimensional Geodetic Model GPS Observables Propagation Media, Multipath, and Phase Center Processing GPS Carrier Phases Network Adjustments Ellipsoidal and Conformal Mapping Models Useful Transformations Datums, Standards, and Specifications Appendices References Abbreviations for Frequently Used References Indexes.

The use of FFT techniques in physical geodesy

Abstract: SUMMARY The fast Fourier transform (FFT) technique is a very powerful tool for the efficient evaluation of gravity field convolution integrals It can handle heterogeneous and noisy data, and thus presents a very attractive alternative to the classical, time consuming approaches, provided gridded data are available This paper reviews the mathematics of the FFT methods as well as their practical problems, and presents examples from physical geodesy where the application of these methods is especially advantageous The spectral evaluation of Stokes’, Vening Meinesz’ and Molodensky’s integrals, least-squares collocation in the frequency domain, integrals for terrain reductions and for airborne gravity gradiometry , and the computation of covariance and power spectral density functions are treated in detail Numerical examples illustrate the efficiency and accuracy of the FFT methods Key words: FFT, physical geodesy, spectral methods 1 INTRODUCTION Physical geodesy is the branch of geodesy which uses measured gradients of the anomalou6gravity potential T to determine a unique and coherent representation of the terrestrial gravity field at the Earth’s surface and in outer space The anomalous potential T is the difference between the actual gravity potential of the Earth and the reference potential of an ellipsoid with the same mass, flattening, and angular rotation rate as the Earth An approximation of T is needed to model geodetic measurements, to predict perturbations of satellite orbits, to determine global ocean circulation patterns, to assist global geophysics, and to support oil and mineral exploration In recent years, the amount of data available for the solution of this problem has increased dramatically, both in quantity and in type This has made the data processing problems more severe and has created a demand for efficient numerical solutions Since much of the data is available in gridded form, the use of fast spectral techniques was clearly appropriate Progress in the application of these methods to geodetic problems has been rapid during the last three years and it is almost certain that, because of their efficiency and accuracy, they will become standard procedures for a number of applications However, it has also become clear that geodetic and, more generally, geophysical data often present specific problems not usually encountered in typical electrical engineering applications The problems are with the heterogeneity of the data, the complicated surface on which they are given, the uneven spatial distribution, and the non-uniformity of the data noise This paper will discuss the use of

Geodetic point positioning with GPS carrier beat phase data from the CASA UNO Experiment

Abstract: The Global Positioning System (GPS) carrier beat phase data collected by the TI4100 GPS receiver has been successfully utilized by the US Defense Mapping Agency in an algorithm which is designed to estimate individual absolute geodetic point positions from data collected over a few hours. The algorithm uses differenced data from one station and two to four GPS satellites at a series of epochs separated by 30 second intervals. The precise GPS ephemerides and satellite clock states, held fixed in the estimation process, are those estimated by the Naval Surface Warfare Center (NSWC). Broadcast ephemerides and clock states are also utilized for comparative purposes. An outline of the data corrections applied, the mathematical model and the estimation algorithm are presented. Point positioning results and statistics are presented for a globally-distributed set of stations which contributed to the CASA Uno experiment. Statistical assessment of 114 GPS point positions at 11 CASA Uno stations indicates that the overall standard deviation of a point position component, estimated from a few hours of data, is 73 centimeters. Solution of the long line geodetic inverse problem using repeated point positions such as these can potentially offer a new tool for those studying geodynamics on amore » global scale.« less

Strategies for Solving the Vertical Datum Problem Using Terrestrial and Satellite Geodetic Data

Abstract: The classical procedure of establishing precise height networks is based upon geodetic levelling, potentially including gravity information along the levelling lines. Since levelling is a relative operation some vertical datum must be fixed in order to determine “absolute” heights of benchmarks. In most cases the vertical datum of a height network has been defined by assigning zero height to the long-term mean value of local sea level observed at a fundamental tide gauge station. The vertical datum of largely extended height networks has often been fixed by employing several tide gauge stations situated along the coastline. In any case the definition of datum of classical vertical networks is connected with the concept of local mean sea level; the equipotential surface of the earth’s gravity field passing through the fundamental tide gauge mark is the reference surface of heights derived from levelling.

A strategy for the analysis of the stability of reference points in deformation surveys

Abstract: Confirmation of the stability of reference points is one of the main problems in deformation analysis. The difficulty lies in the datum defects of monitoring networks. A strategy has been developed...

Geodetic and oceanographic results from the analysis of 1 year of Geosat data

Abstract: The radial error model of Engelis (1987) is extended using the altimeter data obtained by the Geosat Exact Repeat Mission for simultaneous measurements of the stationary sea surface topography and geopotential coefficients. It is shown that the proper utilization of the Geosat altimeter data significantly improved the quality of field modeling at wave numbers by providing, simultaneously, the solution for improved orbits and the oceanographic information on the global scale. A new set of Geosat geophysical data records with the improved orbits was created, which also contains the values of the geoid undulation computed from a recent spherical harmonic expansion to degree 360.

Nonlinear inversion of geodetic and geophysical data : Diagnosing nonlinearity

Abstract: This paper addresses the problem of diagnosing nonlinearity in the nonlinear inversion of geodetic and geophysical data. Measures of nonlinearity are proposed that can be used to assess the amount of nonlinearity in nonlinear models and to test whether a linear (ized) model is a sufficient approximation.

Interannual differences of Geosat altimeter heights and sea level: The importance of a datum

Abstract: Sea surface height data from the Geosat altimeter are compared with island sea level data from 18 gages in the western and central tropical Pacific during December 1986 to November 1987. Care was taken to ensure that the two data sets are referenced to the same mean surface. This was done by requiring that both data sets have a zero mean sea level over the period April 1985 to April 1986. When the annual means are computed at each station in the later time period, it is found that the annual mean sea surface height values have drifted away from the corresponding sea level values by as much as 16 cm. Further, the pattern of differences that develop between the two data sets is not random but is spatially coherent with a strong east-west gradient. These observations illustrate the necessity for sea surface height data to be referred to a well-defined zero point, a datum, in order to reliably monitor interannual changes in the sea surface topography. Until these differences can be eliminated, it will be necessary to use tide gage data as benchmarks for the altimeter sea surface height data.

The North American datum of 1983: Project methodology and execution

Abstract: A new adjustment of the geodetic control networks in North America has been completed, resulting in a new continental datum—the North American Datum of 1983 (NAD 83). The establishment ofNAD 83 was the result of an international project involving the National Geodetic Survey of the United States, the Geodetic Survey of Canada, and the Danish Geodetic Institute (responsible for surveying in Greenland). The geodetic data in Mexico and Central America were collected by the Inter American Geodetic Survey and validated by the Defense Mapping Agency Hydrographic/Topographic Center. The fundamental task ofNAD 83 was a simultaneous least squares adjustment involving 266,436 stations in the United States, Canada, Mexico, and Central America. The networks in Greenland, Hawaii, and the Caribbean islands were connected to the datum through Doppler satellite and Very Long Baseline Interferometry (VLBI) observations. The computations were performed with respect to the ellipsoid of the Geodetic Reference System of 1980. The ellipsoid is positioned in such a way as to be geocentric, and its axes are oriented by the Bureau International de l'Heure Terrestrial System of 1984. The mathematical model for theNAD readjustment was the height-controlled three-dimensional system. The least squares adjustment involved 1,785,772 observations and 928,735 unknowns. The formation and solution of the normal equations were carried out according to the Helmert block method.

Sea surface topography and the geoid

Abstract: and Program.- The Geoid.- Geoid Determination Over Basin-Wide Scales Using a Combination of Satellite Tracking, Surface Gravity and Altimeter Observations.- Geoid Determination Using GPS-Aided Inertial Systems.- The Italian Gravimetric Geoid.- Kernel Modification in Generalized Stokes's Technique for Geoid Determination.- A High Resolution, High Precision Geoid for the British Isles.- A Relative Geoid for the UK.- Towards a Cross-Validated Spherical Spline Geoid for the South-Western Cape, South Africa.- A Gravimetric Geoid in Taiwan Area.- Geoidal Map Improvement in Brazil.- Control and Refinement of the Austrian Geoid Using GPS.- Sea Surface Topography.- Altimetry-Gravimetry Problem: An Example.- Sea Surface Height Determination in the Mediterranean Sea by Local Adjustment of GEOSAT Altimeter Data.- The Mean Sea Level of the Gulf Stream Estimated from Satellite Altimetric and Infrared Data.- The Vertical Datum.- Strategies for Solving the Vertical Datum Problem Using Terrestrial and Satellite Geodetic Data.- Vertical Datum Definition for Oceanographic Studies Using the Global Positioning System.- The Determination of Mean Sea Level Using GPS.- Recent Variations in Mean Sea Level in Southern Africa.- Miscellaneous.- A General Ocean Circulation Model Determined in a Simultaneous Solution with the Earth's Gravity Field.- Christoffel Symbols and the Laplacian in Detailed Studies of the Earth's Gravity Field with Emphasis on Topography and Eccentricity Effects.- Corrections for Atmospheric Refraction in Precise Leveling.- Author Index.

Assessment of observations using minimum norm quadratic unbiased estimation (minque)

Abstract: Estimation of the variances and covariances of observations in geodetic and engineering surveys of high precision is of great importance for proper weighting of the observations in their subsequent...

A practical approach to transformation between commonly used reference systems

Abstract: This article presents some practical solutions to the problem of transformation between local geodetic coordinate sets and also between the local geodetic system and global reference systems. A number of commonly used transformation parameters are given. Particular attention is given to the conversion of ellipsoidal heights to the Australian Height Datum and to the methods of obtaining geoid-ellipsoid separation values.

Global Plate Motion Parameters Derived from Actual Space Geodetic Observations

Abstract: Recent results of time-dependent geodetic SLR and VLBI observations provide us with precise data of global baseline changes and station coordinate shifts. These data are interpreted as plate tectonic motions, and they are used to derive uniform plate rotation vectors. We use four different data sets: VLBI station coordinate shifts, VLBI baseline changes, and SLR baseline changes (all evaluated by NASA GSFC) , as well as SLR coordinate shifts from DGFI/I. The resulting plate rotation vectors of five plates are opposed to current geophysical parameters. The discrepancies are discussed.

Nanosecond GPS Time Transfer Using Precise Geodetic Processing Techniques

Abstract: . Today GPS time transfer is mainly realized with C/A-Code receivers using the common view technique. The method is limited by the gua- lity of the satellite broadcast information, the uncertainities of the receiver antenna position and the resolution of the C/A-code signal. We show that this situation can be improved conside- rably using phase measurements from geodetic GPS receivers, appropriate processing techniques, and the results of a regional or continental orbit determination service.

Estimability of geodetic parameters from space VLBI observables

Abstract: The feasibility of space very long base interferometry (VLBI) observables for geodesy and geodynamics is investigated. A brief review of space VLBI systems from the point of view of potential geodetic application is given. A selected notational convention is used to jointly treat the VLBI observables of different types of baselines within a combined ground/space VLBI network. The basic equations of the space VLBI observables appropriate for convariance analysis are derived and included. The corresponding equations for the ground-to-ground baseline VLBI observables are also given for a comparison. The simplified expression of the mathematical models for both space VLBI observables (time delay and delay rate) include the ground station coordinates, the satellite orbital elements, the earth rotation parameters, the radio source coordinates, and clock parameters. The observation equations with these parameters were examined in order to determine which of them are separable or nonseparable. Singularity problems arising from coordinate system definition and critical configuration are studied. Linear dependencies between partials are analytically derived. The mathematical models for ground-space baseline VLBI observables were tested with simulation data in the frame of some numerical experiments. Singularity due to datum defect is confirmed.

A High Resolution, High Precision Geoid for the British Isles

Abstract: This paper demonstrates how the height of the geoid above the reference ellipsoid can be computed from surface gravity observations with high local precision. The geoid is the equipotential surface, closely approximated by mean sea level, which forms the zero-reference for conventional measurements of topographic height. The Global Positioning System (GPS) can measure the height of a ground-based receiver above the reference ellipsoid with a precision of a few centimetres. Consequently, a combination of GPS measurements with a precise gravimetric geoid can provide a cheap alternative to nationwide geodetic levelling.

Combined Sea Surface Determination Based on Various Geometric and Dynamic Techniques

Abstract: The combined use of Global Positioning System (GPS) differential positioning as well as ERS‐1 altimeter data is considered in implementing geodetic vertical datums and their unification. The article describes concepts, techniques, practical realization, and associated questions and problems. Particular aspects in view of small sea surface perturbations in offshore areas in determining sea surface components (variable and steady state) are discussed. The combinations of tide gauge data with altimetry and (mainly) GPS positioning for geodetic purposes are discussed in detail. Special attention is devoted to the associated reference systems as well as to the combination of dynamic (level and nonlevel surfaces) with geometric quantities. The discussion is based on a specific ERS‐1 project supported by the National Science Foundation. Implications and practical impact of the project are outlined.

Secular upheaval of datum level in relation to tsunamigenic earthquake

Abstract: The relation between annual mean sea level and hazardous tsunamigenic earthquakes in the circum‐Pacific seismic zone is studied. A specific example is secular upheaval of datum level on the coast of the Japanese Islands Arc. At present, it is hard to frequently repeat geodetic surveys for finding the datum level's upheaval, and precision of satellite altimetry is hard to resolve this upheaval. On the other hand, an inversed annual mean sea level could be useful for detecting the upheaval, even though consideration should be given to climatological and oceanographical contamination in the variations of the datum level. The inversed annual mean sea level could be used to predict tsunamigenic earthquakes.

A high-resolution geoid for the United States

Abstract: The National Oceanic and Atmospheric Administrations's National Geodetic Survey is computing a high-resolution geoid height model for the conterminous United States. The model, known as GEOID90, should be available from NGS in December 1990. These geoid heights will be referred to as the Geodetic Reference System of 1980 (GRS 80) and will be appropriate for use with measurements from the Global Positioning System.

Rigorous mathematical models for the densification and integration of geodetic networks

Abstract: Existing position information in a network can be integrated with the densification solution in two ways: One way is to obtain a solution of the densification network followed by a merger of this and all other solutions or vice versa. Alternatively, the existing solutions (not used as weighted constraints) can be taken to be pseudo-observations in a simultaneous adjustment with the “new” observations. In both cases, all existing solutions must first be transformed to the coordinate system of the densified network and be statistically compatible with it. Simultaneous densification and integration is discussed through mathematical adjustment models in which the geometrical strength of networks is underscored. The rationale behind densifying and integrating networks either in two different steps or simultaneously is analyzed. It is concluded that the simultaneous approach should be avoided unless the various solutions turn out to be statistically compatible.

Optimum Positions for GPS-Points and Supporting Fix-Points in Geodetic Networks

Abstract: GPS is actually becoming a standard tool for determining discrete point-configurations within geodetic networks, and the combination of GPS with usual terrestrial measurements is presently coming to be a typical kind of geodetic network. For such GPS—terrestrial as well as for free engineering networks which are to be linked to some fix points e.g. of a superior network, there arises the question of selecting the best point—positions for their fixing with respect to a given discrete number of supporting-points in the sense of an optimality criterion. In this context JAGER and DRIXLER, 1989 show that the optimum positions for fix—points are not receivable and identical, respectively to those points which get the number of maximum weights in the weight—hierarchy of an ‘all—point Third Order Design aproach’. The following recently developped optimization method (JAGER and DRIXLER, 1989) for optimum supporting point—positions is founded in the concept of spectral network—analysis (JAGER, 1988; WIESER, 1988) and spectral optimization of geodetic networks (JAGER, 1988), both referring to spectral criterions and target functions respectively. The decisive spectral target function of a maximum contraction of the eigenvalue—spectrum of the covariance-matrix of network—coordinates provides for the best possible homogenization of the error situation and the best possible stability or strength respectively of the regarding network. The latter criterion is similar to an optimum stiffness of the network in the sense of valuable mechanical analogies (JAGER, 1988).

Doppler geoid in sudan

Abstract: The shape of the geoid in· Sudan is determined by using satellite Doppler observations obtained between 1975 and 1986.Results show that the geoid rises from west to east and from north to south with respect to the local (Adindan) datum based on the Clarke 1880 Spheroid.In both directions the surface of the geoid follows the overall surface of the earth.

Accuracy analysis for the NAD 83 geodetic reference system

Abstract: The North American Datum of 1983 (NAD 83) provides horizontal coordinates for more than 250,000 geodetic stations These coordinates were derived by a least squares adjustment of existing terrestrial and space-based geodetic data For pairs of first order stations with interstation distances between 10km and 100km, therms discrepancy between distances derived fromNAD 83 coordinates and distances derived from independentGPS data may be suitably approximated by the empirical rulee=0008 K07 where e denotes therms discrepancy in meters and K denotes interstation distance in kilometers For the same station pairs, therms discrepancy in azimuth may be approximated by the empirical rule e=0020 K05 Similar formulas characterize therms discrepancies for pairs involving second and third order stations Distance and orientation accuracies, moreover, are well within adopted standards While these expressions indicate that the magnitudes of relative positional accuracies depend on station order, absolute positional accuracies are similar in magnitude for first, second, and third order stations Adjustment residuals reveal a few local problems with theNAD 83 coordinates and with the weights assigned to certain classes of observations

An alternative common view method for time transfer with GPS

Abstract: The standard method of using GPS to compare the times of distant clocks is called the common view technique, by which two or more ground stations simultaneously observe a single GPS space vehicle (SV). In the most widely used form of common view, the positions of the SVs must be known because the signal transit times must be calculated. However, common view is one form of multilateration, by which solutions can be obtained for the positions of both stations and satellites, as well as clock offsets, We show in this paper how to adapt the principles from the classical geodetic literature on multilateration to time transfer. We discuss the need to optimize the geographic distribution of common view stations and the times of observation. A proposal for field tests is also presented, based on numerical simulations.

Recent Variations in Mean Sea Level in Southern Africa

Abstract: The primary reference surface, or vertical datum, for heights is the geoid, which coincides closely with mean sea level. In fact, in many applications the terms are taken as synonymous, although the two may depart from each other by as much as two metres. National or regional vertical datums are usually established by making the implicit assumption that the geoid and mean sea level (MSL) coincide at one or more tide gauge sites, and the tidal records are then used as a basis for heights. As a consequence, any variability in the MSL, temporal or spatial, must be of concern to geodesists and surveyors, as it affects the stability of the vertical datum. Long term trends are also of concern to oceanographers and (taking into account the possible consequences of global warming) also exercise the minds of environmentalists and politicians.

The geoidal heights for nigeria

Abstract: The geoidal heights of a country may be computed from astrogeodetic, gravimetric or doppler satellite data. In this paper, the doppler-derived three-dimensional cartesian coordinates of certain ground stations were combined with heights of the same stations obtained from spirit-levelling in order to determine the geoidal heights of points in Nigeria referred to a geocentric global datum. Transformation equations were applied to obtain the geoidal heights referred to the Minna Datum of the Nigerian geodetic network. The application of this information to the classification and evaluation of the geodetic network has been discussed.

Datum transformation parameters in southern africa

Abstract: A prerequisite for the transformation of co-ordinates between the various southern African datums is a knowledge of the transformation parameters linking them. The necessity for this type of transformation is becoming more common, with the increasing application of satellite positioning techniques to surveying. In this paper we review the relationships between geodetic datums and describe some results obtained for the transformation of southern African co-ordinates to a geocentric coordinate system.