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1.
One of the principal problems in separating the non-tidal Newtonian gravitational effects from other forces acting on the ocean surface with a resolution approaching the 10 cm level arises as a consequence ofall measurements of a geodetic nature being taken eitherat orto the ocean surface. The latter could be displaced by as much as ±2 m from the equipotential surface of the Earth’s gravity field corresponding to the mean level of the oceans at the epoch of observation— i.e., the geoid. A secondary problem of no less importance is the likelihood of all datums for geodetic levelling in different parts of the world not coinciding with the geoid as defined above. It is likely that conditions will be favourable for the resolution of this problem in the next decade as part of the activities of NASA’s Earth and Ocean Physics Applications Program (EOPAP). It is planned to launch a series of spacecraft fitted with altimeters for ranging to the ocean surface as part of this program. Possible techniques for overcoming the problems mentioned above are outlined within the framework of a solution of the geodetic boundary value problem to ±5 cm in the height anomaly. The latter is referred to a “higher” reference surface obtained by incorporating the gravity field model used in the orbital analysis with that afforded by the conventional equipotential ellipsoidal model (Mather 1974 b). The input data for the solution outlined are ocean surface heights as estimated from satellite altimetry and gravity anomalies on land and continental shelf areas. The solution calls for a quadratures evaluation in the first instance. The probability of success will be enhanced if care were paid to the elimination of sources of systematic error of long wavelength in both types of data as detailed in (Mather 1973 a; Mather 1974 b) prior to its collection and assembly for quadratures evaluations.  相似文献   

2.
Simultaneous observations of the GEOS-I and II flashing lamps by the NASA MOTS and SPEOPT cameras on the North American Datum (NAD) have been analyzed using geometrical techniques to provide an adjustment of the station coordinates. Two separate adjustments have been obtained. An optical data—only solution has been computed in which the solution scale was provided by the Rosman-Mojave distance obtained from a dynamic station solution. In a second adjustment, scaling was provided by processing simultaneous laser ranging data from Greenbelt and Wallops Island in a combined optical-laser solution. Comparisons of these results with previous GSFC dynamical solutions indicate an rms agreement on the order of 4 meters or better in each coordinate. Comparison with a detailed gravimetric geoid of North America yields agreement of 3 meters or better for mainland U.S. stations and 7 and 3 meters, respectively, for Bermuda and Puerto Rico.  相似文献   

3.
Vertical datum definition is identical with the choice of a potential (or height) value for the fundamental bench mark. Also the connection of two adjacent vertical datums poses no principal problem as long as the potential (or height) value of two bench marks of the two systems is known and they can be connected by levelling. Only the unification of large vertical datums and the connection of vertical datums separated by an ocean remains difficult. Two vertical datums can be connected indirectly by means of a combination of precise geocentric positions of two points, as derived by space techniques, their potential (or height) value in the respective height datum and their geoid height difference. The latter requires the solution of the linear geodetic boundary value problem under the assumption that potential and gravity anomalies refer to a variety of height datums. The unknown off-sets between the various datums appear in the solution inside and outside the Stokes integral and can be estimated in a least squares adjustment, if geocentric positions, levelled heights and adequate gravity material are available for all datum zones. The problem can in principle also be solved involving only two datums, in case a precise global gravity field becomes available purely from satellite methods.  相似文献   

4.
Contribution of new AAM data source to δLOD excitation   总被引：1，自引：0，他引：1  
N. Yu  D. Zheng  H. Wu 《Journal of Geodesy》1999,73(8):385-390
Data sets of the changes of the length of day (ΔLOD) measured by space geodetic techniques and of the atmospheric angular momentum (AAM) derived from global meteorological data by the National Meteorological Center (NMC) and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) are used to reanalyze and study the excitations of ΔLOD, and to examine and compare the contribution of each AAM series to ΔLOD. The main results are as follows. 1. The AAM reanalyzed with the assimilated global meteorological data by NCEP/NCAR are more accurate and have lower noise than the original AAM derived by NMC. The NCEP/NCAR-based AAM is more consistent with the changes of the length-of-day series. 2. The NCEP reanalysed AAM data may better explain the non-tidal LOD variations on intraseasonal to interannual time scales, especially on the quasi-biennial time scale. The atmosphere cannot, however, explain all LOD variations; other excitation sources are possible. 3. The effects of atmosphere on the estimated values of tides for ΔLOD components up to a month are weak. The preliminary results of the annual and semiannual tides can be estimated after removing the effects of atmosphere from ΔLOD. Received: 27 May 1998 / Accepted: 22 March 1999  相似文献   

5.
Center-of-mass coordinates for 57 tracking sites participating in the GEOS-C mission are given in a unified system having a common origin and scale. The positions were obtained either directly from analyses of satellite observations or from survey ties to colocated sites with coordinates previously determined by various investigators from optical, laser of doppler data. The uncertainty of the positions in nearly all cases is 5 m in each coordinate. Data reductions show that station coordinates of this quality introduce a rapidly changing error into the altitude of a satellite unless global tracking constrains the orbit.  相似文献   

6.
R. Lehmann 《Journal of Geodesy》2000,74(3-4):327-334
 The definition and connection of vertical datums in geodetic height networks is a fundamental problem in geodesy. Today, the standard approach to solve it is based on the joint processing of terrestrial and satellite geodetic data. It is generalized to cases where the coverage with terrestrial data may change from region to region, typically across coastlines. The principal difficulty is that such problems, so-called altimetry–gravimetry boundary-value problems (AGPs), do not admit analytical solutions such as Stokes' integral. A numerical solution strategy for the free-datum problem is presented. Analysis of AGPs in spherical and constant radius approximation shows that two of them are mathematically well-posed problems, while the classical AGP-I may be ill posed in special situations. Received: 2 December 1998 / Accepted: 30 November 1999  相似文献   

7.
C. Hwang 《Journal of Geodesy》2001,75(2-3):117-136
 COSMIC is a joint Taiwan–US mission to study the atmosphere using the Global Positioning System (GPS) occultation technique. Improved formulas are developed for the radial, along-track, and cross-track perturbations, which are more accurate than the commonly used order-zero formulas. The formulas are used to simulate gravity recovery using the geodetic GPS data of COSMIC in the operational phase. Results show that the EGM96 model can be improved up to degree 26 using 1 year of COSMIC data. TOPEX/POSEIDON altimeter data are used to derive a temporal gravity variation. COSMIC cannot reproduce this gravity variation perfectly because of data noise and orbital configuration, but the recovered field clearly shows the gravity signature due to mass movement in an El Ni?o. Received: 3 March 2000 / Accepted: 10 November 2000  相似文献   

8.
Summary.  GFZ Potsdam and GRGS Toulouse/Grasse jointly developed a new pair of global models of the Earth's gravity field to satisfy the requirements of the recent and future geodetic and altimeter satellite missions. A precise gravity model is a prerequisite for precise satellite orbit restitution, tracking station positioning and altimeter data reduction. According to different applications envisaged, the new model exists in two parallel versions: the first one being derived exclusively from satellite tracking data acquired on 34 satellites, the second one further incorporating satellite altimeter data over the oceans and terrestrial gravity data. The most recent “satellite-only” gravity model is labelled GRIM4-S4 and the “combined” gravity model GRIM4-C4. The models are solutions in spherical harmonics and have a resolution up to degree and order 60 plus a few resonance terms in the case of GRIM4-S4, and up to degree/order 72 in the case of GRIM4-C4, corresponding to a spatial resolution of 555 km at the Earth's surface. The gravitational coefficients were estimated in a rigorous least squares adjustment simultaneously with ocean tidal terms and tracking station position parameters, so that each gravity model is associated with a consistent ocean tide model and a terrestrial reference frame built up by over 300 optical, laser and Doppler tracking stations. Comprehensive quality tests with external data and models, and test arc computations over a wide range of satellites have demonstrated the state-of-the-art capabilities of both solutions in long-wavelength geoid representation and in precise orbit computation. Received 1 February 1996; Accepted 17 July 1996  相似文献   

9.
Airborne radiometric measurements at frequencies near 92 GHz and 183 GHz were conducted over two precipitation events near Wallops Island, Virginia during February, 1986. The measured brightness temperatures are compared with those from calculations to estimate the snowfall and rainfall rates for both events. The estimated rates over water surface are within a factor of two of those derived from the concurrent measurements by the SPANDAR radar at the NASA Wallops Flight Facility. These estimated rates, however, suggest that both snowfall events are light and close to the threshold of radiometric detection especially over land surface. Observations of additional snowfall events with lower frequency channels are needed to demonstrate the approach and to account for cloud effects.  相似文献   

10.
After reviewing the overall goals of geodesy, the paper focuses on the unique properties of inertial survey systems in the geodetic arsenal: three-dimensionality; ability to determine relative positions and changes in the anomalous components of the earth’s gravity field; and independence of line-of-sight observations and the effects of refraction, both traditional antagonists in geodetic operations. Inertial survey systems, including field and office computational procedures, are briefly reviewed. Their short-comings are pointed out and certain remedies offered. Future possible improvements in hardware and software, as well as the development of hybrid systems (e.g., with gravity gradiometers), are discussed. “Apart from the refinement of existing techniques through the use of computers and the introduction of electromagnetic and optical distance measurement devices, instrumental research and development has been conducted by scientists and engineers outside the geodetic profession. This separateness of geodetic instrument research and development is seen as a deficiency by some, because of the reduced interaction between measurement techniques and the problems to which they apply. However, geodesy does not seem extraordinarily different from other environmentally oriented sciences in this respect and certainly has been quick to adopt new techniques once the benefits become evident.” (NAS 1978, p. 6) From the Keynote Address presented at Second International Symposium on Inertial Technology for Surveying and Geodesy, June 1–5, 1981, Banff, Alberta, Canada.  相似文献   

11.
为解决世界各国高程基准差异的问题,提出联合卫星重力场模型、地面重力数据、GNSS大地高、局部高程基准的正高或正常高,按大地边值问题法确定局部高程基准重力位差的方法。首先推导了利用传统地面"有偏"重力异常确定高程基准重力位差的方法;接着利用改化Stokes核函数削弱"有偏"重力异常的影响,并联合卫星重力场模型和地面"有偏"重力数据,得到独立于任何局部高程基准的重力水准面,以此来确定局部高程基准重力位差;最后利用GNSS+水准数据和重力大地水准面确定了美国高程基准与全球高程基准W0的重力位差为-4.82±0.05 m2s-2。  相似文献   

12.
A note on frame transformations with applications to geodetic datums   总被引：1，自引：0，他引：1  
Rigorous equations in compact symbolic matrix notation are introduced to transform coordinates and velocities between ITRF frames and modern GPS-based geocentric geodetic datums. The theory is general but, after neglecting higher than second-order terms, it is shown that the equations revert to the formulation currently applied in most major continental datums. We discuss several examples: the North American Datum of 1983 (NAD83), the European Terrestrial Reference System of 1989 (ETRS89), the Geodetic Datum of Australia of 1994 (GDA94), and the South American Geocentric Reference System (SIRGAS). Electronic Publication  相似文献   

13.
The direct method of vertical datum unification requires estimates of the ocean’s mean dynamic topography (MDT) at tide gauges, which can be sourced from either geodetic or oceanographic approaches. To assess the suitability of different types of MDT for this purpose, we evaluate 13 physics-based numerical ocean models and six MDTs computed from observed geodetic and/or ocean data at 32 tide gauges around the Australian coast. We focus on the viability of numerical ocean models for vertical datum unification, classifying the 13 ocean models used as either independent (do not contain assimilated geodetic data) or non-independent (do contain assimilated geodetic data). We find that the independent and non-independent ocean models deliver similar results. Maximum differences among ocean models and geodetic MDTs reach >150 mm at several Australian tide gauges and are considered anomalous at the 99% confidence level. These differences appear to be of geodetic origin, but without additional independent information, or formal error estimates for each model, some of these errors remain inseparable. Our results imply that some ocean models have standard deviations of differences with other MDTs (using geodetic and/or ocean observations) at Australian tide gauges, and with levelling between some Australian tide gauges, of \({\sim }\pm 50\,\hbox {mm}\). This indicates that they should be considered as an alternative to geodetic MDTs for the direct unification of vertical datums. They can also be used as diagnostics for errors in geodetic MDT in coastal zones, but the inseparability problem remains, where the error cannot be discriminated between the geoid model or altimeter-derived mean sea surface.  相似文献   

14.
This paper presents the contemporary motion and active deformation of South America plate and relative motion of Nazca-South America plate using space geodetic data. The South America plate is moving at average 14.5 mm/a with an azimuth of 15.2° and shrinking in the west-east at 10.9 mm/a. The geodetic deformations of sites with respect to the South America plate are in quite good agreement with the estimated deformations from NNR-NUVEL1A, but the deformation of the western South America regions is very large.  相似文献   

15.
The Spaceborne Laser Ranging System is a proposed short pulse laser on board an orbiting spacecraft.1,2,3,4 It measures the distance between the spacecraft and many laser retroreflectors (targets) deployed on the Earth’s surface. The precision of these range measurements is assumed to be about ±2 cm (M. W. Fitzmaurice, private communication). These measurements are then used together with the orbital dynamics of the spacecraft, to derive the relative position of the laser ground targets. Assuming a six day observing period with 50% cloud cover, uncertainties in the baseline for target separations of 50 km to 1200 km were estimated to be on the order of 1 to 3 cm and the corresponding values in the vertical direction, ranged from 1 cm to 12 cm. By redetermining the measurements of the relative target positions, the estimated precision in the baseline for a target separation of 50 km is less than 0.3 cm and for a separation of 1200 km is less than 1 cm. In the vertical direction, the estimated precision ranged from 0.4 cm to 4.0 cm respectively. As a result of the repeated estimation of the relative laser target positions, most of the non-temporal effects of error sources as exemplified by the errors in geopotential are reduced. The Spaceborne Laser Ranging System’s capability of determining baselines to a high degree of precision provides a measure of strain and strain rate as shown byCohen, 1979. These quantities are essential for crustal dynamic studies which include determination and monitoring of strain near seismic zones, land subsidence, and edifice building preceding volcanic eruptions. It is evident that such a system can also be used for geodetic surveys where such precisions are more than adquate.  相似文献   

16.
Within the International VLBI Service for Geodesy and Astrometry (IVS) Monte Carlo simulations have been carried out to design the next generation VLBI system (“VLBI2010”). Simulated VLBI observables were generated taking into account the three most important stochastic error sources in VLBI, i.e. wet troposphere delay, station clock, and measurement error. Based on realistic physical properties of the troposphere and clocks we ran simulations to investigate the influence of the troposphere on VLBI analyses, and to gain information about the role of clock performance and measurement errors of the receiving system in the process of reaching VLBI2010’s goal of mm position accuracy on a global scale. Our simulations confirm that the wet troposphere delay is the most important of these three error sources. We did not observe significant improvement of geodetic parameters if the clocks were simulated with an Allan standard deviation better than 1 × 10−14 at 50 min and found the impact of measurement errors to be relatively small compared with the impact of the troposphere. Along with simulations to test different network sizes, scheduling strategies, and antenna slew rates these studies were used as a basis for the definition and specification of VLBI2010 antennas and recording system and might also be an example for other space geodetic techniques.  相似文献   

17.
New Zealand uses 13 separate local vertical datums (LVDs) based on geodetic levelling from 12 different tide-gauges. We describe their unification using a regional gravimetric quasigeoid model and GPS-levelling data on each LVD. A novel application of iterative quasigeoid computation is used, where the LVD offsets computed from earlier models are used to apply additional gravity reductions from each LVD to that model. The solution converges after only three iterations yielding LVD offsets ranging from 0.24 to 0.58 m with an average standard deviation of ±0.08 m. The so-computed LVD offsets agree, within expected data errors, with geodetically levelled height differences at common benchmarks between adjacent LVDs. This shows that iterated quasigeoid models have a role in vertical datum unification.  相似文献   

18.
The term “entity” covers, when used in the field of electronic data processing, the meaning of words like “thing”, “being”, “event”, or “concept”. Each entity is characterized by a set of properties. An information element is a triple consisting of an entity, a property and the value of a property. Geodetic information is sets of information elements with entities being related to geodesy. This information may be stored in the form ofdata and is called ageodetic data base provided (1) it contains or may contain all data necessary for the operations of a particular geodetic organization, (2) the data is stored in a form suited for many different applications and (3) that unnecessary duplications of data have been avoided. The first step to be taken when establishing a geodetic data base is described, namely the definition of the basic entities of the data base (such as trigonometric stations, astronomical stations, gravity stations, geodetic reference-system parameters, etc...). Presented at the “International Symposium on Optimization of Design and Computation of Control Networks”, Sopron, Hungary, July 1977.  相似文献   

19.
Use of C-Band frequencies for satellite navigation: benefits and drawbacks   总被引：2，自引：0，他引：2  
Although not considered for the first generation of European Galileo satellites, the use of C-Band frequencies for navigation purposes may be taken into account for a future generation of Galileo. For this reason, a frequency band of 20 MHz bandwidth (5,010–5,030 MHz) has been allocated in the course of the World Radio Communications Conference 2000 held in Istanbul, Turkey. The use of C-Band navigation signals offers both advantages and drawbacks. One example is the ionospheric path delay which is inversely proportional to the (squared) carrier frequency and is therefore significantly smaller at C-Band. On the other hand, the use of C-Band frequencies results in increased attenuation effects such as free space loss or rainfall attenuation. It is therefore necessary to provide a detailed analysis of the effects of C-Band frequencies on the navigation process. In order to assess the feasibility of using C-Band frequencies, various aspects of signal propagation and satellite signal tracking at C-Band are examined in the context of this article. In particular, aspects like free space loss, atmospheric effects, foliage attenuation, code and carrier tracking performance, code noise, phase noise and multipath are discussed with respect to their performance at C-Band. In order to allow comparison with the current GPS system, the performance at C-Band is compared to the L-Band performance under similar or identical conditions. The results of this analysis will finally be discussed with respect to their impact on satellite payload and receiver design.
Markus IrsiglerEmail: Phone: +49-89-60043552Fax: +49-89-60043019
  相似文献   

20.
At the beginning of the twenty-first century, a technological change took place in geodetic astronomy by the development of Digital Zenith Camera Systems (DZCS). Such instruments provide vertical deflection data at an angular accuracy level of 0.̋1 and better. Recently, DZCS have been employed for the collection of dense sets of astrogeodetic vertical deflection data in several test areas in Germany with high-resolution digital terrain model (DTM) data (10–50 m resolution) available. These considerable advancements motivate a new analysis of the method of astronomical-topographic levelling, which uses DTM data for the interpolation between the astrogeodetic stations. We present and analyse a least-squares collocation technique that uses DTM data for the accurate interpolation of vertical deflection data. The combination of both data sets allows a precise determination of the gravity field along profiles, even in regions with a rugged topography. The accuracy of the method is studied with particular attention on the density of astrogeodetic stations. The error propagation rule of astronomical levelling is empirically derived. It accounts for the signal omission that increases with the station spacing. In a test area located in the German Alps, the method was successfully applied to the determination of a quasigeoid profile of 23 km length. For a station spacing from a few 100 m to about 2 km, the accuracy of the quasigeoid was found to be about 1–2 mm, which corresponds to a relative accuracy of about 0.05−0.1 ppm. Application examples are given, such as the local and regional validation of gravity field models computed from gravimetric data and the economic gravity field determination in geodetically less covered regions.  相似文献   

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