Geodetic Community Research Articles

On new data validation criteria in differential GPS

In differential GPS, once the observations have been screened for strong biases such as cycle slips, multipath, etc., the statistical modelling of the problem may still be unsatisfactory for some receiver-satellite pairs at certain epochs. The new validation tests presented in this paper are aimed at detecting these failures. The corresponding analysis calls on elementary concepts of algebraic graph theory. The notion of a spanning tree of maximal weight, in particular, proves to be well suited to cope with these failures. The related framework is presented in a concise manner, while making the link with the usual terminology and symbology of the geodetic and satellite positioning communities.

Assimilation of COST 716 Near-Real Time GPS data in the nonhydrostatic limited area model used at MeteoSwiss

Application of the GPS derived water vapor into Numerical Weather Prediction (NWP) models is one of the focuses of the COST Action 716 “Exploitation of Ground based GPS for climate and numerical weather prediction applications”. For this purpose the GPS data covering Europe have been collected within the Near-Real Time (NRT) demonstration project and provided for Observing System Experiments (OSE). For the experiments presented in this manuscript the operational NWP system of MeteoSwiss is used. The limited area nonhydrostatic aLpine Model (aLMo) of MeteoSwiss covers most of western Europe, has a horizontal resolution of 7 km, 45 layers in the vertical, and uses a data assimilation scheme based on the Newtonian relaxation (nudging) method. In total 17 days analyses and two 30 hours daily forecasts have been computed, with 100 GPS sites assimilated for three selected periods in autumn 2001, winter and summer 2002. It is to be noted that only in the last period data from 10 french sites, i.e. west of Switzerland are assimilated. The GPS NRT data quality has been compared with the Post-Processed data. Agreement within 3 mm level Zenith Total Delay bias and 8 mm standard deviation was found, corresponding to an Integrated Water Vapor (IWV) bias below 0.5 kg/m2. Most of the NRT data over aLMo domain are available within a prescribed time window of 1 h 45 min. In the nudging process the NRT data are successfully used by the model to correct the IWV deficiencies present in the reference analysis; stronger forcing with a shorter time scale could be however recommended. Comparing the GPS derived IWV with radiosonde observations, a dry radiosonde bias has been found over northern Italy. Through GPS data assimilation the aLMo analysis bias and standard deviation in the diurnal cycle has been reduced. The negative bias of –0.64 kg/m2 in the reference analysis has been reduced to 0.34 kg/m2 in GPS analysis. However, the diurnal cycle statistic from the forecast does show the characteristic negative bias only slightly reduced starting with the GPS analysis. The GPS IWV impact on aLMo is large in June 2002 and moderate in September 2001 OSE. January OSE is inconclusive due to inconsistent use of humidity data below the freezing point. In June 2002 OSE, a substantial IWV impact is seen up to the end of the forecast. Over Switzerland the dry bias in the reference analysis has been successfully corrected and the 2 m temperature and dew point have been slightly improved over the whole aLMo domain. The subjective verification of precipitation against radar data in autumn 2001 and summer 2002 gives mixed results. In the forecast the impact is limited to the first six hours and to strong precipitation events. A missing precipitation pattern has been recovered via GPS assimilation in June 20 2002 forecast. A negative impact on precipitation analysis on June 23 has been observed. The future operational use of GPS will depend on data availability; European GPS networks belong mainly to the geodetic community. A further increase of GPS network density in southern Europe is welcome. The GPS derived gradient and Slant Path estimates could possibly improve efficiency of IWV assimilation via the nudging technique.

Space-wise approach to satellite gravity field determination in the presence of coloured noise

For many years, the gravity field of the Earth was only seen by satellite geodesy as the main factor affecting the orbit and consequently it was retrieved together with a number of other orbital perturbations. Since the advent of a new generation of accelerometers, non-gravitational perturbations can be separated from the gravity effects and a new era of gravity field estimates from space has been born. During preparatory data analysis for new missions performed by the geodetic community, three approaches have been proposed and numerically tested: the brute force method (direct approach), the semi-analytical (time-wise) method and the space-wise method. In particular, the time-wise method takes advantage of the incoming time flow of data and, after performing a Fourier transform of the observation equations, exploits the prevailing block diagonal structure of the normal equations to estimate the spherical harmonic coefficients of the gravity field. Complementary to this is the space-wise approach, which goes back to the traditional computation of the harmonic coefficients by an integration technique or by least-squares collocation. Some advantages and disadvantages are peculiar to both methods, particularly the space-wise approach, which has for a long time ignored the marked signature of the noise spectrum due to the specific measuring conditions of space-borne accelerometers. The application of a proper Wiener filter, exploiting the correlation along the orbit, embedded into an iterative scheme, seems to be the answer. The solution to this major problem of the space-wise approach is illustrated and simulation results are discussed.

Premières mesures françaises de gravimétrie absolue dans les régions polaires Antarctique (terre Adélie) et Arctique (Spitzberg)

Abstract This study is devoted to the first French determinations of absolute gravity in the Antarctic (Dumont-d'Urville, Terre Adelie) and Arctic (Ny-Alesund, Spitsbergen) polar regions. The measurements in Dumont-d'Urville were performed in 2000 with the help of the FG5#206 absolute gravimeter belonging to the French geodetic community since beginning 1997; they show a strong noise that causes an uncertainty close to 11 μGal in the determination of the mean gravity value, which will be compared, to a new determination planned for 2005. The Ny-Alesund measurements show on the contrary a noise that, although very variable in time, leads to a gravity determination of better than 5 μGal. The comparison of our value with a previous one two years before suggests a gravity decrease of about 4 μGal which is related to the vertical uplift measured by Very Large Baseline Interferometry (VLBI) at the same site. However, the uncertainty does not allow discriminating between height-to-gravity conversion factors originating from different models. To cite this article: J. Hinderer et al., C. R. Geoscience 334 (2002) 819–826.

Avoiding numerical stability problems of long duration DGPS/INS Kalman filters

A current pursuit of the geodetic community is the optimal integration of differential GPS (DGPS) and inertial navigation system (INS) data streams for precise and efficient position and gravity vector surveying. Therein a complete INS and multiple-antenna GPS receiver payload, mounted on a moving platform, is used in conjunction with a network of ground-fixed single antenna GPS receivers. This paper presents a complete, GPS-based, external updating measurement model for the applicable Kalman filter. The model utilizes four external observation types for every GPS satellite in-view: DGPS range differences, single phase differences, and single phase-rate differences; as well as the mobile, multipleantenna GPS receiver's measurement of theerrors in the INS's estimate of the phase difference between any two vehicle-borne GPS antennae. Although not widely conveyed in the geodetic world, the inertial navigation community has long known that traditional Kalman filter covariance propagation recurrences are inherently unstable when such highly accurate external updates are repeatedly applied (every 1 second) over long time durations. A hybrid square root covariance/U — D covariance factorization approach is a numerically stable alternative and is reviewed herein. The hybrid makeup of the algorithm is necessitated by the correlated nature of the fourth type of GPS external measurement listed above (each vehicle-borne GPS antenna formstwo baselines). Such measurement correlations require a functional transformation of the overall external updating model to permit the multiple updates (simultaneously available at each updating epoch) to be sequentially (and efficiently) processed. An appropriate transformation is given. Stable covariance propagation relationships are presented and the transformed Kalman gain is also furnished and its use in the determination of the externally updated error states is discussed. Specific DGPS/INS instabilities produced by the traditional recurrences are displayed. The stable alternative method requires about 25% more CPU time than the traditional Kalman recurrences. With the ever-increasing computational speeds of microprocessors, this added CPU time is of no real concern.

The Sub-bureau for Atmospheric Angular Momentum of the International Earth Rotation Service: A Meteorological Data Center with Geodetic Applications

By exchanging angular momentum with the solid portion of the earth, the atmosphere plays a vital role in exciting small but measurable changes in the rotation of our planet. Recognizing this relationship, the International Earth Rotation Service invited the U.S. National Meteorological Center to organize a Sub-bureau for Atmospheric Angular Momentum (SBAAM) for the purpose of collecting, distributing, archiving, and analyzing atmospheric parameters relevant to earth rotation/polar motion. These functions of wind and surface pressure are being computed with data from several of the world's weather services, and they are being widely applied to the research and operations of the geodetic community. The SBAAM began operating formally in October 1989, and this article highlights its development, operations, and significance.

What is IGS, and why is it important to Australia?

Abstract The International Global Positioning System Geodynamics Service (IGS) aims to be a permanent international service under the auspices of the International Association of Geodesy, (IAG). Using the Global Positioning System (GPS) this service will provide earth science related products, including precise satellite ephemerides and earth rotation parameters as well as station co-ordinates and changes of baselines. These products are considered fundamental for the proper application of GPS in both global and regional geodesy, geodynamics and physical oceanography. Their availability will also contribute to other areas of research. Australian participation in the IGS is extremely high. This participation will provide the regional geodetic, surveying and general scientific communities with IGS products as well as a precision fiducial frame which will give access to the International Terrestrial Reference System.

IMPROVING INTERNATIONAL RELATIONS AMONG THE GEODETIC COMMUNITY

Recent activities of the Soviet, Polish, Bulgarian, Hungarian, Czechosiovakian and East German geodetic communities are discussed in a report on the 16th Conference of Representatives of Scientific-Technical Societies for Geodesy and Cartography (Socialist Countries). Work both within the individual countries and in collaboration with other member states is described and possibilities for strengthening international linkages are outlined. The present and future role of East European socialist countries in international nongovernmental organizations is examined as well. Translated by Edward Torrey, Alexandria, VA 22308 from: Geodeziya i kartografiya, 1988, No. 9, pp. 47-49.

Considerations for Bench Mark Subsidence Monitoring

Geotechnical engineers and engineering geologists are often involved with siting facilities where the potential for ground subsidence exists. The potential for subsidence could be a result of ground-water withdrawal, deep mining, hydrocarbon extraction, or salt solution mining. In order to monitor for detection of subsidence in these environments, bench marks are often installed. The bench mark design, location, and depth are governed by the purpose of the subsidence monitoring program. The geotechnical engineer or engineering geologist generally requires a high order of accuracy of vertical control, which includes repetitive optical leveling over an established network of bench marks to obtain accurate and precise data. However, the standards and widely used specifications for precise leveling recognized throughout the geodetic community consist of many more requirements than generally understood by the geotechnical engineer or engineering geologist.

Radio interferometry

In the previous Quadrennial Report, Clark [1979c] concluded that, after slightly more than one decade of development and refinement, the technique of Very Long Baseline Interferometry (VLBI) had “come of age.” However, by the close of 1978, only the most tentative first steps had been taken by the geodetic community toward the deployment and operation of VLBI geodetic surveying systems. There was not a single dedicated geodetic VLBI observatory in the world. Even at major astronomical observatories, geodetic VLBI observing sessions were elaborate productions involving the very scientists and engineers who designed and built the instrumentation operating the data acquisition systems.

Introduction to the report on geodesy

The achievements of the U.S. geodetic community during the years 1974–1978 between the General Assemblies of the IUGG are summarized in this section. During this quadrennium, American geodesy dramatically moved from the era of primarily meeting the traditional needs of surveying and mapping into one of supporting a broad spectrum of geophysical and planetary disciplines. This movement was aided by modern technology epitomized by the increases in Doppler satellite observations, multifrequency distance measurements, lunar and satellite laser ranging, very long base line interferometry (VLBI), satellite altimetry measurements and inertial positioning.By far the most spectacular achievement in the field of geodesy during this period was the global sampling of sea surface topography with precision radar altimeters aboard the National Aeronautics and Space Administration (NASA) GEOS‐3 and Seasat‐1 satellites. The GEOS‐3 altimetry data set, acquired over a 3‐year period (1975–78), significantly improved our knowledge of the global gravity field of the earth, the oceanic geoid and the related mean gravity anomalies. Had the Seasat‐1 not failed after 3 months, an even more detailed knowledge of the oceanic geoid would be available today. Even with this early demise, Seasat‐1 in 3 months collected more altimetry data with greater accuracy than did GEOS‐3 in 3 years.

The global positioning system (navstar)

In October 1978, the third of a series of prototype navigational satellites of an entirely new design was successfully launched. By the end of the first phase (1979) there will be six of these “NAVSTAR” satellites. They constitute one piece of a major development thrust to deploy a revolutionary navigation system called the Global Positioning System (GPS), or NAVSTAR. This system, being developed by the Air Force for the joint use of al components of the Department of Defense, has been under development since 1972. The purpose of this article is to describe the system, indicate the requirements which have caused the Department of Defense to spend many millions of dollars on the development, outline how the geodetic community can potentially use it, present the test results that have been accumulated to date and finally, indicate what future plans lie in store for this new system.

Satellite altimetry applied to marine geoid determination

The pioneering satellite radar altimeter aboard SKYLAB has provided a wealth of information about ocean surface topography for both the oceanographic and geodetic communities. This report describes typical satellite altimetry concepts and discusses the parameters measured, geometry utilized, and techniques employed to generate ocean geoid estimates. The standard deviation of the noise on the altitude measurements is shown to range from one to three meters when the altimeter antenna is nadir aligned. The altimeter is shown to sense short wavelength ocean surface features which are not included in present conventional global geoids. An estimate of a local geoid in the Atlantic, using only altimeter data, is presented. Finally, results demonstrate that the SKYLAB radar altimeter system capability is less than 10 m RMS.

Introduction to the Report on Geodesy

The following articles summarize the achievements of the U.S. geodetic community during the years 1971–1974, between the General Assemblies of the IUGG. During this quadrennium, American geodesy made the first transition from meeting the traditional needs of surveying and mapping to catering to the geophysical and oceanographic communities, trying to satisfy their ever‐increasing appetite for ultraprecise surveys.Major achievements included the completion of the National Geodetic Satellite Program, probably the largest single effort to establish a global network of satellite tracking stations and the determination of the earth's gravity field to an accuracy heretofore not available; the proliferation of Doppler satellite observations for control surveys and polar motion determination; the affirmation of lunar and satellite laser ranging and very long base line interferometry (VLBI) as the best available geodetic tools for global geophysical investigations, such as plate tectonics, polar motion, and the earth's rotation; the development of the satellite‐borne altimeter and the establishment of its feasibility for a better definition of the surface of the oceans; major improvements in the knowledge of the dynamics, size, shape, gravity field, and topography of the moon and some of the major planets; the extension of the high‐precision transcontinental traverse to a total of 9000 km, only 10% of the network remaining for completion in 1975; the start of the readjustment of the North American Datum, in collaboration with Canada, Mexico, and Denmark; and an unprecedented cross‐fertilization between geodesy, solid earth geophysics, oceanography, and dynamical astronomy.

Positioning of Horizontal Geodetic Datums

This paper treats the classical problem of positioning horizontal geodetic datums. By ‘classical problem’ we mean the problem we face when dealing with the usual horizontal control networks, as opposed to some of the more modern ideas, such as the use of geocentric coordinates, or the ideas of Hotine [1959, 1969]. Our basic assumption is that a coordinate system is a fixed framework (i.e., invariant with respect to network adjustment, readjustment, or expansion) for describing geodetic networks. Our view is that a coordinate system and the network it describes are two different things (this view is not universally accepted within the geodetic community).