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representation of this field is the geoid: the equipotential surface of the gravity field of the earth which most nearly coincides with the undisturbed surface of the oceans which is the surface the seas would maintain if not subjected to the tidal attractions of the sun and moon, the waves, atmospheric disturbances, variations in the water salinity, and circulatory patterns of the oceans. There are small radial differences--seldom exceeding 100m and, for most of the earth, less than 25m--between the geoidal surface and the equipotential ellipsoid of revolution which the former closely approximates; but these differences are significant indicators of internal stress, and they are essential for improving the accuracy of geodetic results. In satellite orbit determination, the largest inaccuracy results from an inadequate knowledge of the gravity field; however, sub-meter accuracy for station location has been achieved with present data and analytic techniques. These activities fall into the category of physical aspects of geodesy. (A) Determination of geodetic quantities for solar system bodies. Though the techniques may vary, the geodetic aims for the earth also pertain to all bodies of the solar system. These are the determination of control networks, size, shape, and topography; the determination of their gravity fields; the determination of their rotation rates; and the detection of dynamic processes. AREAS OF STUDY In the past, the Committee has addressed problems that relate to the objectives of geodesy as previously described. The Committee's past activities are indicated in Appendices I, II, and III. However, the science continually changes due to new instrumentation, new theories, and new problems. In the following sections, areas that merit additional attention by the Committee and the scientific community are outlined. I. Global Positioning System The most challenging issues in geodesy in the coming decades will deal with the transition from the classical geodetic methods of leveling, triangulation, and trilateration to the new space-based methods--Global Positioning System (GPS), Very Long Baseline Interferometry (VLBI), and Satellite Laser Ranging (SLR). Among the available techniques, GPS will be the equipment of choice for regional relative positioning because of cost, ease of use, and portability. With respect to this system, there are questions that need to be addressed and programs that should be initiated:
A. Testing How accurate are the various GPS receivers currently on the market? Are the manufacturer's claims of accuracy realistic? These questions point to the need for improved receiver testing procedures. Currently, the Federal Geodetic Control Committee (FGCC) control grid consisting of base lines up to 50km, located at the National Bureau of Standards facility in Gaithersburg, Maryland, serves as the main testing site. However, it is not clear whether the mix of baseline lengths is representative of that which is encountered in the field on a routine basis. Test lines on the order of 100km to 200km are needed. Standardized procedures should be recommended to manufacturers for test purposes. Moreover, with just the few satellites (6) in operation, observed geometries change depending upon one's geographic location, which results in different Geometric Dilution of Precision (GDOP) numbers and different accuracies. Finally, there is a need for ambiguity resolution testing: currently, it is possible to resolve phase of the signal for code-correlating receivers, but sometimes the cycle number becomes confused (the cycle slip problem). In this regard, it is advisable to institute double-blind testing procedures, so that neither those performing the GPS field work, nor those providing the data, know the position of the monuments being observed. B. Monumentation Should current monument densities be maintained? Can a unified horizontal and vertical monument set be defined? Current practices in monumentation differ depending upon the type of survey. For example, leveling monuments tend to be closely spaced (about 1km spacing) and located along roads, railroad tracks, or other areas of gentle grade. By contrast, triangulation and trilateration surveys tend to require long-distance visibility and are therefore made from positions on high ground, such as mountain tops, or on tall artificial structures. With GPS receivers, the major requirement is a good view of the sky, with good visibility down to about 10° above the horizon. Hence, it will be possible with GPS to unify the horizontal and vertical networks, which may allow for substantial improvement in control and in interpretation of tectonic deformations. Moreover, the density of monuments need not be nearly as great using GPS, provided that appropriate procedures and corrections are applied for points separated by several hundred kilometers.
C. Geodetic Control What is the future of classical geodetic observations for horizontal control? The Global Positioning System is having a substantial impact on the techniques for obtaining horizontal positions; the need to use many of the classical observational techniques is now reduced. What is the optimum method for combining GPS observations with other data for vertical control? Vertical control can be determined from GPS observations, however the accuracy is dependent upon knowledge of variations in the geoid. Nevertheless, changes in vertical position can be detected by GPS independent of a knowledge of the geoid. But the determination of mean sea level heights using GPS does require an accurate knowledge of the geoid. D. Improvement in Accuracy How much can GPS accuracy be improved, how could such improvement be accomplished, and what would be the contributions to science and engineering? Because the Global Positioning System (GPS) is a radio based system, like Very Long Baseline Interferometry (VLBI), the precisely measured carrier phase measurements are sensitive to environmental effects, which are mainly perturbations of the signals due to the ionosphere and troposphere. Removal of ionospheric contributions requires use of multiphase frequency measurements. Currently receivers are available that collect measurements of the LI and L2 frequencies. The troposheric contribution is composed of "dry" and "wet" terms. The "dry" term can be determined very well from surface measurements of atmospheric pressure. However, the "wet" term can only be determined to the order of 25 percent using surface measurements of pressure, temperature, and relative humidity. Portable microwave radiometers that can measure the "wet" contribution are now at the prototype stage. As the cost of GPS receivers drops, the price of a portable microwave radiometer (from $50,000 to $100,000) will certainly limit their use in all but the more accurate science missions. Because the GPS antennas are omni-directional, the measurements are also subjected to degradation due to multipath receptions. This problem is being addressed now, and it is hoped that antennas used in geodetic applications that attenuate this multipath contamination will soon be part of the manufacturers' geodetic package. This will also provide much better calibration of the electrical centers. Using currently available precise positions based upon VLBI and SLR (Satellite Laser Ranging) as fiducial locations (i.e., locations where GPS receivers
are employed but their positions held fixed in the data reduction), GPS determined baselines of more than hundreds of kilometers in length are yielding results at the 10 level when dual frequency receivers are used. Considering the infant stage of the GPS system, the fact that the Block II satellites have not yet been launched, and the sparse global tracking network currently available, one will not be surprised when accuracies approaching those obtained from VLBI and SLR measurements over continental baselines are reported during the next decade. E. Data Formats. Analysis Programs, and Coordinate Systems As the use of GPS in surveying, mapping, and crustal motion studies by divergent groups using different receivers expands, a number of problems will need to be resolved. How can the differences in Data Formats be resolved? The different GPS receivers produce cassettes in varying, generally incompatible, formats. The "Standard Exchange Format" is awkward to use because it is very general and quite verbose. Are data reduction routines compatible? There are many different types of data reduction routines, they need to be validated. What is the appropriate output, in terms of position, needed from GPS software? The primary use of the GPS system will be in the relative mode, where positions are defined relative to another defined position. Should the output be in terms of latitude and longitude, of state plane coordinates, or both? F. Orbits What is the optimum procedure for the determination and dissemination of GPS satellite orbits? Several different groups are involved in the determination of GPS satellite orbits. Although a cooperative agreement between DMA, NOAA, and NASA on the computation and dissemination of orbital data was initiated, the agreement has not yet been ratified. With the large number of GPS users and in consideration of budget constraints, it is important to ensure that U.S. resources are effectively used. The Committee on Geodesy should play a leadership role in bringing together commercial, university, and federal groups to formulate a far reaching plan to satisfy GPS orbit needs.
G. Data Classification Will GPS data be readily available? Portable instruments are being widely used for scientific and engineering purposes. Some of these activities would be adversely affected by degradation of the GPS signals or restrictions on the availability of the ephemerides. The NRC should encourage timely availability and wide dissemination of precise GPS signals and ephemerides. II. Geodesy in Hostile Environments Classically, geodetic activities have been conducted on land; though some of these activities have been in areas that might be considered hostile to man, access was a matter of perseverance. Today, geodetic measurements are needed in ocean areas, space, politically hostile territories, and hazardous environments. These hostile environments require their own suite of instruments and present their own special problems. Can geodetic measurements be made in hostile environments with acceptable accuracies? An important problem of the future will be the accomplishment of precise geodesy in hostile environments. Examples include observation of crustal deformation across oceanic trenches and mid-ocean ridges. Interplate motion studies, the lineation of magnetic anomalies, and seismic observations all imply that, on the scale of hundreds of kilometers, the various plates are in motion relative to one another and that the individual plates behave, to a reasonable approximation, as if they are rigid. However, the variability of appearance of fracturing and lava flow types at spreading centers, as well as the apparent variability of hydrothermal activity, imply that at some small spatial scale the relative motion of the plates is episodic. Even in the simplest steady-state model, there must be a zone in which the newly-formed crustal material is, in some sense, accelerated from zero velocity to that characteristic of the relative motion of the plates. Observations of surface morphology at intermediate-rate spreading centers seem to indicate that a large fraction of the acceleration takes place in a zone that is at most a few kilometers wide. Moreover, the existence of intense fissuring at some of the spreading centers, where volcanic activity is not particularly fresh, implies that spreading may be nearly continuous while volcanism is not. The combination of these factors indicates that a series of strain observations during the course of a few years, within areas several kilometers wide and on the order of ten kilometers along strike, at intermediate-rate ridges should provide useful results. The temporal
and spatial patterns of increasing strain within the acceleration zone, if measured over decades, should provide practical constraints for models of crustal accretion, fissuring as related to hydrothermal activity, and insight into plate edge seismic processes. In addition to strain measurements, measurements of variations in elevation and tilt may indicate processes such as normal faulting, which play a critical role in the translation of newly-formed crust from the rise axis to the flanking abyssal hills. Also, it is sometimes of interest to obtain high-accuracy data at sites that are impractical to visit regularly, such as radioactive waste sites, or politically hostile territories, or hazardous volcanic sites. At such locations, the ability to obtain data in an untended mode over long periods of time would be critical. Will first-order survey control be needed on other planets; for what purpose and at what cost? Other hostile environments might even include planets or satellites other than the earth, such as Mars or lo, where significant crustal deformation is known or inferred to be occurring. Information on these planets would be of great interest to comparative planetologists, who could then study a variety of active processes for comparison with their earthly equivalents. III. Geodesy in Ocean Areas Are geodetic measurements capable of providing solutions to problems in physical oceanography? The primary problem facing oceanographers for which highly precise geodetic data are required is the determination of the general circulation of the world's oceans (both mean and time varying). Although other techniques can be applied, the only feasible means to sample the oceans globally on the required spatial and temporal scales is through the use of satellite-borne instruments such as altimeters and scatterometers. The altimeter data, when combined with the orbit height obtained by tracking the satellite, make it possible to determine the topography or shape of the ocean surface; this surface closely approximates an equipotential surface referred to as the marine geoid. The small deviations of the ocean surface from the marine geoid are caused primarily by quasi-geostrophic currents and tides. The slope of the sea surface is a direct result of that part of the surface flow field that is geostrophic. Measurement of these slopes would provide direct observation of a component of large-scale oceanic flow. In order to calculate these slopes, knowledge of the marine geoid and the dynamic ocean topography relative to the geoid are required. Satellite altimetry, together with accurate knowledge of the satellite orbit, can provide the dynamic topography.
10 A number of satellite altimeter missions are under way or in the planning stage. There is need for an independent scientific group to monitor the development of these missions. In order to assure that they provide the necessary topographic, geodetic, and orbit data required for determination of oceanic circulation, a number of programs are needed. These include: A geodetic mission that improves knowledge of the global marine geoid to the centimeter level for wavelengths greater than 30km. Continued precision and accuracy improvements in altimeter hardware to allow ranging to the centimeter level. Multibeam altimeter technology to remove the spatial and temporal sampling constraints imposed by a single-beam altimeter technology. Improvements in orbit determination technology with emphasis on perfection of satellite-to-satellite tracking techniques being considered for the TOPEX mission. Continued advances in techniques for data processing, handling, storage, and distribution in order to accommodate the needs of future geodetic and oceanographic missions. Continued improvement of mathematical techniques and models in oceanography, geodesy, and orbit determination. IV. Gravity Field Information Gravity field information is needed for a large variety of studies ranging from geophysics to inertial navigation. The interest extends from local to global areas, from static to time varying aspects. New instrumentation is becoming, or will become, available that can have a substantial impact on how we acquire knowledge of the earth's gravity field and quantities that depend upon it, such as the deflection of the vertical. Instrumentation ranges from portable absolute gravity devices to gradiometers (airborne and spaceborne) to satellite-to-satellite tracking missions to inertial surveying systems. These developments raise a number of questions: Where should we be heading in the area of gravity instrumen tat ion? What is the role of cryogenic relative gravity meters? What should be the future mix of local and global measurements? What should be the future of ship gravity measurements?
11 What further studies should be undertaken on the relationships between gravity, sea slope, and tidal benchmarks? Should satellite altimeter data be thought of as the prime data source for gravity-at-sea? What is the role of terrestrial gravity gradiometry in determining the gravity vector? How critical is the Geopotential Research Mission for the solution of geodetic and geophysical research problems? What are the optimum techniques for determining planetary gravity fields? How and where can absolute gravimeters be best applied for scientific purposes? Is there a significant difference between terrestrial and space derived gravity fields? What are the most appropriate ways to represent the gravity field for geodetic, geophysical, and oceanographic purposes? Can further improvements in knowledge of the gravity field be justified if the costs are large? Can inertial surveying systems determine high order deflections of the vertical to sufficient accuracy? What are the best methods for determining time variations in the gravity field? V. International Programs A. Global Crustal Motion Measurements The improvement in geodetic measurement instrumentation and techniques allows scientists to consider the determination of temporal crustal variations. A coordinated world-wide program of measurements along faults, repeated semiannually, would form a baseline for estimating the stability of tectonic plates. The program would not necessarily be limited to faults, but could include regions of subsidence and uplift.
12 B. Earth Rotation Service Proposals for a new International Earth Rotation Service have been requested by the International Astronomical Union and the International Union of Geodesy and Geophysics. A number of organizations in the United States have submitted proposals to participate in the new service. The NRC should take a leading role in ensuring that the scientific and engineering communities are aware of the significance of the new service and that the service meets their needs. C. VLBI. SLR, and GPS Observing Campaigns Scientific experiments utilizing Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR) and Global Positioning System (GPS) data will be the predominant geodetic methods for contributing to the resolution of geodetic and geophysical problems. Suggestions for types of experiments, such as that undertaken to monitor Icelandic rifting, and assignment of priorities should be undertaken by the NRC. VI. Validation of Experimental Methods As geodetic measurements are obtained and used by diverse groups, a procedure should be established to formally review the various experimental techniques in order to validate the systematic error budget claims. Among the questions to be asked are: What information can be recovered from earth tide measurements? Are there systematic errors in leveling that are still hidden? What is the role of GPS in vertical control? What is the usefulness of absolute gravity measurements? VII. Geodetic Data A. Data Bases Geodetic data are of many different types, ranging from positioning information to gravity data to terrain data. In the near future a great deal of GPS data will be available. A coherent plan for the storage and retrieval of a great variety of data needs to be developed. The recently completed report of the Committee on Geophysical Data did not specifically address geodetic data problems.
13 The NRC through its Committee on Geodesy needs to take an active role in examining the current geodetic data base problems and in recommending solutions. B. Data Classification A variety of geodetic studies continues to be adversely affected by security classification policies. The NRC should encourage policies of open dissemination of all scientific data, and of responsiveness to the concerns of the scientific and engineering communities. C. Applications Are there physical measurements that are location and/or gravity dependent, but presently outside the field of classical geodesy, that should be examined for their geodetic content? VIII. Survey and Mapping A. U.S. Vertical Datum The U.S. vertical datum can have a substantial impact on the scientific and engineering users of vertical control data. The Committee on Geodesy submitted a report to the National Geodetic Survey (NGS) that recommended studies to be undertaken on issues related to vertical datum definition. On receipt of a response from NGS concerning these recommended studies, panels of interested and knowledgeable individuals should be formed. The feasibility and usefulness of separate reference systems--one for engineering and one for science--should be explored. Although the two systems should be compatible, the possibility of conflicting user demands may require that the former system not necessarily be just a simplified version of the latter. There is a need to coordinate a smooth transition from the North American Vertical Datum 1929 to the North American Vertical Datum 1988. B. The Impact of Technology on Survey Law The recent advances in Geodesy as a result of improved technology has had an impact on surveying. Is legislation needed in order to take into account advances such as the Global Positioning System and the World Geodetic System?
