G32A-01 INVITED
Making Sense of Evolving Reference Frames for North America
The World Geodetic System 1984 (WGS84) and North American Datum of 1983 (NAD83) are the most widely- used spatial reference systems in North America. NAD83 is the national reference system used for georeferencing by most federal and provincial/state agencies while WGS84 is the default "native" system used by the Global Positioning System (GPS) and commercial GPS receivers. The physical realization of these reference systems have undergone several updates since they were first introduced over two decades ago. NAD83 has evolved from a traditional, ground-based horizontal control network to a space-based 3D realization fully supporting modern GPS techniques and the integration of both horizontal and vertical reference systems. WGS84, on the other hand, has no publicly accessible ground-based network. It is accessible only via broadcast orbits that provide positions with an accuracy of about a meter at best (with augmented corrections). More recently, a new reference systems called the Stable North American Reference Frame (SNARF) has been created primarily in support of Plate Boundary Observatory component of the EarthScope project. We explain the differences between these global and regional reference frames and as well as their relationship to each other. We also discuss some problems that occur when these relationships are not properly represented as done, for example, with NAD83 in the vast majority of GPS receivers.
G32A-02 INVITED
Status and Future Developments of SIRGAS
This paper presents the status and future developments of the SIRGAS (Geocentric Reference System for the
Americas) project. Since its creation, in 1993, SIRGAS has coordinated two continental GPS campaigns in 1995
an 2000, responsible for the establishment of a very accurate 3D reference frame in the region. First focusing on
South America, the project has expanded its scope to Latin America since 2001. Currently the maintenance of the
SIRGAS reference frame is carried out through more than 80 continuous operating GNSS (Global Navigation
Satellite System) stations available in the region, whose data is officially processed by the International GNSS
Service (IGS) Regional Network Associate Analysis Centre for SIRGAS (IGS RNACC-SIR), functioning at the DGFI
(Deutsches Geodatisches Forschungsinstitut), in Munich, to generate weekly coordinates and velocity information
of each continuous GNSS station. Since October 2006, five additional experimental processing centers - located
at the Brazilian Institute of Geography and Statistics (IBGE), National Institute of Statistics, Geography and
Informatics of Mexico (INEGI), Military Geographic Institute of Argentina (IGM), University of La Plata (UNLP),
Argentina, and Geographic Institute Agustín Codazzi, Colombia (IGAC) - have also been processing data from
those stations in order to assume the official processing responsibility in near future.
Many Latin American countries have already adopted SIRGAS as their new official reference system. Besides,
efforts have been carried out in order to have the national geodetic networks of Central American countries
connected to the SIRGAS reference frame, which will be accomplished by a GNSS campaign scheduled for the
first semester of 2007.
In terms of vertical datum, SIRGAS continues to coordinate with each member country all the necessary efforts
towards making the geodetic leveling data available together with gravity information in order to support the
computation of geopotential numbers, to be unified in a continental adjustment.
http:sirgas.igm.gov.ar
G32A-03
A New Tectonic Plate Model Based on ITRF2005: Implications on the Global Kinematics
Tectonic plate models have been routinely computed in the last years using space-geodetic solutions of the motions of discrete points on the Earth's surface. Such models have been referenced with respect to different realizations of the ITRS (International Terrestrial Reference System). In particular, several models were produced based on the different ITRF solutions. This is the case of DEOSVel (Fernandes et al., 2003), which used the official ITRF2000 velocity solutions for 154 sites (supplemented with 14 own-computed solutions in Africa) to estimate the angular velocity for the stable part of eight major tectonic plates. A new realization of ITRS, ITRF2005, was recently (October 2006) released. It presents position/velocity solutions for a total of 338 different sites, based on four different techniques: GPS, VLBI, SLR and DORIS. Although the number of sites decreased (ITRF2000 presented solutions for 487 sites), the derived ITRF2005 velocity solutions are based on a much longer time-series of positions (until the beginning of 2006) than the previous ITRF2000 velocity solutions (until mid 2000). Consequently, the reliability of the estimated solution improved substantially for most of the stations. In this work, we present a new tectonic plate model based on ITRF2005: DEOSVel05a. This model is based exclusively in the ITRF2005 velocity solutions in order to ensure the best internal consistency of the input data. However, the formal uncertainties associated with the ITRF2005 solutions are too optimistic because temporal correlations between observations were not taken into account. Therefore, we investigated the implications of using a rescaled covariance matrix to compute the plate model uncertainties. We compute our own position solutions for a global set of GPS stations. For each station, the spectral index of the power-law noise in the time- series was estimated and new uncertainties of the motion estimates derived. By computing the ratio between these values and the formal errors provided in the ITRF2005 solution, an average scale factor was estimated and applied to the input covariance matrix used to derive the angular velocities in the least squares sense. We will discuss the implications of using DEOSVel05a instead of other space-geodetic derived models to constrain the present-day kinematics of the stable part of the analyzed tectonic plates. In particular, we focus on the variation of the angular velocity estimates for Americas and Africa. We demonstrate that for many plates, the estimates provided by these different models are converging. However, for some plates (e.g., Somalia), the number of available data points is still not enough to provide a robust answer about the present-day kinematics of these tectonic blocks.
G32A-04
Precise Geodesy Programs at the Naval Research Laboratory
For the past decade the Naval Research Laboratory independently and in collaboration with KMS, NOAA, NGA and NAVOCEANO has maintained an active program in developing precise geoid models. The Navy focus has been in the development of precise coastal reference frames for applications to coastal oceanography and hydrography. Our primary tool has been airborne gravimetry used to fill in data gaps, and to serve as a self consistent survey to detect and correct problems such as incorrect tie values in the historical data. Most recently we have been utilizing high-altitude surveys (7.5-10.5 km) flown on the NRL P-3 and the NOAA Citation. The attenuation of short wavelengths with altitude permits wider line spacing without aliasing and minimizes the effects of terrain corrections. At the same time true air-speeds are increased over low altitude operations. Both factors increase the cost effectiveness of geodetic surveys, allow the coverage of larger areas in less time than would be possible at lower altitudes. High-altitude profiles also generally provide a benign environment for the acquisition of good quality gravimetry. We will show recent work conducted over Afghanistan and the Gulf of Mexico and discuss plans for a comprehensive airborne geodetic survey of the southeastern border of the United States.
G32A-05
Naval Reseaarch Laboratory Gulf of Mexico Geoid Model Texas Reference Center Pilot Project
The purpose of this project is to improve elevations in Texas and nearby regions for flooding, mapping and hurricane preparedness. The U.S. Navy Department's Naval Research Laboratory and Texas A&M University- Corpus Christi are planning an aircraft-based gravity data collection effort along the Gulf of Mexico and the southern boundary of the United States. This project will utilize the Naval Airborne Gravity modeling system to improve elevations along the coast and throughout the United States in support of shallow-water navigation, aviation, and ground monitoring systems used by federal, state and local public safety agencies. The current project plan will address the issue of inconsistencies in the gravity field from onshore to offshore, facilitate the identification of systematic problems and otherwise help resolve issues related to existing, legacy gravity data. The widespread and increasing use of GPS for height determinations fundamentally depends on the use of a geoid height model to convert GPS-derived heights into heights above mean sea level. Current geoid models are inadequate. Accurate, homogeneous gravity measurements are essential for the computation of geoid models. To address the inadequate gravity data set for the US, the Naval Research Laboratory (NRL) and National Geodetic Survey (NGS) have developed a new approach based on airborne methods. The new approach yields spatially well distributed high-quality, consistent, and contemporary gravity data. The method to be employed has undergone extensive testing and is now ready for additional investigation. The airborne gravitational data will be used in conjunction with existing ground and marine gravity data sets and data from the Texas Coastal Ocean Observation Network (TCOON), a network of 32 tide gauges, to be used to help identify problems of tying the geoid to mean sea-level. These improved gravity data will lead to the creation of new and improved US geoid models. Accurate geoid models will then allow; (i) improving ocean circulation and tide models in support of shallow-water Naval and Marine operations, (ii) improved hurricane SLOSH models for coastal flooding, where hydrographic and topographic mapping in coastal zone areas are typical land maps which use one height system and ocean charts use another, (iii) improved geospatial mapping for homeland security and defense applications, (iv) accurate heights for engineering and construction projects, (v) air and ground transportation navigation, and (vi) provide a homogeneous gravity database of the US for studying the geology and tectonics in general, and studies of resources, geologic hazards and the environment in particular.
G32A-06 INVITED
Plumb Line Variations (PLV) and Geoid Deformations (GD): approaches be used to describe better the tomparal variation of a gravity field
In order to decribe better the temporal variation of a gravity field where repeated gravimetric gravimetric observations have been performed, an approach of determinating the Plumb Line Variations (PLV) within the field, as well as the corresponding Geoid deformations (GD) there, has been introduced. The suggested approarch has been put into practice in using the repeated gravimatric observations of 1985-1998 performed at Beijing and Yunnan gravimetric Networks. After a comparison with the great earthquakes (M >5) nearby in the same period, it can be seen that the obtained PLV and GD is helpful both in understanding the temporal variations of a gravity field, as well as in the studying of an earthquake.
G32A-07
Earthquake Location Prediction in Using the Measurements of the Variations of Deflection of Vertical at Different Locations
An introduction is given on the formula of measuring temporal variations of the deflection of vertical by the repeated gravity measurements in an area. It is also discussed on the possibility to use these measurements at different station in order to predict the location of the coming earthquake.