摘要:

In January 1988, scientists from over 25 organizations in 13 countries and territories cooperated in the largest Global Positioning System (GPS) campaign in the world to date (Table 1). 43 GPS receivers collected approximately 590 station‐days of data in American Samoa, Australia, Canada, Colombia, Costa Rica, Ecuador, New Zealand, Norway, Panama, Sweden, United States, West Germany, and Venezuela. The experiment was entitled CASA UNO, an acronym for Central and South America – and uno is the Spanish word for one, designating first epoch measurements. The CASA UNO experiment was the first civilian effort implementing a global GPS satellite tracking network.Scientific goals of the project include measurement of strain in the northern Andes, measurement of subduction rates for the Cocos and Nazca plates beneath Central and South America, and measurement of relative motion between the Caribbean plate and South America. A second set of measurements are planned in 1991 (CASA DOS), and should provide preliminary estimates of crustal deformation and plate motion rates in the region. The CASA series of experiments are intended to be carried out over at least one dec

ISSN:0094-8276    

DOI:10.1029/GL017i003p00195

年代:1990

数据来源: WILEY

摘要:

An algorithm has been developed to edit automatically Global Positioning System data such that outlier deletion, cycle slip identification and correction are independent of clock instability, selective availability, receiver–satellite kinematics, and tropospheric conditions. This algorithm, called TurboEdit, operates on undifferenced, dual frequency carrier phase data, and requires (1) the use ofPcode pseudorange data and (2) a smoothly varying ionospheric electron content. The latter requirement can be relaxed if the analysis software incorporates ambiguity resolution techniques to estimate unresolved cycle slip parameters. TurboEdit was tested on the large data set from the CASA Uno experiment, which contained over 2500 cycle slips. Analyst intervention was required on 1% of the station–satellite passes, almost all of these problems being due to difficulties in extrapolating variations in the ionospheric delay. The algorithm is presently being adapted for real time data editing in the Rogue receiver for continuous monitoring applicati

ISSN:0094-8276    

DOI:10.1029/GL017i003p00199

年代:1990

数据来源: WILEY

摘要:

Wet tropospheric path delay can be a major error source for Global Positioning System (GPS) geodetic experiments. We investigate strategies for minimizing this error using data from CASA Uno, the first major GPS experiment in Central and South America, where wet path delays may be both high and variable. We compared wet path delay calibration using water vapor radiometers (WVRs) and residual delay estimation, with strategies where the entire wet path delay is estimated stochastically without prior calibration, using data from a 270 km test baseline in Costa Rica. Both approaches yield centimeter‐level baseline repeatability and similar tropospheric estimates, suggesting that WVR calibration is not critical for obtaining high precision results with GPS in the CASA regio

ISSN:0094-8276    

DOI:10.1029/GL017i003p00203

年代:1990

数据来源: WILEY

摘要:

The CASA UNO Global Positioning System (GPS) experiment (January‐February 1988) included an extended tracking network which covered three continents in addition to the network of scientific interest in Central and South America. The repeatability of long baselines (400–1000 km) in South America is improved by up to a factor of two in the horizontal vector baseline components by using tracking stations in the Pacific and Europe to supplement stations in North America. In every case but one, the differences between the mean solutions obtained using different tracking networks was equal to or smaller than day‐to‐day rms repeatabilities for the same baselines. The mean solutions obtained by using tracking stations in North America and the Pacific agreed at the 2–3 millimeter level with those using tracking stations in North America and Europe. The agreement of the extended tracking network solutions suggests that a broad distribution of tracking stations provides better geometric constraints on the satellite orbits and that solutions are not sensitive to changes in tracking network configuration when an extended network is used. A comparison of the results from the North Andes and a baseline in North America suggests that the use of a geometrically strong extended tracking network is most important when the network of interest is far from Nort

ISSN:0094-8276    

DOI:10.1029/GL017i003p00207

年代:1990

数据来源: WILEY

摘要:

The CASA UNO GPS (Global Positioning System) experiment (January‐February 1988) has provided the first epoch baseline measurements for the study of plate motions and crustal deformation in and around the North Andes. Two dimensional horizontal baseline repeatabilities are as good as 5 parts in 108for short baselines (100–1000 km), and better than 3 parts in 108for long baselines (>1000 km). Vertical repeatabilities are typically 4–6 cm, with a weak dependence on baseline length. The expected rate of plate convergence across the Colombia Trench is 6–8 cm/yr, which should be detectable by the repeat experiment planned for 1991. Expected deformation rates within the North Andes are of the order of 1 cm/yr, which may be detectable with the 1991 exp

ISSN:0094-8276    

DOI:10.1029/GL017i003p00211

年代:1990

数据来源: WILEY

摘要:

The Global Positioning System (GPS) will be used in the coming decade for mm‐level measurements of crustal motion and for sub‐decimeter earth orbiter positioning and navigation. GPS orbits for these applications will be required accurate to 10–20 cm. Although sub‐meter GPS orbit precision has been previously demonstrated with a regional tracking network, several factors are expected to play a role in further improvement to the several decimeter level. These include the use of worldwide ground tracking networks and improved orbit modeling. In this paper, CASA UNO orbit results are presented utilizing data from four continents. Refinements in orbit modeling, combined with the availability oft worldwide tracking network and the dense distribution of tracking sites in North and South America, have improved orbit determination precision to about 60 cm (per component) for four of the seven GPS satellites tracked in CASA UNO. The orbit results are consistent with California baseline repeatabilities, which are at the few mm level in horizontal and length, and 1–2 cm in the vertical. Baseline comparisons with VLBI provide a measure of orbit accuracy, showing sub‐cm agreement in length and 1.5 cm agreement in the

ISSN:0094-8276    

DOI:10.1029/GL017i003p00215

年代:1990

数据来源: WILEY

摘要:

A solution using GPS measurements acquired during the CASA UNO campaign has resulted in 3–4 mm horizontal daily baseline repeatability and 13 mm vertical repeatability for a 729 km baseline, located in North America. The agreement with VLBI is at the level of 10–20 mm for all components. The results were obtained with the GIPSY orbit determination and baseline estimation software and are based on five single‐day data arcs spanning the 20, 21, 25, 26, and 27 of January, 1988. The estimation strategy included resolving the carrier phase integer ambiguities (bias fixing), utilizing an optimal set of fixed reference stations (fiducials), and constraining GPS orbit parameters by applying a priori information (derived from initial multi‐day trajectory fits to broadcast ephemeris data). A multi‐day (January 20–27) GPS orbit and baseline solution has yielded similar 2–4 mm horizontal daily repeatabilities for the same baseline, consistent with the constrained single‐day arc solutions. The application of weak constraints to the orbital state for single‐day data arcs produces solutions which approach the precise orbits obtained with unconstrained multi‐day arc solutions. This work suggests that the general availability of nominal ephemerides based on many days of global tracking data would be very valuable and practical for regional geodesy, allowing for high precision with single‐day network solutions when combined with a continental s

ISSN:0094-8276    

DOI:10.1029/GL017i003p00219

年代:1990

数据来源: WILEY

摘要:

We use a simple mass‐spring model to systematically examine the dynamical behavior introduced by fault zone heterogeneities. The model consists of two sliding blocks coupled to each other and to a constant velocity driver by elastic springs. The state of this system can be characterized by the positions of the two blocks relative to the driver. A simple static/dynamic friction law is used. When the system is symmetric, we observe cyclic behavior. For an asymmetric system where the fractional forces for the two blocks are not equal, the solutions exhibit deterministic chaos. Chaotic windows occur repeatedly between regions of limit cycles on bifurcation diagrams. Our model behavior is similar to that of the one‐dimensional logistic map. In many examples of deterministic chaos, chaotic behavior of a low order system implies chaos in similar higher order systems. Thus, our results provide substantial evidence that earthquakes are an example of deterministic ch

ISSN:0094-8276    

DOI:10.1029/GL017i003p00223

年代:1990

数据来源: WILEY

摘要:

We analyze the source mechanism of a deep earthquake beneath Japan on January 1, 1984, whose Harvard centroid moment tensor (CMT) solution significantly deviates from a double couple mechanism. The broadband P‐wave ground displacement records show the presence of several distinct phases whose relative amplitudes vary among the stations. We perform very broadband waveform analyses over four different frequency bands: CMT inversion (surface waves (4∼5mHz) and long‐period body waves (12∼20mHz)), waveform inversion using the long‐period P, pP and SH waves (∼0.04Hz), and broadband P‐ and pP‐wave waveform analysis (0.03∼1Hz). We show that the large non double couple solution observed at long periods (>25 sec) is very likely to be a manifestation of the presence of subevents with different double couple mechanisms in a single

ISSN:0094-8276    

DOI:10.1029/GL017i003p00227

年代:1990

数据来源: WILEY

摘要:

Coherency filtered seismic sections provide excellent models for estimating residual statics, especially for deep crustal data with low signal to noise. A second coherency filter pass following residual statics collections provides even better structural clarity than is seen on sections with only a single coherency filter pass. Benefits of the technique include: increased lateral extent of individual reflections resulting in fewer isolated events, uniform wavelet shape throughout the section, and an increase in the number of detected events over line drawings. This processing flow results in appealing visual displays and may provide new information on structural complexities not recognized in line drawings based on the input data alone.

ISSN:0094-8276    

DOI:10.1029/GL017i003p00231

年代:1990

数据来源: WILEY