Export 4 results:
Sort by: Author [ Title  (Asc)] Type Year
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z   [Show ALL]
Dong, D, Fang P, Bock Y, Cheng MK, Miyazaki S.  2002.  Anatomy of apparent seasonal variations from GPS-derived site position time series. Journal of Geophysical Research-Solid Earth. 107   10.1029/2001jb000573   AbstractWebsite

[1] Apparent seasonal site position variations are derived from 4.5 years of global continuous GPS time series and are explored through the "peering'' approach. Peering is a way to depict the contributions of the comparatively well-known seasonal sources to garner insight into the relatively poorly known contributors. Contributions from pole tide effects, ocean tide loading, atmospheric loading, nontidal oceanic mass, and groundwater loading are evaluated. Our results show that similar to40% of the power of the observed annual vertical variations in site positions can be explained by the joint contribution of these seasonal surface mass redistributions. After removing these seasonal effects from the observations the potential contributions from unmodeled wet troposphere effects, bedrock thermal expansion, errors in phase center variation models, and errors in orbital modeling are also investigated. A scaled sensitivity matrix analysis is proposed to assess the contributions from highly correlated parameters. The effects of employing different analysis strategies are investigated by comparing the solutions from different GPS data analysis centers. Comparison results indicate that current solutions of several analysis centers are able to detect the seasonal signals but that the differences among these solutions are the main cause for residual seasonal effects. Potential implications for modeling seasonal variations in global site positions are explored, in particular, as a way to improve the stability of the terrestrial reference frame on seasonal timescales.

Wang, MH, Wang JX, Bock Y, Liang H, Dong DA, Fang P.  2019.  Dynamic mapping of the movement of landfalling atmospheric rivers over Southern California with GPS data. Geophysical Research Letters. 46:3551-3559.   10.1029/2018gl081318   AbstractWebsite

Atmospheric rivers (ARs) are long, narrow, and transient corridors of strong horizontal water vapor transport that can result in heavy precipitation. Measuring the movement of these concentrated water vapor bands is important in gaining better insight into AR characteristics and forecasts of AR-caused precipitation. We describe a method to dynamically map the movement of landfalling ARs. The method utilizes high-rate GPS observations from a dense network to derive isochrones that represent the AR arrival time over specific locations. The generated isochrones show that the three ARs, during landfall over Southern California in January 2017, moved southeastward and took about 10 hr to pass over the study area. Overlaying the topography with isochrones reveals that the Peninsular Ranges slow the movement of the landfalling ARs. The large spacing between two adjacent isochrones, reflecting fast AR movement, is closely related to the increased hourly rain rate. Plain Language Summary Atmospheric rivers (ARs), "rivers in the sky," are "rivers" of water vapor rather than liquid water. The landfall of ARs can cause extreme rainfall that in turn induces disasters. We present a method with a dense high-rate GPS network to capture the movement of the landfalling ARs over Southern California. For the three landfalling AR cases in January 2017, results show that the ARs moved southeastward and the durations of AR passing over the study area were about 10 hr. The results also reveal that the landfalling AR movement is affected by local terrain and the fast AR movement is closely related to the large hourly rain rate. The use of the method provides a way to study ARs with high spatial-temporal resolution, which is important in gaining better insight into the forecasts of AR-caused rainfall.

Bock, Y, Nikolaidis RM, de Jonge PJ, Bevis M.  2000.  Instantaneous geodetic positioning at medium distances with the Global Positioning System. Journal of Geophysical Research-Solid Earth. 105:28223-28253.   10.1029/2000jb900268   AbstractWebsite

We evaluate a new method of Global Positioning System (GPS) data analysis, called instantaneous positioning, at spatial scale lengths typical of interstation spacings in a modern crustal motion network. This method is more precise and versatile than traditional GPS static and kinematic processing of multi-epoch batches of data. The key to instantaneous positioning is the ability to resolve integer-cycle phase ambiguities with only a single epoch of dual-frequency phase and pseudorange data, rendering receiver cycle slips irrelevant. We estimate three-dimensional relative coordinates and atmospheric zenith delay parameters independently every 30 s over a 12-week period for baseline distances of 50 m, 14 km: and 37 km. Horizontal precision of a single-epoch coordinate solution is about 15 mm and vertical precision is about 7-8 times worse. Removing that component of each time series which repeats with a period of exactly I sidereal day, and thus manifests signal multipath, reduces the scatter by about 50% in all components. Solution averaging of the high frequency time series can be performed using: ally number of measurement epochs to further improve coordinate precision. We demonstrate that the daily coordinates estimated with instantaneous positioning are more precise (by 20-50% per coordinate component) than those estimated with 24-hour batch processing. Spectral analysis of the single-epoch solutions indicates that the flicker noise characteristic of GPS time series observed in lower-frequency bands also affects GPS solutions in the frequency band 0.01 mHz to 10 mHz. We argue that the flicker noise is induced by tropospheric effects. Since modern GPS receivers are capable of observing at frequencies as high as 10 Hz, our technique significantly overlaps and complements the frequency band of broadband seismology and benefits other research areas such as earthquake geodesy, volcanology, and GPS meteorology.

Williams, S, Bock Y, Fang P.  1998.  Integrated satellite interferometry: Tropospheric noise, GPS estimates and implications for interferometric synthetic aperture radar products. Journal of Geophysical Research-Solid Earth. 103:27051-27067.   10.1029/98jb02794   AbstractWebsite

Interferometric synthetic aperture radar (INSAR), like other astronomic and space geodetic techniques, is limited by the spatially and temporally variable delay of electromagnetic waves propagating through the neutral atmosphere. Statistical analysis of these variations, from a wide variety of instruments, reveals a power law dependence on frequency that is characteristic of elementary (Kolmogorov) turbulence. A statistical model for a major component of the delay fluctuations, the "wet" component, has previously been developed by Treuhaft and Lanyi [1987] for very long baseline interferometry. A continuous Global Positioning System (GPS) network is now in place in southern California that allows estimation of, along with geodetic parameters, the total delay due to the atmosphere above each site on a subhourly basis. These measurements are shown to conform to the Treuhaft and Lanyi (TL) statistical model both temporally and spatially. The TL statistical model is applied to the problem of INSAR and used to produce the covariance between two points separated in time and/or space. The error, due to the atmospheric variations, for SAR products such as topography and surface deformation is calculated via propagation of errors. There are two methods commonly cited to reduce the effect of atmospheric distortion in products from SAR interferometry, stacking and calibration. Stacking involves averaging independent interferograms to reduce the noise. Calibration involves removing part (or all) of the delay using data from an independent source such as total zenith delay estimates from continuous GPS networks. Despite the relatively poor spatial density of surface measurements, calibration can be used to reduce noise if the measurements are sufficiently accurate. Reduction in tropospheric noise increases with increasing number of measurement points and increasing accuracy up to a maximum of root N, where N is the number of points. Stacking and calibration are shown to be complementary and can be used simultaneously to reduce the noise to below that achievable by either method alone.