Global Positioning System Tracking Stations Research Articles

GPS Observations of Crustal Deformation Associated with the 2012 Mw 7.8 Haida Gwaii Earthquake

Abstract On 28 October 2012, an M w 7.8 earthquake occurred off the west coast of Haida Gwaii, British Columbia. Although past large events at this margin reflect strike‐slip motion between the Pacific and North American plates (e.g., 1949 M s 8.1), this earthquake involved low‐angle thrust faulting with a slip direction almost perpendicular to the margin, a consequence of slip partitioning at this obliquely convergent margin. To refine regional source models, we investigated the coseismic and postseismic displacements using a network of Global Positioning System (GPS) sites. Coseismic movement at the campaign site closest to the coast and about 30 km from the epicenter is 115 cm to the south‐southwest, with 30 cm of subsidence. The only continuously recording GPS station on Haida Gwaii at the time of the earthquake, about 80 km from the epicenter, provides a robust coseismic displacement estimate of 22 cm to the south‐southwest. The coseismic results are consistent with a shallow‐dipping thrust rupture underlying the Queen Charlotte terrace, immediately seaward of the Queen Charlotte fault. These results allowed us to update rupture models that were originally derived from seismic waveforms and tsunami data. Estimates of cumulative postseismic horizontal displacements over about one year from seven sites are up to 6 cm, with a systematic along‐strike variation in azimuth from south‐southwest to southeast from north to south in the study area. Besides viscoelastic stress relaxation, these postseismic trends may indicate afterslip on the deeper plate interface beneath northern Moresby Island and possibly aseismic slip along the Queen Charlotte or subparallel faults offshore of southern Moresby Island. Online Material: Figures of three‐component daily position estimates from Bernese analysis at Global Positioning System tracking stations on Haida Gwaii and the adjacent mainland.

High‐resolution station‐based diurnal ionospheric total electron content (TEC) from dual‐frequency GPS observations

AbstractTotal electron content (TEC) estimates derived from Global Navigation Satellite System (GNSS) signal delays provide a rich source of information about the Earth's ionosphere. Networks of Global Positioning System (GPS) receivers data can be used to represent the ionosphere by a Global Ionospheric Map (GIM). Data input for GIMs is dual‐frequency GNSS‐only or a mixture of GNSS and altimetry observations. Parameterization of GNSS‐only GIMs approaches the ionosphere as a single‐layer model (SLM) to determine GPS TEC models over a region. Limitations in GNSS‐only GIM TEC are due largely to the nonhomogenous global distribution of GPS tracking stations with large data gaps over the oceans. The utility of slant GPS ionospheric‐induced path delays for high temporal resolution from a single‐station data rate offers better representation of TEC over a small region. A station‐based vertical TEC (TECV) approach modifies the traditional single‐layer model (SLM) GPS TEC method by introducing a zenith angle weighting (ZAW) filter to capture signal delays from mostly near‐zenith satellite passes. Comparison with GIMs shows the station‐dependent TEC (SD‐TEC) model exhibits robust performance under variable space weather conditions. The SD‐TEC model was applied to investigate ionospheric TEC variability during the geomagnetic storm event of 9 March 2012 at midlatitude station NJJJ located in New Jersey, USA. The high temporal resolution TEC results suggest TEC production and loss rate differences before, during, and after the storm.

Clarifying the Relationship between Quality of Global Positioning System Data and Precision of Positioning

This study selected five indexes to determine the relationship between data quality and positioning precision. These indexes are multipath on L1, multipath on L2, observations by cycle slips, clock offset, and clock stability. To reduce the number of common errors, six types of global positioning system (GPS) receivers are selected and set up in an Ultra-Short Distance Network at the National Measurement Laboratory, Taiwan, for 3 days. The first three indicators are calculated with the Teqc software, and the last two indicators are computed using the Bernese software. Moreover, the observations from three GPS tracking stations, namely, short baseline (about 25 km), middle baseline (about 150 km), and long baseline (around 2000 km), were downloaded from the Internet and processed using Bernese software. Analytical results indicate that clock stability and observations divided by cycle slips are found to be very important positioning factors. Clock offset and multipath effects do not significantly affect th...

Identifying the degraded environment and bad receivers setting by using the GPS data quality indices

To ensure the accuracy of positioning, an accurate and efficient system for calibrating the Global Positioning System (GPS) receivers must be constructed, especially when thousands of GPS tracking stations around the world are established for a variety of applications. In Taiwan, an auto-analytical system for analysing data quality is already established to monitor the GPS stations. However, the relationships between the data quality and positioning precision are not clearly addressed yet, and this is the main goal we want to achieve. In this study, six indices are chosen to characterize the relationship between the data quality and GPS positioning precision. They are the number of observations, multipath on L1, multipath on L2, occurrence of cycle slips, relative frequency offset and frequency instability. To reduce common errors, fourteen types of GPS receivers were selected and set up at an Ultra-Short Distance Network of NML (National Measurement Laboratory, Taiwan) in the years 2002, 2003 and 2004. The results of this study indicate that the frequency instability is the most critical factor among the six indices of interest to influence the GPS positioning precision. The frequency instability may cause a positioning difference of as much as 17.5 mm in the precision of three-dimensional coordinates. The second and third most critical error sources are the number of observations and the multipath effect, respectively, with a maximum difference of 10.7 mm. In contrast, for the method of parameter estimation, the cycle slips and relative frequency offset have minor effects on positioning precision with a few occasional exceptions. The findings of this study will serve as a crucial indicator to classify the positioning error of the tracking stations for long term analysis.

Automatic data-quality monitoring for continuous GPS tracking stations in Taiwan

Taiwan has more than 300 Global Positioning System (GPS) tracking stations maintained by the Ministry of the Interior (MOI), Academia Sinica, the Central Weather Bureau and the Central Geological Survey. In the future, GPS tracking stations may replace the GPS control points after being given a legal status. Hence, the data quality of the tracking stations is an increasingly significant factor. This study considers the feasibility of establishing a system for monitoring GPS receivers. This investigation employs many data-quality indices and examines the relationship of these indices and the positioning precision. The frequency stability of the GPS receiver is the most important index; the cycle slip is the second index and the multipath is the third index. An auto-analytical system for analysing GPS data quality and monitoring the MOI's tracking stations can quickly find and resolve problems, or changes in station environment, to maintain high data quality for the tracking stations.

Geodetic and geophysical results from a Taiwan airborne gravity survey: Data reduction and accuracy assessment

An airborne gravity survey was conducted over Taiwan using a LaCoste and Romberg (LCR) System II air‐sea gravimeter with gravity and global positioning system (GPS) data sampled at 1 Hz. The aircraft trajectories were determined using a GPS network kinematic adjustment relative to eight GPS tracking stations. Long‐wavelength errors in position are reduced when doing numerical differentiations for velocity and acceleration. A procedure for computing resolvable wavelength of error‐free airborne gravimetry is derived. The accuracy requirements of position, velocity, and accelerations for a 1‐mgal accuracy in gravity anomaly are derived. GPS will fulfill these requirements except for vertical acceleration. An iterative Gaussian filter is used to reduce errors in vertical acceleration. A compromising filter width for noise reduction and gravity detail is 150 s. The airborne gravity anomalies are compared with surface values, and large differences are found over high mountains where the gravity field is rough and surface data density is low. The root mean square (RMS) crossover differences before and after a bias‐only adjustment are 4.92 and 2.88 mgal, the latter corresponding to a 2‐mgal standard error in gravity anomaly. Repeatability analyses at two survey lines suggest that GPS is the dominating factor affecting the repeatability. Fourier transform and least‐squares collocation are used for downward continuation, and the latter produces a better result. Two geoid models are computed, one using airborne and surface gravity data and the other using surface data only, and the former yields a better agreement with the GPS‐derived geoidal heights. Bouguer anomalies derived from airborne gravity by a rigorous numerical integration reveal important tectonic features.

A New Ionosphere Monitoring Technology Based on GPS

Although global positioning system (GPS) was originally planned as a satellite-based radio-navigation system for military purposes, civilian users have significantly increased their access to the system for both, commercial and scientific applications. Almost 400 permanent GPS tracking stations have been stablished around the globe with the main purpose of supporting scientific research. In addition, several GPS receivers on board of low Earth orbit satellites fitted with special antennas that focus on Earth's horizon, are tracking the radio signals broadcasted by the high-orbiting GPS satellites, as they rise and set on Earth horizon. The data of these ground and space-born GPS receivers, readily accessible through Internet in a ‘virtual observatory’ managed by the International GPS Service, are extensively used for many researches and might possibly ignite a revolution in Earth remote sensing. By measuring the changes in the time it takes for the GPS signals to arrive at the receiver as they travel through Earth's atmosphere, scientists can derive a surprising amount of information about the Earth's ionosphere, a turbulent shroud of charged particles that, when stimulated by solar flares, can disrupt communications around the world. This contribution presents a methodology to obtain high temporal resolution images of the ionospheric electron content that lead to two-dimensional vertical total electron content maps and three-dimensional electron density distribution. Some exemplifying results are shown at the end of the paper.

Quiet and perturbed ionospheric representation according to the electron content from GPS signals

Signals from Global Positioning System (GPS) satellites received at the surface of the Earth have passed through the terrestrial atmosphere and are therefore affected by refraction in the ionosphere. A large number of permanent dual frequencies GPS tracking stations have been built up in the last years and their data have proved to be suitable to study the ionosphere. The main goal of this paper is to assess the capability of these observations to continuously and routinely monitor the ionosphere at a global scale. The interest was focused on retrieving the coefficients of a spherical harmonics expansion that describe the global distribution of the vertical total electron content (VTEC) from the GPS signal. To test our results, we compare them with other GPS‐derived results, with VTEC values computed with the International Reference Ionosphere (IRI‐95) model, and with direct VTEC determinations provided by Topex‐Poseidon satellite. A large set of global VTEC maps with a time resolution of 2 hours was used to describe the evolution of this ionospheric variable at quiet geomagnetic periods during the year 1997. The outstanding features of the VTEC during the 15 May 1997 geomagnetic storm have also been studied. The results show that our method is able to identify large‐scale features and seasonal variations of the VTEC in quiet conditions, as well as its variations during a large geomagnetic storm.

Comparing vertical total electron content from GPS, Bent and IRI models with TOPEX-Poseidon

The Global Positioning System (GPS) has become a powerful and mature geodetic tool widely used for a broad range of technological and scientific applications. The observations of permanent GPS tracking stations, under the management of the International GPS Service (IGS), seem suitable for ionospheric research, providing continuous and quite good world-wide coverage at low cost for the users. TOPEX-Poseidon surveys sea-level heights by measuring the time required for pulses generated by the onboard radar altimeters to bounce back to the satellite from the sea surface. Free electrons in Earth's ionosphere can delay the return of the radar pulses to the satellite, interfering with the accuracy of sea-level measurements. To correct this delay, the satellite's altimeter makes measurements in two channels. The difference between the two measurements provides a measure of the integrated total electron content, between satellite and sea surface. To analyse the quality of different vertical total electron content (VTEC) maps, we compare our empirical model of the VTEC obtained using GPS data (hereafter called La Plata model) with the VTEC measurements of TOPEX-Poseidon and with the VTEC obtained using the Bent and IRI models. The Bent and IRI models provide monthly averages in the non—auroral ionosphere for magnetically quiet conditions and they are ones of the classical global ionospheric models used as referent in many ionospheric researches. On other hand we have that La Plata model provides a numerical ionospheric model at “any time” using GPS measurements; in disturbed magnetic activity it provides also a mean representation of ionosphere that are very different that in quiet conditions. We make comparison between the different models in quiet geomagnetic conditions because Bent and IRI models were developed to work in these conditions. So in this study we chose some selected quiet geomagnetic days during the year 1997. In our analysis we can see that the GPS model has the best global VTEC representation at any latitude and longitude even without modelling the ionospheric anomaly.