Chalermchon Satirapod

Insight into the 2004 Sumatra–Andaman earthquake from GPS measurements in southeast Asia

Data collected at ∼60 Global Positioning System (GPS) sites in southeast Asia show the crustal deformation caused by the 26 December 2004 Sumatra–Andaman earthquake at an unprecedented large scale. Small but significant co-seismic jumps are clearly detected more than 3,000 km from the earthquake epicentre. The nearest sites, still more than 400 km away, show displacements of 10 cm or more. Here we show that the rupture plane for this earthquake must have been at least 1,000 km long and that non-homogeneous slip is required to fit the large displacement gradients revealed by the GPS measurements. Our kinematic analysis of the GPS recordings indicates that the centroid of released deformation is located at least 200 km north of the seismological epicentre. It also provides evidence that the rupture propagated northward sufficiently fast for stations in northern Thailand to have reached their final positions less than 10 min after the earthquake, hence ruling out the hypothesis of a silent slow aseismic rupture.

MULTIPATH MITIGATION BY WAVELET ANALYSIS FOR GPS BASE STATION APPLICATIONS

Abstract It is well known that multipath disturbance is one of the major error sources impacting on high precision GPS positioning. The multipath disturbance is largely dependent on the receivers environment since satellite signals can arrive at the receiver via multiple paths, due to reflections from nearby objects such as trees, buildings, vehicles, etc. Although the multipath effect can be reduced by choosing sites without multipath reflectors or by using choke-ring antennas to mitigate the reflected signal, it is difficult to eliminate all multipath effects from GPS observations. Since the geometry between the GPS satellites and a specific receiver-reflector location repeats every sidereal day, multipath tends to exhibit the same pattern between consecutive days. This repetition can then be useful for verifying the presence of multipath through the analysis of observations made at a static receiver on different days. In this study, the authors apply a wavelet decomposition technique to extract multipath from GPS observations. The extracted multipath signature is then applied directly to the GPS observations to correct for the multipath effects. The results show that the proposed method can be used to significantly mitigate the multipath effects at a permanent GPS station.

A SIMPLIFIED MINQUE PROCEDURE FOR THE ESTIMATION OF VARIANCE-COVARIANCE COMPONENTS OF GPS OBSERVABLES

Abstract Minimum Norm Quadratic Unbiased Estimation (MINQUE) is one of the commonly used methods for the estimation of variance-covariance components. The MINQUE procedure has been successfully used to estimate the variance-covariance components of GPS observations. However, the MINQUE procedure is a big computational burden, and the requirement of having an equal number of variancecovariance components in the estimation step is a major limitation. It is therefore difficult to implement this procedure when the number of observed satellites has changed during an observation period. In this paper, a simplified MINQUE procedure is proposed in which the computational load and time are significantly reduced The qualify of the results obtained is similar to those from the rigorous procedure. Furthermore, the effect of changing the number of satellites on the computations is effectively dealt with.

GPS SINGLE POINT POSITIONING WITH SA OFF: HOW ACCURATE CAN WE GET?

Abstract The announcement by U.S. President Bill Clinton to turn off ‘Selective Availability’ at midnight (Washington DC time) 1 May 2000 caught everyone by surprise. This decision to stop degrading the accuracy of the Global Positioning systems (GPS) Standard Positioning Service (SPS) will have a significant impact on users all over the world. One important outcome for the geomatics community is that the substantial improvement to GPS Single Point Positioning (SPP) results could make it an attractive alternative to the Differential GPS (DGPS) mode of positioning. In this paper some results from different SPP processing strategies are presented, and the accuracy improvement in the case of averaged static solutions is discussed. Finally, the paper speculates on the attainable SPP accuracy using single-frequency GPS receivers when the ionospheric activity is less intense in the future.

Deformation of Thailand as Detected by GPS Measurements due to the December 26th, 2004 Mega-Thrust Earthquake

Abstract The Mw 9.3 mega-thrust earthquake on December 26th, 2004 off the coast of North Sumatra, Indonesia has resulted in large co- and post-seismic motions throughout SE Asia. As a result, the geodetic network of Thailand has been deformed. The THAICA network has been regularly observed with GPS since 1994. Therefore the continuous tectonic motions in Thailand, which is located on the Sundaland block are well known. The last GPS campaign prior to the mega-thrust earthquake took place in October 2004 as part of the EU-ASEAN funded SEAMERGES project. Shortly after the earthquake, the GPS campaigns were repeated in January 2005 and February 2005 to determine the displacements. The GPS data in Thailand were processed, using the Precise Point Positioning strategy of the GIPSY software package. Data from the International GPS Service (IGS) were included, to obtain the positions and velocities of the Thai GPS sites in the International Terrestrial Reference Frame (ITRF) solution of 2000. Since the coordinate time series of the Thai geodetic network spanning up to a decade are available, the absolute steady-state velocities prior to the earthquake are known with uncertainties below 1 mm/yr. Results obtained from the latest campaign indicated that the earthquake has resulted in the horizontal displacements, ranging from 33 cm in the south, 9 cm in the centre, to about 3 cm in the north and east of Thailand. In addition, it was found that the post-seismic motion due to the earthquake has increased further the displacement at the PHUK station.

Modelling post-seismic displacements in Thai geodetic network due to the Sumatra-Andaman and Nias earthquakes using GPS observations

Abstract It is evident that the 2004 Sumatra-Andaman, 2005 Nias and 2007 Bengkulu earthquakes caused significantly large co-seismic and post-seismic displacements all over the South-East Asian region. The Thai geodetic network has been severely affected by the 2004 and 2005 earthquakes. Largest post-seismic horizontal displacements were observed in the southern part of Thailand, while moderate and small displacements were seen in the central and northern parts of Thailand. This paper will analyse the GPS observations obtained by the Royal Thai Survey Department GPS campaigns up to the end of 2009. The post-seismic displacements have been shown to follow a main direction which is towards the rupture area. A simple filtering technique is proposed to remove noise from the post-seismic displacements prior to the fitting of the post-seismic displacements with the logarithmic decay function. As a result, the τ log values in the logarithmic decay function at each point were found to be more consistent in both north and east directions. The new fitting results can therefore be used to estimate the coordinate of the zero-order Thai geodetic network to any epoch with millimetre accuracy.

Updating Thai Reference Frame to ITRF2005 Using GPS: Diversion Between ITRF2000 and 2005 in Southeast Asia

Abstract The Thai geodetic network has been regularly observed with Global Positioning System (GPS) since 1994 through several collaborative EU-ASEAN projects such as GEODYSSEA, SEAMERGES and RTSD-Delft. This geodetic network has long been served as a reference frame for Thailand. Previous realisations of the Thai coordinate reference frame were therefore tied to the global International Terrestrial Reference Frame (ITRF) at epochs 1994, 1996 and 2000. After the occurrence of the 9.2 Mw Sumatra-Andaman earthquake on the 26th December 2004, horizontal displacements were evident at different magnitudes in many surrounding countries. The geodetic network within Thailand was also significantly deformed during the earthquake at the centimetre to decimetre level. Large co-seismic horizontal displacements were observed in the southern part of Thailand, while moderate and small displacements were seen in the central and northern parts of Thailand. The Royal Thai Survey Department (RTSD) has been carrying out multiple GPS field campaigns to monitor the post-seismic displacements. This paper will analyse the GPS observations obtained from the RTSD GPS campaigns up to the end of 2008 using the Precise Point Positioning (PPP) strategy of the GIPSY-OASIS II software. It has been demonstrated that by employing the state of the art PPP technique, the users could achieve mm-level of repeatability in the horizontal components and centimetre precision in the vertical direction, for a 24-hr data span from a static site occupied by a geodetic-quality receiver. Coordinate results obtained from each campaign are then mapped to ITRF2000 and ITRF2005 using a number of well-determined global International GNSS service (IGS) sites. By comparing coordinate results between ITRF2000 and ITRF2005, it is evident that there is a significant diversion in the north component at a rate of 1.7 mm per year over Southeast Asia region. Finally, ITRF2005 coordinate results obtained from the latest RTSD GPS campaign (November 2008) will be served as a new coordinate reference frame for Thailand.

STOCHASTIC MODELS USED IN STATIC GPS RELATIVE POSITIONING

Abstract In order to achieve high-accuracy positioning results, GPS carrier phase observations have to be used in the data processing step. It is generally known that there are two important aspects to the optimal processing of GPS observations, the definitions of the so-called functional model and the corresponding stochastic model. The functional model describes the mathematical relationship between the GPS observations and the unknown parameters, while the stochastic model describes the statistics of the GPS observations. The functional model is nowadays sufficiently known, however the definition of the stochastic model still remains a challenging research topic. Data differencing techniques are extensively used for constructing the functional model as they can eliminate many of the troublesome GPS biases, such as the atmospheric bias, the receiver clock bias, the satellite clock bias, and so on. However, some unmodelled biases still remain in the GPS observations following such differencing. The challenge is to find a way to realistically incorporate information on such unmodelled biases into the stochastic model. Recently there has been interest in using three types of data, Signal-to-Noise Ratio, satellite elevation and least-squares residual, as quality indicators for a formulation of the stochastic model. In this paper, fundamental equations for processing of GPS data are explained. The three quality indicators used for a construction of the stochastic model are described. The recent development works in stochastic models for static GPS positioning are reviewed, followed by some concluding remarks and recommendations.

Modelling residual systematic errors in GPS positioning: methodologies and comparative studies

Since its introduction to civilian users in the early 1980s, the Global Positioning System (GPS) has been playing an increasingly important role in high precision surveying and geodetic applications. Like traditional geodetic network adjustment, data processing for precise GPS static positioning is invariably performed using the least squares method, which requires both functional and stochastic models. A double-differencing technique is commonly used for constructing the functional model in order to account for systematic errors in the observations. In current stochastic models, it is usually assumed that all the one-way measurements have equal variance, and that they are statistically independent. The above functional and stochastic models have therefore been used in standard GPS data processing algorithms.

Effect of Thai Ionospheric Maps (THIM) model on the performance of network based RTK GPS in Thailand

Abstract In 2008, the DOL-NRTK (Department of Lands-Network-based Real Time Kinematic) GPS system was established to support cadastral surveying applications in Thailand using the Virtual Reference Station (VRS) concept. Currently the DOL-NRTK network consists of 11 reference stations located in the Central Plain region of Thailand with spatial coverage ranging from 30 to 125 km with an average spacing around 60 km. A previous study confirmed that ionospheric errors are the main error sources that affect the NRTK performance. Also the ambiguity fixing success rate was significantly improved when the final Global Ionospheric Maps (GIM) from the Center for Orbit Determination in Europe (CODE) were applied. However, the final GIM is the global model and may not perfectly fit with smaller regional test areas in Thailand. Therefore, an investigation on the use of a regional ionospheric model was required. In this paper, the Thai Ionospheric Map (THIM) generated by the Bernese software is described. An investigation on the performance of NRTK with the aid of the THIM and the final GIM in various different reference station spacing, i.e. 10–20, 30–50, 50–60 and 60–80 km are compared. This investigation is performed using a large number of GPS observations (31 consecutive days) and all available Continuous GPS (CGPS) stations in the Central Plain region of Thailand. Test results indicate that the NRTK positioning performance with the aid of the THIM yields the best solutions when compared to the use of the final GIM and no model, especially in the case of the middle reference station spacings (i.e. 30–50 and 50–60 km spacing). Therefore, it can be concluded that the THIM can effectively mitigate the ionospheric bias in the NRTK mode in Thailand.