Dingfa Huang

Detecting Land Subsidence in Shanghai by PS-Networking SAR Interferometry

Existing studies have shown that satellite synthetic aperture radar (SAR) interferometry has two apparent drawbacks, i.e., temporal decorrelation and atmospheric contamination, in the application of deformation mapping. It is however possible to improve deformation analysis by tracking some natural or man-made objects with steady radar reflectivity, i.e., permanent scatterers (PS), in the frame of time series of SAR images acquired over the same area. For detecting land subsidence in Shanghai, China, this paper presents an attempt to explore an approach of PS-neighborhood networking SAR interferometry. With use of 26 ERS-1/2 SAR images acquired 1992 through 2002 over Shanghai, the analysis of subsiding process in time and space is performed on the basis of a strong network which is formed by connecting neighboring PSs according to a distance threshold. The linear and nonlinear subsidence, atmospheric effects as well as topographic errors can be separated effectively in this way. The subsidence velocity field in 10 years over Shanghai is also derived. It was found that the annual subsidence rates in the study area range from -2.1 to -0.6 cm/yr, and the averaged subsidence rate reaches -1.1 cm/yr.

A multi-antenna GPS system for local area deformation monitoring

A new method of using GPS for monitoring local area deformations such as landslides is presented. Unlike the standard method of using GPS for deformation monitoring where a GPS receiver is required for each point to be monitored, the new method allows multiple points to be monitored with one receiver. A system that implements the concept has been developed. It uses a specially designed electronic component that allows a number of GPS antennas to be linked to a single GPS receiver. The receiver takes data sequentially from each of the antennas attached to the receiver. A distinctive advantage of the approach is that one GPS receiver can be used to monitor more than one point. The cost per monitored point is therefore significantly reduced. The design of the system, as well as the data management and processing strategies will be introduced in detail. Results from some preliminary tests will also be given.

Measurement of vibrations of tall buildings with GPS : a case study

Experimental studies have been carried out to employ GPS to measure the vibrations of the 384 m tall Di Wang building in Shenzhen, China. Two GPS receivers were employed in the experiment with one set on the top of the building and the other on a reference station on the ground near-by. The GPS data were collected at a rate of 4 sets per second. The experiment was conducted under different weather conditions over two 24-hour periods. This paper describes the GPS equipment used, the field operations, the data processing and analysis strategies, and the results obtained from the study. It is shown from the study that GPS can be successfully applied for monitoring structural vibrations. However cautions have to be exercised in analyzing GPS data to remove biases such as the so-called GPS multi-path errors. Besides, some practical issues still need resolved before GPS can be applied routinely for this type of applications.

An Integrated System for Slope Monitoring and Warning in Hong Kong

Publisher Summary This chapter presents an integrated system for monitoring and warning of landslides. The system consists of an automatic conventional slope monitoring package and a multiantenna global positioning system (GPS) package. The automated slope-monitoring package has different types of sensors and a remote data acquisition/control system connecting an office computer to datalogger for data communication and control with visual software. The sensors that are employed include in-place inclinometers, piezometers, a rain gauge, and a time domain reflectometer (TDR). The multiantenna GPS is a special technology newly developed for monitoring surface movements of a slope. The two packages or subsystems can work independently or jointly to monitor a slope. When working together, the two subsystems can be set up at the same slope site. The conventional subsystem mainly measures the subsurface ground movements and pore water pressures and the GPS subsystem mainly measure the surface ground movements. The data from the two subsystems can be transferred to the same office computer, processed, and analyzed together. The integrated data can be automatically retrieved, temporarily stored, and transmitted from the two subsystems. Software can perform data download, storage, analysis, and realtime display. All data from in-place inclinometers, piezometers, a rain gauge, a TDR, and the GPS are integrated in the process.

A New Generation of Multi-antenna GPS System for Landslide and Structural Deformation Monitoring

Publisher Summary GPS—an advanced navigation and measurement technology—has been extensively applied to many fields and disciplines over the previous years. One of the prominent applications of GPS is the precise monitoring of deformations of engineering structures and the crust of the Earth. A limiting factor for large-scale use of GPS in such applications is however its high hardware cost. For continuous monitoring of deformations, each point to be monitored is to be equipped with a set of geodetic quality GPS instruments. This makes many applications such as routine monitoring of landslides too expensive in most cases. This chapter proposes a multiantenna GPS system aimed at reducing the cost of GPS when used for monitoring deformations of objects such as slopes. The system uses special hardware and software components to allow one GPS receiver to be connected to a number of GPS antennas. One set of such equipment can therefore be used to monitor a number of points. The system reduces the cost of GPS hardware significantly. The chapter finally describes a new generation of the multiantenna GPS system that consists of integrated hardware and software components for data acquisition, transmission, processing, analysis, and visualization.

AMBIGUITY ONLINE ESTIMATION BY COMBINING DUAL-FREQUENCY PHASE AND CARRIER SMOOTHED CODE MEASUREMENTS: PERFORMANCE ON SHORT BASELINES

Abstract For both static and kinematic relative positioning with GPS, the success of baseline resolution depends closely on the reliable fixing of integer ambiguities of carrier phase measurements. Low noise code correlated dual-frequency GPS receivers offer accurate pseudo-range measurements on both carrier frequencies. With the help of a combination of code and carrier phase measurements, wide-lane ambiguities can be easily determined and used for the improvement of ambiguity resolutions on both L1 and L2 frequencies. A carrier smoothing technique is utilized to eliminate the noise of code measurements. Baselines ranging from 1.5 km to 48 km are used to test the proposed method.

The Analysis and Modeling of Intersystem Biases Between GPS and BDS

Receiver hardware delay is one of the error sources which affect ambiguity resolution and positioning. In this contribution, differential receiver hardware delay between GPS and BDS, namely differential phase and code intersystem bias (ISB), are estimated together with other parameters using single differential (SD) observation, and analyzed by Fast Fourier Transform (FFT). The experiment shows that the differential ISBs are close to zero for the baseline using two identical receivers, and can be neglected when taking the measurement noise of phase and code into account. However they become significant for receivers which are of different types. According to FFT, the ISB time series only contains the constant term, and no trend and periodic terms are included. The model can be established on the basis of the characteristics of ISB and is used for prediction and calibration. Taking into account the ISBs correction, the success rate of ambiguity resolution is up to 99%. The mean values are −0.3, 0.6, −1.7 mm and standard deviations are 2.9, 3.4, 6.3 mm for baseline component in North, East, and Up directions, respectively. The result shows that the prediction and calibration for ISB are feasible.

Kinematic crustal deformation features of Wenchuan earthquake obtained by GPS

Global Positioning System (GPS) plays an important role in monitoring crustal deformation. Traditional methods for GPS crustal deformation monitoring is mainly based on static positioning, which cannot be used to obtain instantaneous crustal deformation. To monitor crustal deformation in Sichuan, 12 continuous operation GPS reference stations (CORS) have been setup in Sichuan, China. There are 5 GPS stations located at the east side of Longmen mountain fault. GPS data for a period of about 60 seconds has been recorded during Wenchuan earthquake. Using data with 1-second interval from Sichuan GPS network, we compute the kinematic crustal deformation caused by MS 8.0 Wenchuan earthquake to study the crustal deformation characteristics during Wenchuan earthquake. GPS baselines before and after the earthquake were processed by GAMIT, and then the coordinates of all stations were combined by GLOBK using China local frame. After selecting relative stable points from the 12 GPS stations based on GPS data of 4 consecutive days from May 11 to May 14, 2008, we have obtained the kinematic crustal deformations lasting about 60 seconds by using Yaan as a reference station. Results show that the principal displacement direction of the stations in Sichuan GPS network was approximately orthogonal to Longmen mountain fault direction. The principal deformation pattern was expansion towards northwest with periodical vibrations. Mianyangs maximum kinematic horizontal displacement was 49.2cm towards NW50°, Pixians maximum kinematic horizontal deformation was about 114.1cm towards NW45° and Chengdus deformation was 21cm toward NE45°. Decomposing the deformation series into Longmen mountain fault direction and its orthogonal direction, the kinematic deformation features of several stations were obtained. Results also show that Mianyan and Pixian moved towards Northwest with small amplitude of swinging during the earthquake. The height of Mianyan decreased 2 cm after uplifting 10 cm in the vertical direction. The height of Pixian fluctuated periodically and the fluctuation range was about ±13 cm.

GPS phase measure cycle-slip detecting and GPS base-line resolution based on wavelet transformation

The errors in GPS measurement generally consist of systematic error (such as clock bias, iron-sphere, trop- sphere effect, etc.), random error (such as measuring error) and outlier. Systematic error can be canceled by differencing technique or adding parameters into the equation system. Outlier may be detected by adding parameters or by statistic method such as expectation shifting or variance inflating. Because wavelet analysis has many good natures both in the time domain and in the frequency domain, as can automatically zoom in or out with different scales (frequencies), the arbitrary details of a signal can be observed and analyzed by the aid of Wavelet analysis. Based on the above features, Wavelet analysis is reputed as a mathematical microscope.

Fast baseline resolution algorithm for GPS kinematic positioning based on probability computation

GPS precise positioning depends on the right determination of phase ambiguity vector. Generally there are three methods for integer ambiguity resolution, including direct rounding to integer method, searching method and ambiguity function method. Direct rounding to integer method is the fastest algorithm among the three methods, but it requires highly precise approximation of the ambiguities, which usually can be only obtained through long term of measurement. The efficiency (speed) of searching method has a very close relationship to the precision of real-valued ambiguity solution, and usually this method is very slow, especially for the data which comes from short time measuring. The efficiency of ambiguity function method is determined by the accuracy of approximation position and the volume of the cube centered at the initial position. The original ambiguities are transformed into another space by a modified ambiguity transformation algorithm purposed by this paper, which is the most effective one compared to the others. In the new ambiguity space, new ambiguities are far more precise than their original partners. Then we compute the successful probability of direct rounding to integer method both in original space and new space. If the success probability is larger than a given value, then direct rounding to integer method is used to obtain the true ambiguities, otherwise go to a subroutine for the searching method.