Shaowei Han

Multipath Mitigation of Continuous GPS Measurements Using an Adaptive Filter

Though state-of-the-art dual-frequency receivers are employed in the continuous Global Positioning System (CGPS) arrays, the CGPS coordinate time series are typically very noisy due to the effects of atmospheric biases, multipath, receiver noise, and so on, with multipath generally being considered the major noise contributor. An adaptive finite-duration impulse response filter, based on a least-mean-square algorithm, has been developed to derive a relatively noise-free time series from the CGPS results. Furthermore, this algorithm is suitable for real-time applications.Numerical simulation studies indicate that the adaptive filters is a powerful signal decomposer, which can significantly mitigate multipath effects. By applying the filter to both pseudorange and carrier phase multipath sequences derived from some experimental GPS data, multipath models have been reliably derived. It is found that the best multipath mitigation strategy is forward filtering using data on two adjacent days, which reduces the standard deviations of the pseudorange multipath time series to about one fourth its magnitude before correction and to about half in the case of carrier phase. The filter has been successfully applied to the pseudorange multipath sequences derived from CGPS data. The benefit of this techniques is that the affected observable sequences can be corrected, and then these corrected observables can be used to improve the quality of the GPS coordinate results.

Improving the computational efficiency of the ambiguity function algorithm

Techniques are described in this paper for improving the Ambiguity Function Method (AFM) for differential GPS positioning using phase observations, (a) that take advantage of optimal dual-frequency observable combinations to improve thereliability of the AFM, and (b) that significantly shorten the computation time necessary for the AFM. The procedure can be used for kinematic positioning applications if a Kalman filter predicted position is accurate enough as an initial position for the suggested AFM searching procedure, or pseudokinematic mode using say a triple-difference solution as an initial position for static positioning if the baseline length is short (typically <5km).

Single-Epoch Ambiguity Resolution for Real-Time GPS Attitude Determination with the Aid of One-Dimensional Optical Fiber Gyro

High-accuracy real-time GPS-based attitude determination requires that integer ambiguities be resolved very quickly so that the attitude angles can be output with minimum delay. This article describes an attitude determination algorithm that can resolve integer ambiguities instantaneously, relative to one antenna of a multi-antenna array configuration. The carrier phase and pseudorange observations are used with fixed baseline length constraints and fiberoptic gyro data. Real-time stochastic model improvement using empirical elevation-dependent standard deviation function and an estimated scale factor are a feature of this algorithm. Integer ambiguity search using the LAMBDA method, sophisticated validation criteria, and an adaptive procedure has also been implemented within the software. An experiment was carried out using four Leica dual-frequency GPS receivers (but only the L1 carrier phase and pseudorange data were used) and a low-cost fiberoptic on a car. The results indicate that integer ambiguities can be resolved on a single-epoch basis with a 98.9% success rate.

GPS seismometers with up to 20 Hz sampling rate

The large near-field displacements before and during an earthquake are invaluable information for earthquake source study and for the detection of slow/silent quakes or pre-seismic crustal deformation events. However due to bandwidth limitations and saturation current seismometers cannot measure many of these displacements directly. In a joint experiment between the University of New South Wales (UNSW) and the Meteorological Research Institute (MRI), two Trimble MS750 GPS receivers were used in the Real-Time Kinematic (RTK) mode with a fast sampling rate of up to 20 Hz to test the feasibility of a “GPS seismometer” in measuring displacements directly. The GPS antenna, an accelerometer, and a velocimeter were installed on the roof of an earthquake shake-simulator truck. The simulated seismic waveforms resolved from the RTK time series are in very good agreement with the results from the accelerometer and the velocimeter, after integrating twice and once respectively. Moreover, more displacement information are revealed in the GPS RTK results although they are noisier.

GPS MULTIPATH CHANGE DETECTION IN PERMANENT GPS STATIONS

Abstract We have proposed a technique based on an adaptive filter using the least-mean-square algorithm for detection of multipath change in permanent, continuously operating GPS (CGPS) stations. We have tested the technique on some experimental data, indicating that the multipath change smaller than the receiver noise level will go undetected. We have also conducted further tests with some CGPS data from the Japanese GEONET when there was a snowfall. The results show that when there is a change in the antenna environment it will indeed be detected in both the pseudo-range and carrier phase data. This technique provides an easy-to-implement, quality assurance tool for CGPS antenna environmental sensing after events such as typhoons, cyclones, snowfalls, volcano eruptions, earthquakes, etc. Other possible applications include the monitoring of slope stability and ground subsidence.

Low-cost densification of permanent GPS networks for natural hazard mitigation: First tests on GSI’s GEONET network

Researchers from The University of New South Wales (UNSW), Australia, and from the Geographical Survey Institute (GSI), Japan, have commenced a joint project to develop, deploy and test an innovative hardware/software system design for an automatic, continuously-operated ground deformation monitoring system based on low-cost GPS receiver technology. Conventional continuously-operated GPS (CGPS) networks, such as the one established in Japan by GSI to precisely measure earth surface movement, are very expensive. The high cost being primarily due to the fact that dual-frequency receivers are used. Japan’s nationwide GEONET network is the world’s largest, numbering nearly 1000 receiver stations, with an average station spacing of the order of 30 km. In order to densify such CGPS networks (important when high spatial resolution for the monitoring of the deformation phenomenon is required), and to promote the use of the CGPS technique in lesser developed countries, a significantly cheaper system architecture is needed. The proposed design is an integrated, dual-mode network consisting of low-cost, single-frequency GPS receivers across the area of interest, surrounded by a sparser network of dual-frequency GPS receivers. Initial tests of data collected at selected stations in the GEONET network have already shown that through enhanced data processing algorithms a CGPS network containing both single-frequency and dual-frequency receivers would be able to deliver better than centimetre level accuracies, at considerably lower cost than present systems based exclusively on dual-frequency instrumentation. This paper reports the results of the first field test of this new CGPS system design, in the Tsukuba area of Japan, in August 1999. The test network consisted of: (a) several stations of the GEONET network surrounding (b) an inner network of four single-frequency Canadian Marconi GPS receivers installed by UNSW researchers. The data from both the GEONET and the UNSW receivers were processed using a specially modified version of the Bernese GPS Software Package. The software first processes the GEONET GPS station data in order to generate empirical corrections which are then applied to the double-differenced data of the GPS baselines located within the test area enclosed by the dual-frequency CGPS stations. These corrections have the effect of improving baseline solution accuracy by up to an order of magnitude, even for baselines ranging up to 100 km in length. The baselines connecting the inner network to the surrounding GEONET stations are processed in a number of modes, including 24 hr files (as is the standard practice for geodynamic applications) and hourly data files (as in volcano deformation monitoring applications). The results indicate that single-frequency-with-correction processing can achieve accuracies of better than 5 mm in the horizontal components and 3 cm in height, while the dual-frequency results can achieve accuracies better than 2 mm in the horizontal components and 6 mm in height. In the authors’ opinion, for certain geodynamic applications there are no significant differences between the single-frequency-with-correction results and the dual-frequency results, especially for the horizontal components.

Pseudo-Satellite Applications in Deformation Monitoring

In this article, three general classes of potential pseudolite applications for deformation monitoring are described. The first is GPS augmentation with pseudolite(s), which is suitable for circumstances such as urban canyons, or for monitoring in valleys and deep open-cut mines. The second is indoor applications of pseudolite deformation monitoring systems. Pseudolite arrays can, in principle, completely replace the GPS satellite constellation. This could extend the “satellite-based” deformation monitoring applications into tunnels or underground, where GPS satellite signals cannot be tracked. The last case is an inverted pseudolite-based deformation monitoring system, where a “constellation” of GPS receivers with precisely known “orbits” track a mobile pseudolite. The system consists of an array of GPS receivers, the base reference pseudolite, the mobile pseudolite, and a central processing system. However, in the case of such pseudolite-only or hybrid pseudolite-GPS deformation monitoring systems, some additional issues need to be addressed. These include multipath, atmospheric delay effects, and pseudolite location-dependent biases. To address deformation monitoring applications, some practical procedures to mitigate or eliminate their influence are suggested. Some experiments were carried out using NovAtel GPS receivers and IntegriNautics IN200CXL pseudolite instruments. The experimental results indicate that the accuracy of the height component can indeed be significantly improved – the RMS of the vertical component has been reduced by a factor of 4, to the same level as the horizontal components. Their performance will be demonstrated through case study example.

GPS MULTIPATH MITIGATION USING FIR FILTERS

Abstract All applications of the GPS technology, from those requiring the highest accuracy (such as for deformation monitoring, or for establishing survey control), to those with modest accuracy requirements, suffer due to the presence of signal multipath. This paper describes the effects of multipath on pseudo-range and carrier phase observations. An analysis has been made of the multipath signature at static sites and a technique based on finite impulse response (FIR) filters is proposed for extracting or modelling multipath from GPS observations, which has been used to subsequently correct multipath effects. Two experiments, one focussing on pseudo-range multipath and the other on carrier phase multipath, have shown that the multipath effects can be significantly reduced using the proposed method.

Sea Surface Determination Using Long-Range Kinematic GPS Positioning and Laser Airborne Depth Sounder Techniques

Precise long-range kinematic GPS positioning requires the use of carrier phase measurements, the data processing of which suffers from the technical challenges of ambiguity resolution and cycle slip repair. In this paper, the combination of an ambiguity recovery technique and a linear bias correction method has been used to overcome such problems. An experiment was conducted to test the utility of this technique to determine aircraft height to high accuracy, over very long baselines (of the order of one thousand kilometres), in support of the Laser Airborne Depth Sounder (LADS). From a comparison of four independently derived trajectories, this airborne GPS kinematic positioning experiment has confirmed that the sea surface can be determined to centimetre accuracy. The sea surface profiles thus obtained can be used to correct the errors introduced by long period ocean swells.

Instantaneous Ambiguity Resolution for Medium-Range GPS Kinematic Positioning Using Multiple Reference Stations

A linear combination functional model, formed from the single-differenced functional equation for baselines from a mobile receiver to three or more reference receivers, is proposed, in which the orbit bias and ionospheric delay can be eliminated, and, in addition, the tropospheric delay, multipath and observation noise can be significantly reduced. As a result, the ambiguity resolution technique that can be employed for medium-range GPS kinematic positioning is similar to that used for the short-range case. Kinematic tests have been carried out in Sydney, Australia, with separations from the nearest reference stations greater than 30km. The carrier phase ambiguities can be resolved for every epoch and the success rate (correct identification of the integer ambiguities) was 100%. This technique is well suited to real-time precise GPS kinematic positioning.