Journal of Global Positioning Systems

Vol. 4, No. 1& No. 2, 2005

Journal of Global Positioning Systems
Vol. 4, No. 1& No. 2, 2005
ISSN 1446-3156 (Print Version)
ISSN 1446-3164 (CD Version)

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JGPS Team Structure, Copyright

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Table of Contents

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Chris Rizos

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Ahmed El-Mowafy

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The use of multiple real-time reference stations (RTK Networks) for positioning during the aircraft's precision approach and airport surface navigation is investigated. These existing networks can replace the proposed airport LAAS systems and have the advantage of improving coverage area. Real-time testing of the proposed technique was carried out in Dubai, UAE, with a helicopter and a small fixed-wing aircraft using a network known as the Dubai Virtual Reference System (DVRS). Results proved the feasibility of the proposed approach as they showed that cm to sub-meter positioning accuracy was achieved most of the time. For some periods, only meter-level positioning accuracy was available due to temporary breaks in reception of the network carrier-phase corrections. Some solutions to improve availability of the corrections are discussed. It is also proposed to integrate the GPS with an IMU. The inertial system aids positioning during periods when the corrections are lost, as well as providing attitude information. The GPS and IMU systems were integrated using a decentralized adaptive Kalman filtering technique. The measurement noise covariance matrix and the system noise matrix are adaptively estimated, taking the aircraft dynamics changes into account. Tests of the integrated system show that it has a good overall performance, and navigation at categories III and II can be achieved during short outages of RTK-GPS network corrections.

Allison Kealy and Stephen Scott-Young

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Augmented reality (AR) technologies enable digitally stored information (virtual objects) to be overlaid graphically on views of the real world. As such, they are able to significantly enhance decision-making and operational efficiency in complex environments. AR technologies typically comprise a fusion of positioning and attitude sensors with visualisation capability and an information processing system. The decreasing size and cost of visualisation and positioning hardware and the increasing portable processing power of laptop and handheld computers now offer enormous potential for the development of intelligent solutions based around real-time, mobile AR technologies.

For any application built around AR technologies, its effectiveness lies in the accuracy to which the virtual objects can be aligned with views of the real world. For many of these applications, this is directly a function of the accuracy to which the position and orientation of the operation platform can be determined. This paper presents an integrated positioning system that combines an array of dual frequency GPS receivers, a fibre optic gyroscope and vehicle odometer within a centralised Kalman filter. It assesses the accuracy of the filter outputs of position and attitude as appropriate to supporting real-time, mobile AR applications. The design and testing of an AR prototype that combines the Kalman filter state with real-time imagery containing augmented objects will also be presented. Finally, approaches adopted to tune the filter and reduce inherent sensor noise, as well as results from a case study undertaken within the land mobile environment will be described.

Binghao Li, Chris Rizos and Hyung Keun Lee

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Network mobilephone-based positioning experiences degradation of location accuracy due to localised non-line-of-sight (NLOS) signal propagation. This is well known to be a major source of error in network-based mobilephone positioning. NLOS error systematically causes the Mobile Station (MS) to appear further away from the base station than it actually is, thereby increasing the positioning error. One method to mitigate the effect of NLOS error is to generate a NLOS error correction map, and then use the correction map to rectify the distorted MS location. The correction map can be generated using the following procedure: (1) estimating the NLOS errors at points where the real positions can be obtained utilising other information such as the points very near BTS (Base Transceiver Station) and the intersections of streets, or the location where the measurement has been made; and (2) interpolating or extrapolating the errors to specific points that we are interested in. Assuming some reference points have been obtained, this paper utilises kriging, an estimation technique that is widely used in mining, to generate the correction map. Theoretically kriging can also be used wherever a continuous measure is made on a sample at a particular location in space or time. Using simulations with a typical dense urban environment assumption, the feature of the NLOS error variogram is analysed and different models of the variogram are compared. The correction map of NLOS error is generated using some 'sampled' points, and compared with the 'true' NLOS error map to show the efficiency of kriging.

Chalermchon Satirapod and Prapod Chalermwattanachai

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It is generally known that the atmospheric effects on the GPS signals are the most dominant spatially correlated biases. The atmosphere causing the delay in GPS signals consists of two main layers, ionosphere and troposphere. The ionospheric bias can be mitigated using dual frequency receivers. Unlike the ionospheric bias, the tropospheric bias cannot be removed using the same procedure. Compensation for the tropospheric bias is often carried out using a standard tropospheric model. Most standard tropospheric models were experimentally derived using available radiosonde data, which were mostly observed on the European and North American continents. In order to determine the best-fit standard tropospheric model with the GPS data collected in Thailand, investigations on the impact of different standard tropospheric models on GPS baseline accuracy are therefore needed. This paper aims to compare the GPS positioning results derived from the use of three different standard tropospheric models, namely the Saastamoinen model, Hopfield model and Simplified Hopfield model. In this study, both short and medium length baseline data sets were tested. In addition, each baseline data set is further divided into two scenarios, flat terrain and rough terrain. Overall results indicate that there are no statistically significant differences in the performance of the three tropospheric models. However, the use of the Saastamoinen and the Hopfield models tends to produce more reliable results than the use of the Simplified Hopfield model.

Daniel Woo, Nick Mariette, Nigel Helyer and Chris Rizos

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Syren, a location-based, multi-speaker augmented audio reality installation was presented as a shipboard exhibit at the 12th International Symposium on Electronic Art in August 2004. It was conceived as a continuous 3-day spatial audio experience that augments the landscape through the Baltic archipelago with location-based audio media, spatialised through a 12-channel speaker array. As the ship tracks between Helsinki, Mariehamn, Stockholm and Tallinn, listeners on the upper deck hear sounds that are perceived to originate from geographic features. Our custom GIS is derived from electronic nautical charting information that includes coastlines, buoys and beacons. A handheld GPS provides both position and direction data that was used by a software system to drive parameters of the spatial audio presentation. The sound production for the artwork was created using the custom application that enabled the artist to place sound media in relation to a real-world map. An important component to this software was the ability to audition the audio experience without ever taking the journey.

Dinesh Kumar-Mills, John Homer, Kurt Kubik and Matt Higgins

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The most advanced and proven implementation of the networked RTK is the VRS network concept. Its requirement of bi-directional communications is a critical disadvantage as this limits the robustness of the system. High costs and coverage limitations are also associated with the types of technology (i.e. UHF, GSM and GPRS) required for VRS communications. The Virtual Reference Cell (VRC) approach can be used to mitigate the disadvantages of the VRS network. This approach generates corrections for a fixed number of cells that are broadcast to the rovers. The drawback of the VRC system is a lower positioning accuracy due to the use of DGPS corrections instead of RTK. This paper proposes an RTK-VRC system whereby advantages of the VRC are maintained while achieving RTK level accuracy, mitigating high communications costs and supporting kinematic applications. The RTK-VRC system is an integration of the VRS network, to provide RTK positioning, and the WCDMA wireless network, to provide the cell structure and communications. For this system a novel communications link will be implemented using the pilot channel of the WCDMA network to minimise the communications costs. The results of a field experiment that utilises the NR&M VRS network in Australia shows that RTK positioning accuracy is achievable for VRS baselines of up to 2 km. This supports the idea of using the WCDMA cells with the RTK-VRC system.

Francis Chan, Jinho Choi and Gyu-In Jee

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A novel multipath mitigation technique for Global Positioning System (GPS) receivers using the Expectation-Maximization (EM) algorithm is proposed. It is well-known that conventional propagation delay estimation using parallel sliding correlators is only optimal in additive white Gaussian noise channel. In practical positioning systems, the weak GPS line-of-sight signal is generally embedded in the multipath signals and other source of interference. Although the GPS direct sequence spread spectrum (DS-SS) signal has inherent resistance to interference, the received superimposed multipath signals, which are possibly coherent, are the dominant source of the propagation delay estimation errors. From the parameter estimation point of view, the problem of multipath mitigation is equivalent to estimating the unknown phases, propagation delays and amplitudes of the superimposed multipath signals. The joint maximum likelihood (ML) estimation of all the unknown parameters is optimal and asymptotically efficient. However it involves multi-dimensional search which is computationally expensive. The proposed coarse/acquisition (C/A) code acquisition system using the EM algorithm is an iterative maximum likelihood estimator which decomposes the multi-parameter estimation problem into a number of separate ML optimizations. The performance of the proposed EM algorithm has been tested by simulations. We have observed that the proposed acquisition system is significantly superior to the conventional correlating receiver in a multipath fading channel.

Hiroshi Isshiki

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In the previous report (Isshiki, 2004b), theory and algorithm of a new dual frequency long baseline kinematic positioning method was discussed. In the theory, the wide-lane coordinates are used as a constraint for obtaining the correct L1 ambiguities by solving the ionosphere free equations. The effectiveness was verified by some numerical examples. A precise positioning for baseline of several hundred kilometeres are possible. In the present report, the effect of epoch interval and observation length is investigated by numerical calculations. The relationship between the positioning error and the baseline length is also discussed. Furthermore, an algorithm for the real time application is shown.

Hongping Zhang, Jinling Wang, WenYao Zhu and Cheng Huang

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Usually, ionospheric Total Electron Content (TEC) variation with time can be viewed as a stationary random process under quiet conditions. However, sudden events of the Sun and the Earth such as solar flare and sudden commencement of geomagnetic storms may induce the disturbances of the ionosphere, so that the stationary random process is broken; the statistical change much. Based on this fact, here we make use of the time series of TEC and the auto-covariance function of the stationary process to construct independent identical distribution Gauss sample so that the test can be used to detect the abnormity hidden in the sequence. In addition, GPS data from several IGS sites in China during the severe solar flare occurred on 14th July, 2000 are used to verify the method. The results indicate that the disturbances caused by the solar flare can be effectively detected.

InSu Lee, Hsing-Chung Chang and Linlin Ge

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Interferometric Synthetic Aperture Radar (InSAR) is a rapidly evolving technique. Spectacular results that are obtained in various fields, such as the monitoring of earthquakes, volcanoes, land subsidence and glacier dynamics, as well as in the construction of Digital Elevation Models (DEMs) of the Earth's surface and the classification of different land types, have demonstrated its strength.

As InSAR is a remote sensing technique, it has various error sources due to the satellite positions and attitude, atmosphere, and others, so it is important to validate its accuracy, especially for the DEM derived from SAR images before it can be used for various applications such as disaster prevention, flood mapping, and emergency map.

In this study, Real Time Kinematic (RTK) GPS positioning and Kinematic GPS positioning were chosen as tools for the validation of InSAR derived DEM. The results showed that Kinematic GPS positioning had greater coverage at field test, i.e. larger number of usable sampling points than RTK GPS. However, tracking satellites and transmitting a data between reference-rover, under trees are still pending tasks to be overcome in GPS positioning techniques. Additionally, Airborne Laser Scanning (ALS) is expected to be an alternative as an effective tool for the validation of DEMs.

Jaewon Seo, Jang Gyu Lee, Chan Gook Park, Hyung Keun Lee and Seong-Baek Kim

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The application of smoothing to integrated navigation system of 4S-Van is considered. 4S-Van is a mobile mapping system, which provides the position information of various objects on the road. For navigation purpose, it has various sensors such as an inertial measurement unit, a GPS receiver and an odometer. It is also equipped with CCD cameras, laser scanners and video cameras, for mapping purpose. The navigation system of 4S-Van is an inertial navigation system, which is aided by GPS and the odometer. Because it can adopt post-processing method for more accurate and reliable result, the nonlinear smoothing is applied. The nonlinear smoothing, which consists of a forward filter, a backward filter and a smoother, is implemented. For the forward filter, the extended Kalman filter is designed, and for the backward filter, the linearized Kalman filter is constituted. In the smoother stage, the results of two filters are combined. The algorithm is applied to experimental data and the obtained result shows the effectiveness and good performance of the nonlinear smoothing.

Jason Zhang, Kefei Zhang, Ron Grenfell, Yong Li and Rod Deakin

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Precise GPS velocity and acceleration determination relies on Doppler and/or Doppler rate observations. There are no direct Doppler rate measurements in GPS. Although every GPS receiver measures Doppler shifts, some receivers output only "raw" Doppler shift measurements and some don't output any at all. In the absence of raw Doppler and Doppler rate measurements, a differentiator is necessary to derive them from other GPS measurements such as the carrier phase observations. For real-time dynamic applications, an ideal differentiator should have a wideband frequency response to cover all the dynamics. It should also have a group delay as short as possible. In addition, a low-order differentiator is more favourable for easy implementation.

This paper provides an overview of methods in differentiator design for applications of GPS velocity and acceleration determination. Low-order Finite Impulse Response (FIR) differentiators proposed by Kavanagh are introduced. A class of first-order Infinite Impulse Response (IIR) differentiators are developed on the basis of Al-Alaoui's novel differentiator. For noise attenuation, it is proposed to selectively use Kavangagh's FIR differentiators, and the first-order IIR filters derived for adaptation to different dynamics.

Jianguo Jack Wang, Jinling Wang, David Sinclair, Leo Watts and Hung Kyu Lee

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Pseudolites, ground-based GPS signal transmitters, can significantly enhance the GPS satellite geometry or can even be an independent positioning system. However, as pseudolites are very close to the receivers, error effects are different from the traditional GPS and should be considered and modeled in a different way. Tropospheric delay is one of the largest error sources of pseudolite positioning, as pseudolite signal propagates through the lower troposphere which is very difficult to be modeled due to spatial variations in atmosphere. The objective of this research is to analyse pseudolite tropospheric delay modelling methods and to select the optimal tropospheric delay models for different applications.

Several methods to estimate the tropospheric delay for pseudolite positioning are introduced and compared. One approach is to utilize single-differenced GPS tropospheric models. Another one is to compute the tropospheric delay as a function of the local refractivity along the pseudolite signal path. The ratio method used for Electronic Distance Measurement (EDM) can also be applied to estimate tropospheric delay.

Experiments with simulation and real flight test data are conducted in this study to investigate the proposed methods. The advantages and limitations of each method are analysed. The mode defined by RTCA and its modification are suitable for a low elevation and short range application, such as LAAS and local ground based applications. Models derived from single-differenced NMF and Saastamoinen models perform well in long range and high elevation but have big bias in low elevation. And the model derived from the Hopfield model performs relatively well in all the range and elevation.

Jinling Wang, Hung Kyu Lee, Young Jin Lee, Tajul Musa and Chris Rizos

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Baseline length-dependent errors in GPS RTK positioning, such as orbit uncertainty, and atmospheric effects, constrain the applicable baseline length between reference and mobile user receiver to perhaps 10-15km. This constraint has led to the development of network-based RTK techniques to model such distance-dependent errors. Although these errors can be effectively mitigated by network-based techniques, the residual errors, attributed to imperfect network functional models, in practice, affect the positioning performance. Since it is too difficult for the functional model to define and/or handle the residual errors, an alternative approach that can be used is to account for these errors (and observation noise) within the stochastic model. In this study, an online stochastic modelling technique for network-based GPS RTK positioning is introduced to adaptively estimate the stochastic model in real time. The basis of the method is to utilise the residuals of the previous segment results in order to estimate the stochastic model at the current epoch. Experimental test results indicate that the proposed stochastic modelling technique improves the performance of the least squares estimation and ambiguity resolution.

Jonghyuk Kim and Salah Sukkarieh

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This paper presents the results of augmenting 6DoF Simultaneous Localisation and Mapping (SLAM) with GNSS/INS navigation system. SLAM algorithm is a feature based terrain aided navigation system that has the capability for online map building, and simultaneously utilising the generated map to constrain the errors in the on-board Inertial Navigation System (INS). In this paper, indirect SLAM is developed based on error analysis and then is integrated to GNSS/INS fusion filter. If GNSS information is available, the system performs feature-based mapping using the GNSS/INS solution. If GNSS is not available, the previously and/or newly generated map is now used to estimate the INS errors. Simulation results will be presented which shows that the system can provide reliable and accurate navigation solutions in GNSS denied environments for an extended period of time.

Kefei Zhang, Youjian Hu, Gangjun Liu, Falin Wu and Rodney Deakin

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This paper investigates the feasibility using continuously operating reference stations (CORS) in Victoria (termed GPSnet) for deformation monitoring and analysis. A number of critical issues associated with the suitability, geological stability, data quality of the GPS networks system, the precision and reliability of the GPSnet solution are investigated using geological information. Appropriate strategies for GPS data processing and deformation analysis are investigated. The absolute and relative displacement of selected GPSnet stations are analysed using chronological GPS CORS data and delicated high precision scientific GPS data processing software packages. The latest International Terrestrial Reference Frame is used for deformation analyses. Detailed data-processing strategies and results of deformation analyses are presented and some useful conclusions are drawn.

Results show that the methodology of deformation analysis and data processing based on the regional CORS network data is feasible and effective. It is concluded that high-precision continuous tracking data from GPSnet is a very valuable asset and can provide a technically-advanced and cost-effective geoscientific infrastructure for deformation monitoring analysis. By mining the data from the GPSnet, not only reliable and high precision deformation information can be potentially obtained, but also high expenditure required for establishing dedicated deformation monitoring networks in this area can also be spared.

K J Parkinson

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The precise time available from the atomic clocks orbiting the earth in GPS satellites is used in many systems where time synchronization is important. The satellite clocks are monitored and adjusted by ground based control telemetry to within one microsecond of Universal Time. A number of commercial GPS receivers have the ability to provide a time synchronised output, typically one pulse per second, that is locked to this precise time base. This easily accessible timing source is often the justification for including a GPS receiver as an integral component of a complex system. There are additional benefits to be gained from integrating a GPS receiver as an embedded component of a mobile radio telemetry system, where GPS information can also be used to enhance the overall performance. This paper examines some research into combining some transmission techniques with time synchronisation from GPS receivers located in the mobile and in the base equipment to improve a digital radio channel. Using this combined approach, a reverse data channel can be eliminated where a single direction data stream is the predominant requirement.

Kyung Ryoon Oh, Jong Chul Kim and Gi Wook Nam

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In this paper, the linearly interpolated PRC (Pseudorange Correction) regenerating algorithm was applied to improve the DGPS positioning accuracy at user's position by using the various PRC information obtained from multi-DGPS reference stations. The unknown user's position can be calculated from the regenerated PRC which can be expressed as the linear combination of multi-DGPS reference station's known position and PRC values of common satellite from multi-DGPS reference stations. Two sets of 3 DGPS reference stations were selected to compare the performance of the linearly interpolated PRC regenerating algorithm. To test the performance, linearly interpolated PRC regenerating algorithm adopted multi-channel DGPS receiver was developed. The results show that the DGPS positioning accuracy is improved by about 40% and with the modification of the navigation solution software of GBAS receiver, GBAS positioning accuracy improvement is expected without any modification of GBAS reference station's equipment.

Maurizio Fantino, Fabio Dovis and Jinling Wang

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The ability to monitor and detect any disturbances on the PRN code signals transmitted from the navigation satellite constellation is of primary importance. It is known that the tracking performance of a navigation receiver stems from the correlation property of the PRN code signals transmitted. These anomalies can be detected in several different ways, either observing the outputs of navigation user receivers, or processing the received signal within the receiver. Quality control is the process that defines how well the solution of a problem is known and in the context of navigation, it consists of assuring an agreed level of accuracy, reliability and robustness for the measurements. In this work a modified version of the conventional tracking scheme will be proposed with the aim of monitoring the quality of the measurements at the signal processing level. The proposed tracking scheme is able to give a measure of the distortion of the correlation function and consequently, of the reliability of the signal tracked. In particular the problem of multipath distortion is considered The amplitude and multipath delay can be estimated with an extension of the linear Kalman Filter which can be implemented inside the traditional DLL architecture. Simulations show that due to its prediction capability, Kalman Filter enhances the robustness of the system when weak signals are present or there is loss of lock on the signals, trading off the performance improvement with an increase in complexity of the new architecture. The recognition of a multipath corrupted signal estimating the amplitude and delay of the reflection can be used to select the more reliable pseudo-range measurements for the evaluation of the positioning equations. Mitigation of the multipath effects may be performed where the number of tracked signals is not sufficient.

Mingquan Lu, Jinling Wang, Ravindra Babu, Dan Li and Zhenming Feng

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In order to improve signal reception performance in GPS/INS/PL (Pseudolite) integration applications, a semi-sphere antenna array is proposed in this paper. It inherits the wide coverage characteristic of conventional spherical arrays and utilizes only about half the number of elements compared to a planar array to cover the upper-semi-sphere space above earth plane. It can process signals from both overhead and horizontal directions at the same time. Thus, unlike common planar arrays, this novel antenna array with a special geometry can receive satellite and Pseudolite (PL) signals from all directions, even from the horizon. Both Capon’s and constraint methods have been used in the simulations of Direction of Arrival (DOA) estimation and beam-forming. These simulation results have demonstrated the advantages of the new array.

P. Alves and G. Lachapelle

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The multiple reference station approach to carrier phase-based positioning uses a network of GPS reference stations to model the correlated errors in a geographic region. This paper compares two methods for multiple reference station positioning under a low and a high level of ionosphere. The first method tested is the conventional method for multiple reference station positioning, which is usually a three-step process, namely (1) estimation of the carrier phase ambiguities in the network, (2) prediction of the measured network errors at the location of the rover, and (3) application of the corrections in a practical format. The second method is called the tightly coupled or in-receiver approach, which uses the data from the rover and integrates it with the network solution to better model the effect of the ionosphere. In this approach there are no explicit corrections. These two methods are compared with the single reference station approach for data from two days collected from the Southern Alberta Network in Canada, a medium scale network with inter-stations distance of 34 to 59 km.

Pavel Kovář, František Vejražka, Libor Seidl and Petr Kačmařík

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The modern satellite navigation system Galileo is developed by European Union. Galileo is a completely civil system that offers various levels of services especially for civil users including service with safety guarantee. Galileo system employs modern signal structure and modern BOC (Binary Offset Carrier) modulation. The Galileo Receiver is investigated in the frame of the GARDA project solved by consortium under leadership of Alenia Spacio - LABEN. The aim of Galileo Receiver Core Technologies subtask is to investigate the key problems of the Galileo receiver development. The Galileo code and carrier tracking subtask of the Galileo Receiver Core Technologies is carried out at the Czech Technical University. The problem was analysed and split to the particular tasks. The aim of this paper is focused on BOC correlator architecture. The correlation function of the BOC modulation is more complex with a plenty of correlation peaks. The delay discriminator characteristic of such signal has several stable nodes, which cause stability problem. The standard solutions of this problem like BOC non-coherent processing, very early - very late correlator and deconvolution correlator are analysed. The new correlator architecture for BOC modulation processing has been developed. The developed correlator has two outputs, one for fine tracking and the second one for correct node detection. The second output is based on comparison of the correlation function envelopes. The simplified method of correlation function envelope calculation is described in this paper. The correlator is planned to be tested in the GRANADA software simulator including a sophisticated method of correlator output combination.

Pinggen Zhou, Baishen Zhou, Jingjun Guo, Donghang Li, Zhigang Ding and Yanming Feng

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In China, the geological disasters of landslide and mud-rock flows cause losses of over 1000 lives and total economic losses of over 10 billions of RMB each year. There have been about 90,000 identified landslide sites, mostly distributed in several southern and north-western provinces of China. In the reservoir area of the Three-Gorge project only, over 1000 landslide sides have been identified. A joint research was launched involving authors from a number of scientific institutions to explore technologies and methodologies for landslide monitoring with focus on the characteristics of the geological disasters at the up and middle reaches of Yangtze River. This paper studies the combined technologies for landslides monitoring and presents a demonstrative automatic landslide monitoring system in a chosen ancient landslide site, where the creeping movement process continues since its latest large sliding on August 25, 1981. The landslide test-bed is 500 m long and 300 m wide located in the Ya’an-Xiakou area in Sichuan province. To study the mechanism of the sliding process, 15 permanent GPS monuments were built in the area for regular observations. Automatic ombrometers, digital thermometers, underground water-level-meter and ground fissure-displacement-meter were set-up as well. The data from these automatic sensors are collected and automatically sent to the data process centre in Beijing via the Beidou-1 communication satellite. The paper also compares the landslide results from three GPS observing campaigns, demonstrates the feasibility to identify the displacements at the accuracy level of 2 mm using the dual-frequency GPS receivers. The results are encouraging and further analyses will be conducted, considering influences of non-GPS measurements.

S.Ravindra Babu and Jinling Wang

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Ultra-tight architecture plays a key role in improving the robustness of the integrated GPS/INS/PL (Pseudolite) system by aiding GPS receiver's carrier tracking loops with the Doppler information derived from INS (Inertial Navigation System) velocity measurements. This results in a lower carrier tracking loop bandwidth and subsequent improvement in measurement accuracy. Some other benefits using this architecture include: robust cycle-slip detection and correction, improved anti-jam performance, and weak signal detection.

Typically the integration/navigation filter run at a rate of 1 to 100 Hz, which is insufficient to aid the carrier tracking loop as such loops normally run at about 1000 Hz. Two approaches were envisioned to solve this problem. One approach is to run the navigation Kalman filter at a higher rate, and the other is to run the filter at a lower rate and interpolate the measurements to the required rate. Although the first approach seems to be straightforward, it is computationally very intensive and requires a huge amount of processing power, adding to the cost and complexity of the system.The second method interpolates the low rate Doppler measurements from the navigation filter using multirate signal processing algorithms.Due to its efficiency and simpler architectures the interpolation method is adopted here.

Filtering is the key issue when designing interpolators as they remove the images caused in the upsampling process. Although direct form of filtering can be adopted, they increase the computations.To reduce the computational burden, two efficient ways of implementing the interpolators are proposed in this paper: Polyphase and CIC (Cascaded Integrator Comb). The paper summarizes the design and analysis of these two techniques, and our initial results suggest that CIC is relatively better in terms of performance and computational requirements.

Sebum Chun, Chulbum Kwon, Eunsung Lee, Young Jae Lee, Teasam Kang and Gyu-In Jee

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The integer ambiguity should be resolved at the beginning stage of GPS carrier phase positioning. In this procedure, many kind of additional information can improve integer resolving performance. For example, the positioning information of INS/GPS integrated system and baseline constraint of two adjacent antennas can improve GPS positioning and attitude information. This improvement is well known by experiment or simulation. But its quantitative characteristic is not known yet. In this paper, we analyse this improvement quantitatively using the success rate.

Shenglin Fan, Kefei Zhang and Falin Wu

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Reliable determination of integer ambiguities is a critical issue in high-precision global positioning system (GPS) applications such as kinematic positioning, fast control surveying and attitude determination. This paper discusses the integer ambiguity resolution procedures in attitude determination using single frequency carrier phase measurements. An optimised ambiguity search algorithm is proposed. This method can not only improve the computation efficiency and reduce the time for resolving ambiguities, but also improve the reliability of the ambiguity solution. The ambiguity search space is determined using float solutions and their variance and covariance matrices estimated by applying Kalman filter algorithm. The integer Gaussian transformation is then used to reduce the size of the search space and Cholesky factorisation algorithm is used to improve the efficiency of the integer ambiguity searching process. Finally, an ambiguity validation method by using the known baseline length and the relationship between the primary and secondary ambiguity groups is presented. The algorithms have been implemented within two low-cost Allstar GPS OEM boards. A number of field experiments have been conducted and the results show that a valid integer ambiguity solution in cold start mode can be identified within 3 minutes.

Shun-Ichiro Kondo, Nobuaki Kubo and Akio Yasuda

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An algorithm is developed to process IF signal data from a GPS RF front-end module, which consists of a down converter and an ADC. The down converter converts the signal from RF to IF, the ADC samples the IF signal. All the other processing including signal acquisition, tracking, data decoding and solving position are implemented in software using base-band signal processing techniques. The local C/A codes and carrier replica signal are pre-generated, stored in memory, and used respectively during signal acquisition and tracking. In order to evaluate the algorithm, this paper demonstrates standalone positioning using the measured pseudoranges of which accuracy depends on DLL parameter of the correlator of early/late spacing. This paper presents the explanation and evaluation of the algorithm.

Simon Fuller, Philip Collier, Allison Kealy

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The growing use of real time high accuracy Global Positioning System (GPS) techniques has resulted in an increase in the number of critical decisions made on the basis of a GPS derived position. When making these decisions mobile users require assurance that the GPS position quality meets their requirements. Providers of Continually Operating Reference Stations (CORS), whom mobile users are generally reliant upon, must also be able to assure users that their data meets agreed quality standards. Unfortunately, the realistic and reliable description of position and data quality is an area in which GPS has traditionally been weak. Research being undertaken as part of the Cooperative Research Centre for Spatial Information (CRC-SI) is attempting to address this problem by assessing and reporting on the quality of raw GPS observations in real time. This paper examines a number of existing approaches to assessing the quality of raw GPS observations and presents a conceptual architecture for the development of a real time quality control system.

S. Skone and V. Hoyle

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By using a regional network of Global Positioning System (GPS) reference stations, it is possible to recover estimates of the slant wet delay (SWD) to all GPS satellites in view. SWD observations can then be used to model the vertical and horizontal structure of water vapor over a local area, using a tomographic approach. The University of Calgary currently operates a regional GPS real-time network of 14 sites in southern Alberta. This network provides an excellent opportunity to study severe weather conditions (e.g. thunderstorms, hail, and tornados) which develop in the foothills of the Rockies near Calgary. In this paper, a 4-D tomographic water vapor model is tested using the regional GPS network. A field campaign was conducted during July 2003 to derive an extensive set of truth data from radiosonde soundings. Accuracies of tomographic water vapor retrieval techniques are evaluated for 1) using only ground-based GPS input, and 2) using a ground-based GPS solution augmented with vertical wet refractivity profiles derived from radiosondes released within the GPS network. Zenith wet delays (ZWD) are computed for both cases, by integrating through the 4-D tomography predictions, and these values are compared with truth ZWD derived from independent radiosonde measurements. Results indicate that ZWD may be modeled with accuracies at the sub-centimeter level using a ground-based GPS network augmented with vertical profile information. This represents an improvement over the GPS-only approach.

Thomas Yan

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For many years, telecommunications technology has assisted GPS users in accomplishing their tasks. Dial-up system over copper phone line enables users to download data from base station at remote locations. Radio modem provides wireless communications link between a base station and a rover to enable surveyors to carry out RTK-surveys. While these techniques are still very much in use, developments in telecommunications technology over the last decade or so has brought more services providing easier use, faster speed and wider coverage. Fast spread of Internet has made TCP/IP protocols ubiquitous resulting in more devices being IP-enabled and Internet-connected. Wireless technology such as GPRS and 3G make better use of bandwidth providing faster speed and better coverage to mobile users. This paper looks at these new emerging technologies and how they could have impact on GPS users. It also discusses recent GPS-related protocols such as Ntrip and RTCM 3.0 which were designed in response to these new developments. Examples will be presented based on local trends, settings and conditions in Australia.

Troy A. Spencer , Rodney A. Walker and Richard M. Hawkes

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While GPS is a relatively mature technology, its susceptibility to radio frequency interference (RFI) is substantial. Various investigations, including the Volpe Report (Volpe, 2001) which was the result of a US Presidential Decision Directive (PDD-63) assigned to the Department of Transportation (DOT), have recommended that methods should be developed to monitor, report and locate interference sources for applications where loss of GPS is not tolerable. With GPS becoming an integral utility for developed society, the significance of research projects that enhance and expand the capabilities of GPS RFI localisation is highly important.

In response to this requirement for GPS interference localisation, a novel technique called "Inverse Diffraction Parabolic Equation Localisation System" (IDPELS) has been developed. This technique exploits detailed knowledge of the local terrain and an inverse diffraction propagation model based on the Parabolic Equation method (PEM). In wave-propagation theory, an inverse problem may involve the determination of characteristics concerning the source, from field values measured at a certain point or certain regions in space. PEM is an electromagnetic propagation modelling tool that has been extensively used for many applications. This paper will present simulation and field trial results of IDPELS. Simulation results show that this technique has good promise to be useful in locating GPS jamming sources in highly-complex environments, based on networks of GPS sensing antennas. Results also show that the method is capable of locating multiple interference sources. Trials concerning the practical application of IDPELS are also provided. With measured lateral field profiles recorded with a single moving sensor platform in a van, results indicate IDPELS to be a pragmatic localisation technique.

W C Cheuk, M Trinkle and D A Gray

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The implementation of a null-steering antenna array using dual polarised patch antennas is considered. Several optimality criterion for adjusting the array weights are discussed. The most effective criteria minimises the output power of the array subject to maintaining a right hand circular polarisation (RHCP) response on the reference antenna. An unconstrained form of this criteria is derived, in which the reference channel is the RHCP output of the reference antenna and the LHCP output of the reference antenna is included as one of the auxiliary channels. An FPGA implementation of the LMS algorithm is then described. To prevent weight vector drift a variant of the circular leakage LMS algorithm was used. The implementation details of a simplified circular leakage algorithm more suited to an FPGA implementation are presented. This simplified leakage algorithm was shown to have a similar steady state weight vector as the full algorithm.

WenBin Shen and Jinsheng Ning

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Two approaches to determining the Earth's external potential field by using GPS technique are proposed. The first one is that the relation between the geopotential difference and the light signal's frequency shift, between two separated points, is applied. The second one is that the spherical harmonic expansion series and a new technique dealing with the "downward continuation" problem are applied. Given the boundary value provided by GPS "geopotential frequency shift" on the Earth's surface, the Earth's external field could be determined based on the "fictitious compress recovery" method. Given the boundary value derived by on-board GPS technique on the satellite surface, the Earth's external field could be determined by using a new technique for solving the "downward continuation" problem, which is also based on the "fictitious compress recovery" method. The main idea of the "fictitious compress recovery" is that an iterative procedure of "compress" and "recovery" between the given boundary (the Earth's surface or the satellite surface) and the surface of Bjerhammar sphere is executed and a fictitious field is created, which coincides with the real field in the domain outside the Earth. Simulation tests support the new approach.

Werner Enderle, Cameron Boyd and Jan A. King

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JAESat is a joint micro-satellite project between Queensland University of Technology (QUT), Australian Space Research Institute (ASRI) and other national and international partners, i.e. Australian Cooperative Research Centre for Satellite Systems (CRCSS), Kayser-Threde GmbH, Aerospace Concepts and Auspace which contribute to this project. The JAESat project is conducted under the leadership of the Queensland University of Technology. The main objectives of the JAESat mission are the design and development of a micro satellite in order to educate and train students and also to generate a platform in space for technology demonstration and conduction of research on a low cost basis. The main research objectives of JAESat are the in-orbit test and validation of the SPARx receiver and its performance, the performance of the on-board Orbit Determination (OD) concept, the test of an integrated GPS-Star Sensor system concept for a 3-axis Attitude Determination (AD) and its related algorithms and also various aspects of Relative Navigation. The aspects of atmospheric research will not be addressed within this article. This article will describe the overall JAESat concept and concentrate on the QUT space applications receiver SPARx and related GPS software concepts for OD and AD. The test environment for the development of GNSS space applications will be outlined and finally simulations and respective results including GPS hardware in the loop will be presented and discussed.

Yi Zheng, Yanming Feng and Zhengdong Bai

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Tropospheric delay is one of the major error sources in GPS positioning. The delay of radio signals caused by the troposphere can range from 2 m at the zenith to 20 m at lower elevation angles. In a wide area differential system, tropospheric delays are corrected locally by users using an empirical tropospheric model, with or without meteorological observations. This can easily result in residual tropospheric errors of several centimetres to a few decimetres in positioning solutions. In this paper, the residuals between GPS-derived zenith tropospheric delays (ZTD) and model-computed ZTDs at reference stations of a continuously operating network are obtained. From these residuals, grid residual ZTDs are generated over the network coverage through Ordinary Kriging interpolation. Users can obtain the additional residual corrections for troposphere errors for improved differential positioning over a regional area. Experimental data from one week ZTD estimates from 17 GPSnet reference stations were analysed. Results show that the RMS ZTD accuracy of about 1cm is generally achievable over the Land Victoria GPS network coverage, using the proposed grid tropospheric correction strategies, which can support centimetre level positioning in the region.

Zemin Wang, Yue Wu, Kefei Zhang and Yang Meng

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New opportunities for the refinement of ionospheric modelling and reduction of the ionospheric error in GPS measurements arise since a third-frequency will be introduced for the modernised GPS system. This paper investigates theoretical models of the ionospheric refractive error. A triple-frequency method of correcting the 1st and 2nd order ionospheric refraction is presented and a triple-frequency ionosphere-free combination method is proposed for GPS modernization. These new methods can be equally applied to the European GALILEO system. In addition, typical effects of the 2nd and 3rd order ionospheric effects are also investigated, and a correct formula for the 3rd order ionospheric error is derived in a simple way for easy implementation.

It is anticipated that the proposed refraction correction methods will play an important role in both the modernized GPS and GALILEO systems. Results show that the proposed triple-frequency methods can correct the ionospheric refraction effects to a millimetre level. Since the models are given in simple forms, these corrections can be easily implemented in many real-time applications and the triple-frequency methods for correcting high-order ionospheric can significantly eliminate the error remained in the current ionospheric models. The method developed will potentially contribute to a better long-range baseline ambiguity resolution and an accuracy improvement in precise point positioning.

Corporate Members of CPGPS

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Instructions to Authors

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CPGPS Management Team (2005) Structure

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