Journal of Global Positioning Systems

Vol. 5, No. 1-2, 2006

Journal of Global Positioning Systems
Vol. 5, No. 1-2, 2006
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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Wu Chen
Hong Kong Polytechic University, Hong Kong

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I am pleased to be the guest editor for these issues of the Journal of Global Positioning Systems to publish the selected papers from the 2005 International Symposium on GPS/GNSS (GNSS-2005), 8-10 December 2005, organized by the Hong Kong Polytechnic University (HKPU), the Hong Kong Institute of Surveyors (HKIS), and the Hong Kong Institution of Engineering Surveyors (HKIES).
The GNSS-2005, attended by over 190 delegates from 16 countries, was the largest symposium in Asia/Pacific in 2005 dedicated to GNSS and wireless positioning. I would like to thank all the delegates for participating this event. The symposium included one-day common sessions and two-day parallel sessions. On the first day common sessions, several invited speakers delivered keynotes speeches on the recent developments on GNSS. The International Information Committee of the US Civil GPS Service Interface Committee (CGSIC) convened an information session on GNSS developments. In this session, the representatives from the USA informed the latest policy and development on GPS and modernization. The representatives from countries in the Asia-Pacific region also reported their GNSS programmes. A panel discussion session was also organized to discuss future trends of GNSS development.
Over 170 papers presented at the symposium. However, given limited space pages in these special issues, many high quality papers could not be selected for publication. I wish to thank all reviewers for their efforts in reading and selecting papers.

R.E. Phelts
Stanford University, USA
D.M. Akos
University of Colorado, USA

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Satellite-based navigation requires precise knowledge of the structure of the transmitted signals. For GPS, accurate knowledge of the shape of the code correlation peaks is required to ensure no biases are introduced into the position solution. It is generally presumed that all GPS-like satellite signals are virtually identical. However, in 1993 a satellite malfunction introduced significant distortion onto one of the satellite C/A codes. That distortion caused range errors to vary with receiver filter characteristics and code tracking loop implementation. As a result, high-integrity systems such as the Wide Area Augmentation System (WAAS) must implement signal deformation monitors to detect and remove signals that become anomalously distorted. In the future, WAAS will rely on modernized signals from both GPS (L5) and Galileo (E1/L1/E2 and E5A/E5B). This should increase performance for users; however they must still protect against potential signal deformations. Although the International Civil Aviation Organization (ICAO) has agreed on a threat scenario for GPS L1 signals, no such agreement exists for modernized signals. In addition, each of these signals will have different chipping rates and their correlation peak structures will be quite different from that of the GPS C/A code. Their code tracking loop implantations are as yet not well-defined, but may differ somewhat from traditional architectures. An additional complication is the unknown receiver filter characteristics that the new receivers will employ. Each of these factors may render a given signal and/or receiver configuration more or less sensitive to signal deformations. This paper analyzes the range error sensitivity of several modernized signals subjected to distortions of the type considered in the ICAO threat model for signal deformations. To isolate the effects of the signal-in-space deformation errors, it assumes an ideal, wideband receiver filter and basic early-minus-late code tracking implementations for the new codes. It also compares the distortion-induced range errors for the new codes to those currently modeled for the C/A code. Finally, these results are used to motivate threat model refinements and receiver tracking loop constraints that minimize the affects of this error source for the modernized GNSS signals.

S. Feng, W.Y. Ochieng and R. Mautz
Imperial College London, United Kingdom

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Receiver Autonomous Integrity Monitoring (RAIM) is a method implemented within the receiver to protect users against satellite navigation system failures. Research has shown that traditional methods for the determination of RAIM holes (i.e. places where less than five satellites are visible and available) based spatial and temporal intervals (grids) compromise accuracy due to the constraint of computation load. Research by the authors of this paper has addressed this and developed a new algorithm to determine RAIM holes using bounded regions instead of approximation based on grid points. This paper uses the new algorithm and proposes an area based method for the computation the RAIM satellite availability statistic based on the ratio of the total area of RAIM holes and the coverage area (regional or global area). Assessment over time is based on the interpolation using a model generated from snapshot spatial statistics at a relatively long temporal interval. Test results show that the area-based method for the calculation of the RAIM satellite availability statistic is significantly more accurate with lesser computational load than the traditional grid points based approach.

H. Setan and R. Othman
Universiti Teknologi Malaysia, Malaysia

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Offshore platforms undergo subsidence, especially due to production activities. The monitoring of such subsidence requires specialised technique (i.e. GPS) as the platforms are situated at sea (i.e. hundreds of kms from mainland). Unfortunately standard GPS processing procedure is unable to achieve the required accuracy (several cm) over long baselines. The research focuses on the development of processing strategy (i.e. estimation and subsidence analysis) for monitoring of offshore platform subsidence using three permanent GPS stations of the Malaysia Active GPS system (MASS). The estimation of each epoch employs Bernese GPS software, followed by subsidence analysis using in-house software. Preliminary results are also shown.

K.K. Fan, X.L. Ding
The Hong Kong Polytechnic University, Hong Kong

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This paper presents a method that is based on Electromagnetic Modeling (EM) technique for modelling GPS carrier phase multipath signals. A commercial software plus modules developed in-house are used for modeling and processing carrier phase multipath error parameters. Static multipath modeling experiments show that up to about 35% carrier phase errors and about 25% 3D positioning errors can be reduced.

D. Manandhar
The University of Tokyo and GNSS Technologies Inc., Japan
R. Shibasaki
The University of Tokyo, Japan
H. Torimoto
GNSS Technologies Inc., Japan

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A reflected GPS signal is normally considered as noise (multipath). However, we believe that a reflected signal contains information about the reflecting object. Though, this information may not be useful for accurate position computation, it may help us to identify the reflecting object itself, which is a type of remote sensing. Besides, by measuring the time delay of the reflected signal, it is possible to estimate the extra path length the reflected signal has to travel. Hence, in general, the analysis of reflected signal can be used for two broad categories of altimetry and remote sensing. However, this type of analysis is rather limited with current commercial GPS receivers. We are in the process of developing a software-based GPS receiver that is capable of reflected signal analysis for remote sensing purpose. The receiver consists of multi-polarization (RHCP and LHCP) array of GPS antennas, front-end device, a PC and necessary software for signal processing. In this paper, we discuss about the system architecture and algorithms, results of reflected signal analysis observed at different places in different conditions and antenna types. Finally, we will discuss the possibility of the system for remote sensing applications using GPS signal.

H.Y. Chen, S.B. Yu, L.C. Kuo
Academia Sinica, Taiwan, ROC
H.Y. Hu
Kun San University of Technology, Taiwan, ROC

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The MW 6.5 Chengkung earthquake occurred at 04:38 UTC on 10 December 2003. Thirty continuously recording GPS stations (CORS) have been set up at different geological sites and distributed throughout a 140 km by 140 km area in southern Taiwan beginning in 2000. The GPS data is recorded daily for the CORS in 30 seconds sampling rate. The GPS data is utilized to study the coseismic and postseismic deformation associated with the Chengkung earthquake. The coordinates of the daily solution for each station were extracted from SINEX (Software INdependent EXchange) files to establish time series in the topocentric north-east-up (NEU) coordinate system. The secular crustal deformation of the station during the ne year period was removed by applying the 2000-2003 interseismic velocities. The CORS near the Chihshan fault, which are located at the southern segment of the Longitudinal Valley Fault, indicated the largest postseismic displacement in eighteen months approached 86 mm (station SHAN) and 91 mm (station TAPO) in the horizontal and vertical components, respectively. The results of the CORS provide detailed information for the temporal process of postseismic deformation.

M. Yu, Z. Li, Y. Chen, and W. Chen
Hong Kong Polytechnic University, Hong Kong

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Map-matching (MM) is a technique that attempts to locate an estimated vehicle position on road network. Many map-matching algorithms have been developed and widely incorporated into GPS/DR vehicle navigation systems for both commercial and experimental ITS applications. However, the reliability of these systems is still a problem because vehicle position may be located to an incorrect road section due to large vehicle positioning errors which occur frequently in urban areas. This incorrect locating is called a mismatch. To improve map matching techniques, it is necessary to enhance the ability of mismatch detection and to reduce the chance of mismatch, which are referred as integrity and reliability respectively. New techniques are developed in this paper to improve the integrity and reliability of map matching techniques. The new techniques have been integrated with a GPS/DR system and extensively tested in Hong Kong. Testing results demonstrate that the performance of the new integrated GPS/DR system is significantly improved in terms of its accuracy, coverage and reliability.

B.K. Choi, J.U. Park, J.K. Chung
Korea Astronomy & Space Science Institute, South Korea

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The vertical profiles of electron distribution near the low and mid-geomagnetic latitudes have been investigated by the computerized tomography method using Algebraic Reconstruction Technique (ART). The slant total electron contents (STEC) data for ionospheric tomography were measured at a regional GPS reference network of nine stations that have been operated by Korea Astronomy and Space Science Institute (KASI). The results from tomographic reconstruction method are in good agreement with profiles obtained by Ionosonde near the KASI GPS stations. The electron density profiles reconstructed by the tomographic method were compared with the results from Ionosonde and IRI-2001 model. GPS tomography reconstruction plays an important role of complementary measurements of Ionosonde in ionospheric structure.

S. Wu, K. Zhang, F. Wu
RMIT University, Australia
Y. Yuan
RMIT University, Australia and Chinese Academy of Sciences, China

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The atmospheric effects, especially the ionosphere, are the key limiting factors for real-time high accuracy positioning using the network RTK technique with a medium-to-long-range baseline separation. To investigate suitable approaches to improve ionospheric modeling towards a real-time CM-level positioning using the Victorian continuously operating reference stations network (i.e. GPSnet) system under various ionospheric conditions, this paper investigates both temporal and spatial variations of the ionospheric total electrons content (TEC) over Victoria through analysing GPS dual frequency data from the GPSnet over a period of two years. Diurnal and seasonal ionospheric variations, and winter anomaly of the ionosphere in Victoria are investigated based on GPS-derived TEC values. Results suggest that the temporal and spatial TEC variations over Victoria are complicated. This complex nature of the ionosphere suggests that it is a challenging task to precisely represent the behaviours of the ionosphere if only a single and simple ionospheric model is used for all the time for RTK uses. It is therefore, necessary to develop new mathematical models or new procedures for precise representation of the ionospheric TEC variations in Victoria using a long period of GPS dual frequency observations, particularly the predictability of the ionosphere changes. It is expected that the new approach will provide a better guidance for the state-wide network-RTK solutions.

J. van Cranenbroek, R. Keenan
Leica Geosystems AG, Switzerland
V. Lui
Leica Geosystems Ltd, Hong Kong

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The authors have acquired extensive experience in marketing and deploying GPS RTK Network infrastructures worldwide, including design, implementation, training and support. They have been regularly confronted by some crucial questions asked by potential GPS network operators on ways to retrieve investment and on how this infrastructure could be turned into a profitable business. In many ways, the GPS RTK Network infrastructure-based services are similar to the mobile communication business, and more particularly like a mobile information content provider business. The apparent difficulty in turning this high tech infrastructure into a profit-making venture is mainly due to the fact that the actual service/product providers are in a direct relationship with their users. Both are focused on technology, not on the business aspects. The benefits of GPS RTK Networks are numerous and cost savings are the most important of the adoption drivers. But for how long? New operator profiles are appearing and they would like to understand how to justify their investment. They are also looking to generate new services based on the GPS data streams. Today, with the advent of the second generation of GPS RTK Network software like the Leica GPS SpiderNET, things could change significantly. The network correction messages format will be soon standardized by the RTCM organization, but these represent only a fraction of the products that such software/infrastructure can generate. The widespread and easy access of high-speed Internet and various forms of wireless connection are now cutting significantly the fixed costs associated with running such infrastructures, and for accessing the GPS real-time data products in the field. New lower cost GPS RTK receivers and GPS-integrated Total Stations are increasing the number of users. Moreover, the ease of use of such devices by non-survey operators will also enlarge the customer community. The topic of this paper is the new business models, new coming business operators, and a totally fresh vision of what this technology can offer, so as to sustain the development of new products and services that most people are ignoring today. The new GNSS constellations like GALILEO will just amplify this trend that will capitalize on the accuracy, reliability and high-speed location addicts.

K.H. Kim, J.G. Lee
Seoul National University,Korea
C.G. Park
Seoul National University, Korea

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This paper proposes an adaptive two-stage extended Kalman filter (ATEKF) for estimation of unknown fault bias in an INS-GPS loosely coupled system. The Kalman filtering technique requires complete specifications of both dynamical and statistical model parameters of the system. However, in a number of practical situations, these models may contain parameters, which may deviate from their nominal values by unknown random bias. This unknown random bias may seriously degrade the performance of the filter or cause a divergence of the filter. The two-stage extended Kalman filter (TEKF), which considers this problem in nonlinear system, has received considerable attention for a long time. The TEKF suggested until now assumes that the information of a random bias is known. But the information of a random bias is unknown or partially known in general. To solve this problem, this paper firstly proposes a new adaptive fading extended Kalman filter (AFEKF) that can be used for nonlinear system with incomplete information. Secondly, it proposes the ATEKF that can estimate unknown random bias by using the AFEKF. The proposed ATEKF is more effective than the TEKF for the estimation of the unknown random bias. The ATEKF is applied to the INS-GPS loosely coupled system with unknown fault bias.

K.J. Parkinson, A.G. Dempster, P. Mumford, C. Rizos
The University of New South Wales, Australia

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A project to build a GPS receiver using an FPGA for base-band processing began in 2004. The new receiver platform uses a commonly available RF front end ASIC to convert the GPS signals to a suitable IF. The digital design for baseband processing is normally a reasonably straight forward task. However, because the received GPS signals are at such low levels this presents some challenges. One of the main considerations is to avoid contamination of the incoming signals with interference that can be generated from the digital electronics when using an FPGA. In this paper we describe the hardware design process with a focus on avoiding interference while still allowing complex FPGA logic to operate alongside sensitive GPS RF signal processing.

E. Wei, H. Chai, Z. An, J. Liu
Wuhan University, China

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In the past few years, network RTK positioning technology, especially the VRS?virtual reference stations?technology, has been widely used in some parts of China and many countries of the world. In this paper, the authors mainly discuss the principle of VRS technology with corresponding formula deduction, and give detailed descriptions of VRS corrections and virtual observations generation algorithm as well as their applications.

W.H. Kim, J.G. Lee
Seoul National University, South Korea
H.K. Lee
Hankuk Avigation University, Korea
C.G. Park
Seoul National University, South Korea

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When GPS blockage occurs for a loosely coupled GPS/INS system, its navigation error diverges. To deal with such cases, this paper introduces an integration scheme for GPS, INS, and an image sensor. The proposed integration scheme is attractive in that it accomplished the position and velocity accuracy improvement by the angular information only. The angular information is provided by the gimbal angles of the image sensor. A realistic scenario is studied by a simulation to demonstrate that the GPS/INS/Image integrated navigation system works effectively.

Y. Li, A. Dempster, B. Li, J. Wang, C. Rizos
The University of New South Wales, Australia

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This paper describes a prototype system for attitude and heading determination. A L1-only GPS receiver is integrated with microelectromechanical gyroscopes, accelerometers and magnetometers. In contrast to a multi-antenna/multi-receiver GPS attitude determination system, this system uses a single antenna/single receiver configuration to derive standalone velocity and acceleration solutions from the GPS L1 carrier phase measurements. No reference station is needed to form differences of carrier phase measurements for the velocity and acceleration calculation. The GPS-derived acceleration is further used in the attitude determination by combination with the three-dimension acceleration sensed by the accelerometers. The magnetometers sense the Earth's magnetic field intensity, and can give the heading estimation regardless of the status of the host platform. To satisfy real-time applications, infinite impulse response differentiators instead of finite impulse response differentiators are used to derive the acceleration from GPS. The algorithms have been implemented and their efficiency demonstrated by experiments.

D. Li, J. Wang, S. Babu
The University of New South Wales, Australia

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The integration of GPS, PL and INS sensors can be implemented at three different levels. Compared with loose and tight integration, ultra-tight integration offers numerous advantages including increased robustness under high dynamics, and improved anti-jamming performance. In current ultra-tight integration scenarios, a centralised Kalman filter is commonly employed to fuse either In-phase (I) and Quadrature (Q) data from the tracking loop or the pseudorange measurement and Position, Velocity, Attitude (P, V, A) measurements from the Inertial Navigation System (INS). Though relatively simple, this centralised filter structure has some disadvantages. Firstly, to reduce the computational load, the filter only makes coarse estimates of Inertial Measurement Unit (IMU) random errors, which significantly degrades the system performance. Secondly, for more accurate estimates, the filter becomes much more complicated, resulting in a large increase in the computation time. All of these hinder the performance of ultra-tight integration considerably. This paper proposes a federated filter structure for the ultra-tight integration of GPS, PL and INS sensors. The new filter structure distributes the computing tasks to different Kalman filters, leading to reduced filter complexities and improved system performance. IMU random errors are estimated separately by the pre-filter at a high data rate, whilst the main filter has a simplified structure, i.e. no estimation of the IMU random errors, and operates at a relatively slow rate. This paper will discuss the dynamic modelling method based on the Walsh function transform for implementing the pre-filter and the simplification of the main filter. Simulation tests were performed to compare the performance of the federated filter with that of the usual centralised Kalman filter in the estimation of the IMU random errors. The results show that with the simplification of the Kalman filter structure, the federated filter design can achieve the almost equally precise estimates as the centralised Kalman filter does but with less computational burden. Hence the federated design is more suitable for implementing the ultra-tight integration for real-time applications. Finally, the simulated high dynamic flight test results of ultra-tight integration based on the federated Kalman filter are presented.

M. Lu, X. Shen, H.C. Lam, W. Chen
Hong Kong Polytechnic University, HKSAR, P.R.China

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The escalating scale and growing complexity of ready-mixed concrete (RMC) delivery operations have challenged current methods for monitoring RMC delivery and dispatching truckmixers by two-way radio voice-based communication and experience-driven management. This paper describes an integrated navigation system, based on Global Positioning System (GPS)/Dead Reckoning (DR)/Bluetooth Beacon (BB) and Global System for Mobile Communication (GSM) technologies. The application of the integrated navigation system in tracking RMC truckmixers on and off Hong Kong construction sites is presented. The system performance in terms of positioning precision in tracking RMC truckmixers is analyzed. In addition, the readily available data of event times resulting from tracking RMC truckmixers is found conducive to high-end simulation modeling for productivity analysis and improvement in construction.

G. Retscher
Vienna University of Technology, Austria

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In the research project "Pedestrian Navigation Systems in Combined Indoor/Outdoor Environements" (NAVIO) we are working on the development of modern intelligent systems and services for pedestrian navigation and guidance. In the project modern and advanced intelligent mobile multi-sensor systems should be employed for 3-D position determination of a user. Due to the fact that satellite positioning with GNSS (Galileo, GPS, etc.) does not work under any environmental condition (e.g. in urban "canyons" with no satellite visibility and indoor) a combination and integration with other sensors (e.g. dead reckoning sensors, inertial navigation systems (INS), indoor location techniques, cellular phone positioning, etc.) is essential. In our approach a loose coupling of the employed sensors should be achieved and it is proposed to develop a multi-sensor fusion model which makes use of knowledge-based systems. As far as we can see now knowledge-based systems can be especially useful. Thereby the decision which sensors should be used to obtain an optimal estimate of the current user's position and the weightings of the observations shall be based on knowledge-based systems. The new algorithm would be of great benefit for the integration of different sensors as the performance of the service would be significantly improved. In this paper the basic principle of the new approach will be described. To test and to demonstrate our approach and results, the project takes a use case scenario into account, i.e., the guidance of visitors to departments of the Vienna University of Techology from nearby public transport stops. The results of first field tests could confirm that such a service can achieve a high level of performance for the guidance of a pedestrian in an urban area and mixed indoor and outdoor environments. Standard deviations in the range of few meters can be achieved for 3-D positioning in urban areas although obstructions cause frequent loss of lock for satellite positioning. Thereby GPS outages of up to 150 m can be bridged using dead reckoning observations with the required positioning accuracy. For indoor areas satellite positioning can be replaced by indoor positioning systems (e.g. WiFi, UWB). Due to the development of advanced sensors it can be expected that such multi-sensor solutions will be deployed in pedestrians navigation services. We believe that these services will play an important role in the field of location-based services in the near future as a rapid development has already started which is driven by their possible applications.

E. P. Glennon
SigNav Pty Ltd, Australia
A.G. Dempster, C. Rizos
The University of New South Wales, Australia

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The feasibility of using GPS as a bistatic radar illuminator for the purposes of air target detection is examined. The power budget analysis is first performed assuming the use of a single satellite, but is followed by a discussion of the expected improvements when multiple satellites are employed. The analysis includes the effect of GPS signal strength dynamic range, also known as the 'near-far' problem. The difference between the radar cross-section (RCS) of a typical air target and ground-based clutter reflections is discussed, followed by an estimation of the effect of ground clutter on the operation of such a system.

W.J. Kellar, M.P. Moody
Queensland University of Technology, Australia

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The provision of data services by wireless telecommunications providers has spurred interest in using these data links to transmit differential GPS corrections. Technologies such as General Packet Radio Service (GPRS) are able to satisfy the bandwidth requirements for distributing corrections in a network RTK system, however many mobile service providers charge a per-byte cost for the quantity of data transmitted. This contributes significantly to the total cost of operation for a GPRS based RTK system. Previous research by the author has shown that a significant reduction in data volume requirements can be achieved through differential entropy coding of CA code differential GPS corrections. The technique has been expanded to include carrier phase and P code pseudorange information. The paper presents the design and implementation of a data compression scheme based on differential entropy coding. The bandwidth requirement of this data format is compared with that of several other popular RTK data protocols. A study of the information content of dual frequency RTK corrections was undertaken in order to estimate the maximum reduction in data size that may be achieved through the use of optimal codes. Several polynomial prediction strategies are compared. Results show that the volume of transmitted data for carrier phase and pseudorange measurements can be reduced by as much as 90 percent when compared to RTCM 2 messages, without any reduction in precision or decimation of the sample rate.

N. Brown, I. Geisler and L. Troyer
Leica Geosystems, Switzerland

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The Master-Auxiliary Concept, jointly proposed by Leica Geosystems and Geo++, is the basis of the soon to be released RTCM 3.0 network messages, the first industry standard for network RTK. The new standard, in addition to promoting increased compatibility and innovation in the industry, offers some distinct advantages to the end user over the previous generation of network corrections, such as VRS. With the Master-Auxiliary Concept complete information on the prevailing errors sources is made available to the rover, thereby facilitating the use of more intelligent positioning algorithms in the determination of the rover's position. The net result is an increased robustness of the system and increased performance in terms of time to fix, reliability of the ambiguity fix and position accuracy. Empirical data from both Leica and third party reference station software and rover receivers is used to demonstrate the real world benefits of the Master-Auxiliary Concept in general and the Leica solution in particular. Clear improvements can be seen when combining the Leica GPS Spider network RTK software with the Leica System 1200 GPS receivers, even when using network correction data at a sampling rate of only 5s.

N. Brown, L. Troyer, O. Zelzer and J. van Cranenbroek
Leica Geosystems, Switzerland

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In many monitoring applications, such as tall buildings, bridges and volcanoes, GPS offers significant advantages over other measurement techniques. GPS allows a high rate of measurement and long distances between the control and monitoring points and does not require line of sight to the control points. The traditional dual-frequency GPS receivers used in surveying are high accuracy but also relatively high cost per monitored point and therefore often prohibitive for the deployment of a GPS monitoring network. Until recently, cost effective L1 sensors and software have not been able to provide the necessary level of accuracy and reliability. A new solution from Leica Geosystems provides real time and post processed RTK positioning with L1 only GPS receivers for monitoring applications. The solution is built on Leica's RTK positioning algorithms, which have proven world-class performance in the surveying industry. The positioning algorithm used in the Leica GX1230 RTK GPS receivers has been implemented in the GPS Spider reference station software and tuned for monitoring applications, giving GPS Spider the capability to compute real time ambiguity fixed solutions for single and dual frequency GPS in addition to its powerful site configuration and data management tools. A direct link has been made between GPS Spider and Leica GeoMoS, Leica's geodetic monitoring software, so that users can combine GPS with the sophisticated terrestrial measurement capabilities of Leica's robotic total stations and utilize GeoMoS's flexible messaging and data analysis capabilities. In addition the RINEX data logged by GPS Spider may be automatically post processed for users with the highest accuracy and reliability requirements. This paper presents results from the system, including using a new ambiguity resolution technique, called quasi-static initialisation, designed for single frequency monitoring. Results from L1 processing are compared to a dual frequency solution in terms of accuracy and reliability. Data was collected with a range of baseline lengths up to 20km in medium multipath environments, typical of many monitoring applications. The L1 system is shown to have remarkable accuracy and reliability, especially in terms of price versus performance.

Corporate Members of CPGPS

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Instructions to Authors

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CPGPS Management Team (2006) Structure

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